Bob Evans
Administrator
OLD STYLE PISTON
Now we are ready to set up the piston rings. The largest contributor to horsepower loss through internal friction is from ring and piston friction(drag). Something in the neighborhood of 70 - 80 % of total friction HP lost on any motor. Calculations have shown as much as 1.07 HP lost from total friction within a Briggs engine. 70% lost for the rings and the piston is near 3/4 of a horsepower! The largest single contributor to this ring drag/friction is the oil ring. Now that WKA has dropped the check for the minimum inside diameter of the oil ring, it is not necessary to use old or cut rings. The easiest method for reducing drag is to heat shrink the ring to reduce the wall tension. This process applies ONLY for the older style piston/rings. The new RAPTOR III rings do not need to be shrunk in my opinion. They come with very little tension straight from the factory. But back to modifying the older style rings. You can do this by placing the ring into an old cylinder making sure it is square in the bore. You can use the piston with the second ring installed to push the oil ring down just far enough to square it up. Now take a propane torch and heat the ring to a dull read and work your way around the ring moving along as each portion obtains the dull red appearance. When you have finished let the ring air cool. Using the block to do this is a very, very slow operation, as it takes quite a while to heat up such a large heat sink.
An alternate method, if you are going to do many of these, is to obtain a replacement sleeve for the Briggs from a supply house. You will find that this will heat up much quicker and yet retain the roundness you need. I simply have taken the sleeve and had it cut by a machine shop into several pieces and bored the pieced to the exact size needed for each bore size marking the piece .010, .020, etc. It is very important to use a round fixture for this, no matter whether you use the block or a sleeve. You can also use some of the aluminum quick ring compressors from MISI that are available in all piston sizes for this operation. These are available from Goodson for around $18 each. No honing or sizing is necessary with these but you can only do one ring at a time.
We currently shrink the oil ring that comes with the piston whether it’s a standard or .030 over. Always use a new oil ring for this operation. If you compare a new ring against an older used ring, you will note the difference in the size of the actual surface contacting the bore. The new ring will have very small edges and thus contribute to less drag after we shrink it. You should also shrink the second ring on the Briggs in the same fixture with the same method. The top ring gets a little tricky. I do not suggest you shrink the top ring at all. Simply use the next largest size top ring for the cylinder such as a .010 for a standard size cylinder and cut it for the proper end gap. Pre shrunk rings are also available from several cam manufactures if you don’t want to experiment with this process. Dyno and Precision are two that offer these sets.
Cutting the end gap on the top ring is a slow tedious process. You can cut the end gap with a file, but the trick is to keep the end square. If you want to try and use a file, get the thinnest one you an find such as a jewelers file. Place the file in a vice and then compress the ring against the file with both ends against the file body. You can then move the ring down the file cutting both end in unison. This will help keep both sides square to each other.
I currently use a ring end gap tool available from most auto supply stores. It is much quicker to use and does a very good job of keeping the end gap square. I have also actually used my Dremel tool with a sanding disc installed to accomplish the same gapping during my earlier years of building. I simply squeezed the ring together on the rotating disc. This will cut both sides at the same time and keep them concentric. Work slowly, as you don’t want to cut too much and end up with excessive end gap to start with. We work to have .001 - .002 starting end gap on the top ring. You can go lower than this if you are sure that your cylinder is very round. A good test is to push the ring, once it’s gapped, completely through the cylinder using your piston. If you feel no substantial increase or decrease in pressure while doing this, you can be sure your cylinder is round. Also take ring gap measurements in several spots down the cylinder. A quick way to determine if your bore has taper.
The final step in ring preparation is slightly hone the edges and ends of the ring to break the sharpness. I use a small(3/8”) wet stone in fine grit to accomplish this on both the top and second ring. You are not trying to create any angles just breaking the sharp edges. I start on one side of the ring and slightly radius the sharp end around the en gap. Then move around the complete edge of the rings breaking the sharp surface. This process will help to eliminate the ring scratching the cylinder during break-in. Many builders will also surface the flat sides of the rings on a jig and wet/dry sandpaper prior to installing them on the piston. MISI makes a specific tool for this operation and it is available from most kart shops or mail orders houses.
