Block Prep

Bob Evans

In this chapter we will break down each piece of block preparation for any motor type you are going to be building. We will cover porting, cylinder preparation, valve seats, decking, and other areas.

Porting of any motor takes careful thought and generally not a heavy hand. This is very true with the Briggs motor in the stock classes. BIGGER is not better, flat out. What you are always looking for with porting is improved AIR FLOW and VELOCITY. Big ports do not mean the best air flow as they will decrease velocity. Keep in mind that we have a small .695 restrictor on the face of the port called a carburetor!

General rule for porting the Briggs stock class motor is to leave the initial size of the intake port alone and work on the upper left hand corner of the port near the valve seat. I use a easy method for getting started on any intake port. Take an old carburetor that you don’t care about and bolt it up to the port with the back plug knocked out. If you have done carb work out of my previous chapter, you will have in your possession a .690 reamer. Take the reamer and insert it through the carb bore until you hit the left hand side wall of the intake port. Now rotate the reamer using a wrench or vice grip until you have cut a good bit of metal out on this side and the reamer has gone to the back wall of the port. What you are doing is making a straight shot through the bore of the carb into the port, removing the left wall restriction.

After roughing out the basic cut, we want to move to a sanding disc or a small carbide bit for your Dremel. You can use the smaller 1/2” disc that come for the Dremel tool or I really prefer the longer sanding arbors available from MSC. These are great for smoothing out the cut and slightly radius the upper left portion of the intake port. This upper portion on a Briggs flat head in commonly called the short turn radius. You just want to radius(smooth) the edge directly under the central portion of the valve seat for a slight improvement in this area. Be careful not to damage the leading edge of the port. With the smaller disc you want to work on the very most upper left hand portion of the port. You want to open this area up in line with the left side of the valve seat . If you take a pencil or pointer and place it on the left hand side of the port you can readily visualize how much of the port to remove to bring it inline with the left most portion of the valve seat.

Don’t mess with the hump on the right hand side of the intake. This is a support area for the carb bolt. If you remove this, you WILL kill air flow. You will also do damage if you hog out the lower right hand side of the intake. It’s OK to lightly remove casting flaws, but nothing heavy here. Again don’t enlarge the overall size of the intake runner. As it comes stock, it is less than the .880 maximum size allowed by WKA but still much large than the bore of a blueprinted carb (.695).

Keep in mind that as air and fuel move through the carb bore and hit the wider opening at the front of the carburetor and then enter the even larger port, air speed will DECREASE! If you don’t believe this look at a stream sometime where a small area of flow runs into a larger still area. You will see how the flow of water slows when it hits the open area and spreads out. Air flow is the same.

To finish up the port you can use two tools to smooth out the rough finish left by the sanding discs or carbide bits. The small Cratex rubber abrasives bits(240 grit) available from MSC do a fine job of smoothing out the rough finish and you can also make yourself a simple tool for using scotch bright to polish the inside of the port. I like the finish scotch bright puts on the port. It’s not super smooth and this is exactly what you want. A smooth surface will not help your