Today, I only check the flatness of the rings as too much sanding will create a new problem with the rings. RING FLUTTER. Ring flutter is an uncontrolled vibration of the top ring in the piston groove during high RPMs. What is happening is instead of the ring laying flat against the piston land it is being moved up and down(vibrating) in the piston land due to cylinder pressure. When this occurs you will be loosing compression(HP) as the ring is not sealing the cylinder wall as well as it should. Most Briggs rings and standard pistons will have a side gap of .004 -.006 out of the box, so sanding the ring will worsen this potential condition. The ideal set-up would be around .0015 to .0025 clearance.
Burris has attempted to help motor builders with this condition with the introduction of their new PLUS 4 ring. This ring is .004 thicker than their standard ring allowing the builder to custom fit the top ring to the piston ring land. Motors that have this problem will exhibit a ‘graying’ of the upper portion of the cylinder. Typically these motors will also get to a given RPM and just go flat in the power band. Very similar to uncontrolled valve float.
Another area to work on is the rod and crank clearance. Typically a new crankshaft from Briggs comes in at .999 or .998 in diameter. A standard rod out of the box will usually be at 1.000 to 1.001. So you can see that you can possibly have from .001 to .003 clearance if you do not check the components. Using a combination of a quality dial caliper and micrometer check the clearance. I work to have .003 on a new motor. If you have above .006 you are going to start breaking parts(rod!!) or pulling the wrist pin out of the piston(really bad news). If the crank is down to .996 use it in the fun motor on your neighbors yard Kart!!! It seems expensive but really a replacement crank is cheaper than a whole new motor!. Also, I would suggest using a hardened crank.. This is just a durability issue no speed here. If you can find some of the older Briggs heat treated cranks from their industrial motors of days gone by, get them. You can recognize them by the dark edges around the crank journal. These are hard as a rock and just never seem to wear out!. The new crank on the Raptor II and III motors is quite good as it has been inductive hardened in the journal area and seems to be as good as older cranks in this area.
As to obtaining the proper clearance, you can use the Flex-hones from Brush Research Manufacturing mentioned in my carb chapter. These can be obtained in 1” as well as 1/2” for use on the large and small ends of the rod. Take the rod and torque the bolts down to exactly 100 inch lbs. before honing the journal end. This will insure roundness after you are finished. Simply use the same honing oil as you used in cylinder preparation. Hone the journal end for the .003 clearance we talked about and then hone the piston pin end of the rod. If the rod is already at the correct size for clearance, lightly hone it to develop a cross hatch. A simple rod honing fixture can be made from 1” ID pipe to align the rod and hone. This insures proper alignment of the journal end. The 1” pipe is welded to a piece of 1” angle iron and then the middle is cutout to allow for the rod to be inserted. On my unit, one side is built to slide in and out. This allows for a tight fit up against the large end of the rod. The wrist pin hole should be honed just enough to allow the pin easy travel back and forth. Don’t get too much clearance here as you don’t want the wrist pin slapping around while you’re running.(.001 is about right). Do the same operation to the piston. Hone the pin holes just enough with the 1/2” hone to obtain free movement of the pin. This will give you a free floating pin and give a nice cross hatch for oil retention. The oiling hole on the rod can also be enlarged to .177 max(WKA). I use a .173 reamer for this operation and do it prior to honing the crankshaft end of the rod so any burrs left by the reamer are removed.
The only modifications to perform on the crank are making the bearing a slip fit if you have an non RAPTOR crankshaft and polishing the rod journal. Any kart shop can turn the shaft down to allow for a press fit. WKA has a minimum dia. for this operation of .775. You can also perform this operation on a 1” belt sander if you like, just be careful of the minimum diameter. Removing the bearing on a pressed fit can be problem. The easiest method is to use a bearing separator to hold the back side of the bearing in a vice, while you pound the end of the crank with a soft lead hammer, driving the bearing off the shaft. Sounds crude but this works very well.
Polishing of the crankshaft journal can be done by locking the crank in a vice and polishing the journal with a 1” strip of fine grit Scotch Brite using a lapping motion. Very light operation here as too vigorous an operation may get you tossed in the tech barn. All you want is a lightly polished surface.
NEW RAPTOR III Piston:
The same style piston and ring set has been used since the introduction of Briggs and Stratton 5hp racing. All of this has changed this year(99) with the introduction of the new RAPTOR III piston. This piston was designed for Briggs by the A.E. Goetze division of Federal Mogul. Major changes in the design and makeup of the piston have introduced a whole new ball park for the engines builders this year. In this portion of the book, we will go over the specifications of the new piston compared to the older style and give specific installation recommendations for it’s use in racing.
The new piston is made of a new alloy called Eutectic. The term Eutectic refers to the composition of the piston as it relates to the percentage of Silicon in the aluminum alloy. Silicon addition to aluminum is best related to adding sugar to iced tea. You can continue to add and mix sugar until the mixture reaches it’s saturation point. At some point adding additional sugar will result in it not mixing with the tea and simply falling to the bottom of the glass. Silicon in aluminum is very similar.
Silicon can be added to the aluminum up to the point of saturation. Any additional silicon added will simply precipitate out in the form of hard particles. The point of saturation in aluminum is known as the Eutectic and occurs around the 12% level. Aluminum alloys with less than the 12% are known as Hypoeutectic and alloys with silicon contents above the 12% level are called Hypereutectic. The new piston is Eutectic in makeup, so it contains right around 12% silicon.
Eutectic pistons show improved strength and are economical to produce. In addition to improved strength, the silicon reduces the expansion of the alloy when compared to straight aluminum and aids to it’s ability to reduce friction and scuffing. This reduced expansion will come to light a bit later in this chapter when we discuss cylinder clearance.
As you can see in the accompanying pictures of the two pistons, the new styles design is vastly different from the old style. The barrel shape leads itself to several improvements.
Improved stability in the bore
Aids in creating an oil film against the cylinder
Reduced friction(less surface area against the bore)
Allows for closer piston/cylinder wall clearances.
In addition to the physical makeup of the piston, the wrist pin hole has been offset to reduce torsional stress throughout the length of piston travel. The design also incorporates a shorter (less weight) wrist pin to help control flex and yield improved strength. The recommended installation of the piston has the arrow on top pointing to the Flywheel side of the block.
The ring package on the new piston also is new and unique to the piston. While the rings appear to be of similar design when compared to the old style, they are much smaller and lighter. They also have much less initial sidewall pressure eliminating the need to shrink the rings to reduce friction. The top ring has a bevel face to improve break-in, while the second and oil rings are very similar in design to the older style. Other than setting the end gap and breaking the sharp edges of the rings very lightly they are good to go from the factory.
The new pistons are also approximately .002 larger than the old style for a given size piston. For example the standard bore new pistons measure 2.5610 at it’s largest point(1.378 inches from the top of the piston) compared to an average of 2.5589 for the old style. This .002 size difference runs through the entire new line of RAPTOR III pistons. Speaking of the line of pistons, we now have several new sizes to play with. The old pistons came in standard, .010, .020 and .030 over. With the introduction of the new parts, we now have seven sizes of replacement pistons. This should give us more life out of our motors as we move from one bore size to the next. The new pistons and their Briggs part numbers are listed in the following table.
Size Part # Ring Set Size
Std. 555478 555485 Std.
.010 555479 (Use .015 ring set)
.015 555480 555486 .015
.020 555481 (use .025 ring set)
.025 555482 555487 .025
.030 555483 (use .035 ring set)
.035 555484 555488 .035
As you can see Briggs is not making a ring set for each new piston size. Given the fact that engine builders are going to fit each piston and ring set to a given motor, simply using the new larger size ring set will work in most cases. The one missing piece today is a .040 over top ring for the .035 piston. The Briggs .035 ring set top ring has too much end gap for most builders (.011). If you purchase the pistons as parts be aware that for the standard, .015, .025 and .035 sizes the ring sets that come with them in the box, will have a top ring with too much end gap. This forces you to use the next size up top ring. Top rings are available from Briggs as a unique part # these days. Also Burris and Power Products have aftermarket top rings for the new piston. These will be legal in 2000 for WKA.
End gap recommendations should go along the same guidelines as the older style piston with the top ring being installed with something around .0015 - .002 end gap(be sure you have a straight bore). The second ring should be around .002 - .004 and the oil ring around .008-.010 although it can be tighter. As to cylinder clearance, that’s a whole other story folks. Big changes here. Due to the design and makeup(Eutectic) of the piston it will not expand under heat as much as the older piston, so we can run much tighter bore clearances than we are used to doing. The offset wrist pin also comes into play here as too much clearance will allow the piston to rock in the bore ,creating several problems(excessive ring wear and scuffing).
Briggs recommends the following procedure for installation of the new style piston. First, measure down from the top of the piston 1.378 inches to the center of the trust faces(90 degrees from the piston pin). Due to the sides of the piston being barrel shaped, this will be the point of the largest diameter. From this point measure the diameter of the piston with a quality 3” micrometer.
Using the size found above, finish the bore to a size from .0005(yes that’s 1/2 thousand of an inch) to .0025 clearance. They recommend using these clearances on both cool bore and I/C blocks. Now folks, .0005 scares me to death, given the many warped cylinders I’ve seen over the years. The Briggs blocks have a natural tendency to settle(warp) a bit as they are heat cycled during their early life. They also seem to settle down after this initial heating and cooling. So what does our experience to date show. Somewhere around .002 to .0025 starting cylinder clearance seems to work very well in both style blocks. My personal experience, as well as, the experience of other builders I know, has seen everything from .007 to .001 tried with varying degrees of success. The larger clearances seem to work, but ring wear is very rapid and forces you to rebuild the motor more often than necessary. I did have a good cool bore that was run with the new piston at .005. After three races the cylinder had an actual imprint of the piston shirt at BDC!! I’m currently using .002 with good results. After a bit of experimentation, it seems the new rings like a very fine cylinder finish. Something around 320-400 to 600 grit stones should be used for the final finish. Either honing stones or the Flex-Hones work well in achieving this plateau finish.
As I noted earlier, the piston is recommended to be installed with the arrow(on crown) pointed toward the Flywheel side. We have run them in the opposite direction with no failures. Which way is better for a given style of motor(restricted or stock)? I’ll leave that up to your testing but I like to point it toward the PTO.
One of the reasons for the new piston was the relative high failure rate of the old piston pulling out the wrist pin at higher RPM levels. As the new Slapper cams (Dyno 95-3,99-3,96-3,96-2 ,PC 145 and others) have come into popularity, we are turning the motors to RPM levels we would not have dreamed of 5 years ago. With these higher RPMs came more piston failures. The increased strength of the new piston should address this issue and in fact we have used the new piston in Super Stock motors turning them over 7000+ with no failures to date(kiss of death). They are not a cure all for sloppy clearances but they are an improvement.
The one thing to be very CAREFUL with on the new piston is the wrist pin retaining clip installation. The new piston has very little free space between both clips and the wrist pin when installed on the rod, making a bit more difficult to properly install the clips into their slots. Make darn sure you have both clips installed correctly. I like to examine both very closely and rotate them in their slots to insure they are seated properly. Seems the grooves are a bit shallower than the old styles to further complicate the issue. The latest pistons are coming with thinner clips which should help.
One last bonus for the new piston is, that as a total package, it’s lighter than the old style. Below is a chart of weights and measurements taken from two standard size piston of both designs. I would like to thank Jim Stone of Stone Racing Engines in Greenville SC, for his time and effort to gather these figures.
PISTON CHART
NEW OLD NEW OLD
Weight(Grams)
Piston 136.9 131.7 Wrist Pin
Rings 27.2 30.5 Length 1.735 1.945
Wrist pin 14.8 29.6
Circlips .8 .8
Total 179.7 192.6
Dimensions(inches)
Piston height 1.674 1.878
Diameter(max.) 2.560 2.558
Pin bore .4895 .4895
Top to pin bore .941 .941
Compression Ring
Height .106 .124
Width .0585 .0781
Second Ring
Height .106 .124
Width .0585 .0781
Oil Ring
Height .085 .116
Width .1000 .1857
Following is the current WKA tech inspection specifications for the new piston and ring set. I obtained these from Morgan Whitaker’s website (http://members.tripod.com/~KartTech/karting.htm). Morgan is currently the head inspector for the WKA dirt series. This web site is a great place to keep up to date on the latest rule changes and ask Morgan specific tech questions.
Piston minimum length 1.673
Top of piston to top of wrist pin minimum .937
Ring lands for top two rings .0603 to .0612
Ring land - oil ring .1020 to .1032
Top two rings minimum width .090
Top two rings thickness .058 + or - .005
Oil ring minimum width .070
Oil ring thickness .100 + or - .005
Wrist pin length 1.732 + or - .005
Outside Diameter maximum .490
Inside diameter maximum .281
Today the new style is a proven piston/ring combination and should always be used in a stock engine.. Overall, thanks Briggs.
Now we are ready to set up the piston rings. The largest contributor to horsepower loss through internal friction is from ring and piston friction(drag). Something in the neighborhood of 70 - 80 % of total friction HP lost on any motor. Calculations have shown as much as 1.07 HP lost from total friction within a Briggs engine. 70% lost for the rings and the piston is near 3/4 of a horsepower! The largest single contributor to this ring drag/friction is the oil ring. Now that WKA has dropped the check for the minimum inside diameter of the oil ring, it is not necessary to use old or cut rings. The easiest method for reducing drag is to heat shrink the ring to reduce the wall tension. This process applies ONLY for the older style piston/rings. The new RAPTOR III rings do not need to be shrunk in my opinion. They come with very little tension straight from the factory. But back to modifying the older style rings. You can do this by placing the ring into an old cylinder making sure it is square in the bore. You can use the piston with the second ring installed to push the oil ring down just far enough to square it up. Now take a propane torch and heat the ring to a dull read and work your way around the ring moving along as each portion obtains the dull red appearance. When you have finished let the ring air cool. Using the block to do this is a very, very slow operation, as it takes quite a while to heat up such a large heat sink.
An alternate method, if you are going to do many of these, is to obtain a replacement sleeve for the Briggs from a supply house. You will find that this will heat up much quicker and yet retain the roundness you need. I simply have taken the sleeve and had it cut by a machine shop into several pieces and bored the pieced to the exact size needed for each bore size marking the piece .010, .020, etc. It is very important to use a round fixture for this, no matter whether you use the block or a sleeve. You can also use some of the aluminum quick ring compressors from MISI that are available in all piston sizes for this operation. These are available from Goodson for around $18 each. No honing or sizing is necessary with these but you can only do one ring at a time.
We currently shrink the oil ring that comes with the piston whether it’s a standard or .030 over. Always use a new oil ring for this operation. If you compare a new ring against an older used ring, you will note the difference in the size of the actual surface contacting the bore. The new ring will have very small edges and thus contribute to less drag after we shrink it. You should also shrink the second ring on the Briggs in the same fixture with the same method. The top ring gets a little tricky. I do not suggest you shrink the top ring at all. Simply use the next largest size top ring for the cylinder such as a .010 for a standard size cylinder and cut it for the proper end gap. Pre shrunk rings are also available from several cam manufactures if you don’t want to experiment with this process. Dyno and Precision are two that offer these sets.
Cutting the end gap on the top ring is a slow tedious process. You can cut the end gap with a file, but the trick is to keep the end square. If you want to try and use a file, get the thinnest one you an find such as a jewelers file. Place the file in a vice and then compress the ring against the file with both ends against the file body. You can then move the ring down the file cutting both end in unison. This will help keep both sides square to each other.
I currently use a ring end gap tool available from most auto supply stores. It is much quicker to use and does a very good job of keeping the end gap square. I have also actually used my Dremel tool with a sanding disc installed to accomplish the same gapping during my earlier years of building. I simply squeezed the ring together on the rotating disc. This will cut both sides at the same time and keep them concentric. Work slowly, as you don’t want to cut too much and end up with excessive end gap to start with. We work to have .001 - .002 starting end gap on the top ring. You can go lower than this if you are sure that your cylinder is very round. A good test is to push the ring, once it’s gapped, completely through the cylinder using your piston. If you feel no substantial increase or decrease in pressure while doing this, you can be sure your cylinder is round. Also take ring gap measurements in several spots down the cylinder. A quick way to determine if your bore has taper.
The final step in ring preparation is slightly hone the edges and ends of the ring to break the sharpness. I use a small(3/8”) wet stone in fine grit to accomplish this on both the top and second ring. You are not trying to create any angles just breaking the sharp edges. I start on one side of the ring and slightly radius the sharp end around the en gap. Then move around the complete edge of the rings breaking the sharp surface. This process will help to eliminate the ring scratching the cylinder during break-in. Many builders will also surface the flat sides of the rings on a jig and wet/dry sandpaper prior to installing them on the piston. MISI makes a specific tool for this operation and it is available from most kart shops or mail orders houses.
Today, I only check the flatness of the rings as too much sanding will create a new problem with the rings. RING FLUTTER. Ring flutter is an uncontrolled vibration of the top ring in the piston groove during high RPMs. What is happening is instead of the ring laying flat against the piston land it is being moved up and down(vibrating) in the piston land due to cylinder pressure. When this occurs you will be loosing compression(HP) as the ring is not sealing the cylinder wall as well as it should. Most Briggs rings and standard pistons will have a side gap of .004 -.006 out of the box, so sanding the ring will worsen this potential condition. The ideal set-up would be around .0015 to .0025 clearance.
Burris has attempted to help motor builders with this condition with the introduction of their new PLUS 4 ring. This ring is .004 thicker than their standard ring allowing the builder to custom fit the top ring to the piston ring land. Motors that have this problem will exhibit a ‘graying’ of the upper portion of the cylinder. Typically these motors will also get to a given RPM and just go flat in the power band. Very similar to uncontrolled valve float.
Another area to work on is the rod and crank clearance. Typically a new crankshaft from Briggs comes in at .999 or .998 in diameter. A standard rod out of the box will usually be at 1.000 to 1.001. So you can see that you can possibly have from .001 to .003 clearance if you do not check the components. Using a combination of a quality dial caliper and micrometer check the clearance. I work to have .003 on a new motor. If you have above .006 you are going to start breaking parts(rod!!) or pulling the wrist pin out of the piston(really bad news). If the crank is down to .996 use it in the fun motor on your neighbors yard Kart!!! It seems expensive but really a replacement crank is cheaper than a whole new motor!. Also, I would suggest using a hardened crank.. This is just a durability issue no speed here. If you can find some of the older Briggs heat treated cranks from their industrial motors of days gone by, get them. You can recognize them by the dark edges around the crank journal. These are hard as a rock and just never seem to wear out!. The new crank on the Raptor II and III motors is quite good as it has been inductive hardened in the journal area and seems to be as good as older cranks in this area.
As to obtaining the proper clearance, you can use the Flex-hones from Brush Research Manufacturing mentioned in my carb chapter. These can be obtained in 1” as well as 1/2” for use on the large and small ends of the rod. Take the rod and torque the bolts down to exactly 100 inch lbs. before honing the journal end. This will insure roundness after you are finished. Simply use the same honing oil as you used in cylinder preparation. Hone the journal end for the .003 clearance we talked about and then hone the piston pin end of the rod. If the rod is already at the correct size for clearance, lightly hone it to develop a cross hatch. A simple rod honing fixture can be made from 1” ID pipe to align the rod and hone. This insures proper alignment of the journal end. The 1” pipe is welded to a piece of 1” angle iron and then the middle is cutout to allow for the rod to be inserted. On my unit, one side is built to slide in and out. This allows for a tight fit up against the large end of the rod. The wrist pin hole should be honed just enough to allow the pin easy travel back and forth. Don’t get too much clearance here as you don’t want the wrist pin slapping around while you’re running.(.001 is about right). Do the same operation to the piston. Hone the pin holes just enough with the 1/2” hone to obtain free movement of the pin. This will give you a free floating pin and give a nice cross hatch for oil retention. The oiling hole on the rod can also be enlarged to .177 max(WKA). I use a .173 reamer for this operation and do it prior to honing the crankshaft end of the rod so any burrs left by the reamer are removed.
The only modifications to perform on the crank are making the bearing a slip fit if you have an non RAPTOR crankshaft and polishing the rod journal. Any kart shop can turn the shaft down to allow for a press fit. WKA has a minimum dia. for this operation of .775. You can also perform this operation on a 1” belt sander if you like, just be careful of the minimum diameter. Removing the bearing on a pressed fit can be problem. The easiest method is to use a bearing separator to hold the back side of the bearing in a vice, while you pound the end of the crank with a soft lead hammer, driving the bearing off the shaft. Sounds crude but this works very well.
Polishing of the crankshaft journal can be done by locking the crank in a vice and polishing the journal with a 1” strip of fine grit Scotch Brite using a lapping motion. Very light operation here as too vigorous an operation may get you tossed in the tech barn. All you want is a lightly polished surface.
NEW RAPTOR III Piston:
The same style piston and ring set has been used since the introduction of Briggs and Stratton 5hp racing. All of this has changed this year(99) with the introduction of the new RAPTOR III piston. This piston was designed for Briggs by the A.E. Goetze division of Federal Mogul. Major changes in the design and makeup of the piston have introduced a whole new ball park for the engines builders this year. In this portion of the book, we will go over the specifications of the new piston compared to the older style and give specific installation recommendations for it’s use in racing.
The new piston is made of a new alloy called Eutectic. The term Eutectic refers to the composition of the piston as it relates to the percentage of Silicon in the aluminum alloy. Silicon addition to aluminum is best related to adding sugar to iced tea. You can continue to add and mix sugar until the mixture reaches it’s saturation point. At some point adding additional sugar will result in it not mixing with the tea and simply falling to the bottom of the glass. Silicon in aluminum is very similar.
Silicon can be added to the aluminum up to the point of saturation. Any additional silicon added will simply precipitate out in the form of hard particles. The point of saturation in aluminum is known as the Eutectic and occurs around the 12% level. Aluminum alloys with less than the 12% are known as Hypoeutectic and alloys with silicon contents above the 12% level are called Hypereutectic. The new piston is Eutectic in makeup, so it contains right around 12% silicon.
Eutectic pistons show improved strength and are economical to produce. In addition to improved strength, the silicon reduces the expansion of the alloy when compared to straight aluminum and aids to it’s ability to reduce friction and scuffing. This reduced expansion will come to light a bit later in this chapter when we discuss cylinder clearance.
As you can see in the accompanying pictures of the two pistons, the new styles design is vastly different from the old style. The barrel shape leads itself to several improvements.
Improved stability in the bore
Aids in creating an oil film against the cylinder
Reduced friction(less surface area against the bore)
Allows for closer piston/cylinder wall clearances.
In addition to the physical makeup of the piston, the wrist pin hole has been offset to reduce torsional stress throughout the length of piston travel. The design also incorporates a shorter (less weight) wrist pin to help control flex and yield improved strength. The recommended installation of the piston has the arrow on top pointing to the Flywheel side of the block.
The ring package on the new piston also is new and unique to the piston. While the rings appear to be of similar design when compared to the old style, they are much smaller and lighter. They also have much less initial sidewall pressure eliminating the need to shrink the rings to reduce friction. The top ring has a bevel face to improve break-in, while the second and oil rings are very similar in design to the older style. Other than setting the end gap and breaking the sharp edges of the rings very lightly they are good to go from the factory.
The new pistons are also approximately .002 larger than the old style for a given size piston. For example the standard bore new pistons measure 2.5610 at it’s largest point(1.378 inches from the top of the piston) compared to an average of 2.5589 for the old style. This .002 size difference runs through the entire new line of RAPTOR III pistons. Speaking of the line of pistons, we now have several new sizes to play with. The old pistons came in standard, .010, .020 and .030 over. With the introduction of the new parts, we now have seven sizes of replacement pistons. This should give us more life out of our motors as we move from one bore size to the next. The new pistons and their Briggs part numbers are listed in the following table.
Size Part # Ring Set Size
Std. 555478 555485 Std.
.010 555479 (Use .015 ring set)
.015 555480 555486 .015
.020 555481 (use .025 ring set)
.025 555482 555487 .025
.030 555483 (use .035 ring set)
.035 555484 555488 .035
As you can see Briggs is not making a ring set for each new piston size. Given the fact that engine builders are going to fit each piston and ring set to a given motor, simply using the new larger size ring set will work in most cases. The one missing piece today is a .040 over top ring for the .035 piston. The Briggs .035 ring set top ring has too much end gap for most builders (.011). If you purchase the pistons as parts be aware that for the standard, .015, .025 and .035 sizes the ring sets that come with them in the box, will have a top ring with too much end gap. This forces you to use the next size up top ring. Top rings are available from Briggs as a unique part # these days. Also Burris and Power Products have aftermarket top rings for the new piston. These will be legal in 2000 for WKA.
End gap recommendations should go along the same guidelines as the older style piston with the top ring being installed with something around .0015 - .002 end gap(be sure you have a straight bore). The second ring should be around .002 - .004 and the oil ring around .008-.010 although it can be tighter. As to cylinder clearance, that’s a whole other story folks. Big changes here. Due to the design and makeup(Eutectic) of the piston it will not expand under heat as much as the older piston, so we can run much tighter bore clearances than we are used to doing. The offset wrist pin also comes into play here as too much clearance will allow the piston to rock in the bore ,creating several problems(excessive ring wear and scuffing).
Briggs recommends the following procedure for installation of the new style piston. First, measure down from the top of the piston 1.378 inches to the center of the trust faces(90 degrees from the piston pin). Due to the sides of the piston being barrel shaped, this will be the point of the largest diameter. From this point measure the diameter of the piston with a quality 3” micrometer.
Using the size found above, finish the bore to a size from .0005(yes that’s 1/2 thousand of an inch) to .0025 clearance. They recommend using these clearances on both cool bore and I/C blocks. Now folks, .0005 scares me to death, given the many warped cylinders I’ve seen over the years. The Briggs blocks have a natural tendency to settle(warp) a bit as they are heat cycled during their early life. They also seem to settle down after this initial heating and cooling. So what does our experience to date show. Somewhere around .002 to .0025 starting cylinder clearance seems to work very well in both style blocks. My personal experience, as well as, the experience of other builders I know, has seen everything from .007 to .001 tried with varying degrees of success. The larger clearances seem to work, but ring wear is very rapid and forces you to rebuild the motor more often than necessary. I did have a good cool bore that was run with the new piston at .005. After three races the cylinder had an actual imprint of the piston shirt at BDC!! I’m currently using .002 with good results. After a bit of experimentation, it seems the new rings like a very fine cylinder finish. Something around 320-400 to 600 grit stones should be used for the final finish. Either honing stones or the Flex-Hones work well in achieving this plateau finish.
As I noted earlier, the piston is recommended to be installed with the arrow(on crown) pointed toward the Flywheel side. We have run them in the opposite direction with no failures. Which way is better for a given style of motor(restricted or stock)? I’ll leave that up to your testing but I like to point it toward the PTO.
One of the reasons for the new piston was the relative high failure rate of the old piston pulling out the wrist pin at higher RPM levels. As the new Slapper cams (Dyno 95-3,99-3,96-3,96-2 ,PC 145 and others) have come into popularity, we are turning the motors to RPM levels we would not have dreamed of 5 years ago. With these higher RPMs came more piston failures. The increased strength of the new piston should address this issue and in fact we have used the new piston in Super Stock motors turning them over 7000+ with no failures to date(kiss of death). They are not a cure all for sloppy clearances but they are an improvement.
The one thing to be very CAREFUL with on the new piston is the wrist pin retaining clip installation. The new piston has very little free space between both clips and the wrist pin when installed on the rod, making a bit more difficult to properly install the clips into their slots. Make darn sure you have both clips installed correctly. I like to examine both very closely and rotate them in their slots to insure they are seated properly. Seems the grooves are a bit shallower than the old styles to further complicate the issue. The latest pistons are coming with thinner clips which should help.
One last bonus for the new piston is, that as a total package, it’s lighter than the old style. Below is a chart of weights and measurements taken from two standard size piston of both designs. I would like to thank Jim Stone of Stone Racing Engines in Greenville SC, for his time and effort to gather these figures.
PISTON CHART
NEW OLD NEW OLD
Weight(Grams)
Piston 136.9 131.7 Wrist Pin
Rings 27.2 30.5 Length 1.735 1.945
Wrist pin 14.8 29.6
Circlips .8 .8
Total 179.7 192.6
Dimensions(inches)
Piston height 1.674 1.878
Diameter(max.) 2.560 2.558
Pin bore .4895 .4895
Top to pin bore .941 .941
Compression Ring
Height .106 .124
Width .0585 .0781
Second Ring
Height .106 .124
Width .0585 .0781
Oil Ring
Height .085 .116
Width .1000 .1857
Following is the current WKA tech inspection specifications for the new piston and ring set. I obtained these from Morgan Whitaker’s website (http://members.tripod.com/~KartTech/karting.htm). Morgan is currently the head inspector for the WKA dirt series. This web site is a great place to keep up to date on the latest rule changes and ask Morgan specific tech questions.
Piston minimum length 1.673
Top of piston to top of wrist pin minimum .937
Ring lands for top two rings .0603 to .0612
Ring land - oil ring .1020 to .1032
Top two rings minimum width .090
Top two rings thickness .058 + or - .005
Oil ring minimum width .070
Oil ring thickness .100 + or - .005
Wrist pin length 1.732 + or - .005
Outside Diameter maximum .490
Inside diameter maximum .281
Today the new style is a proven piston/ring combination and should always be used in a stock engine.. Overall, thanks Briggs.
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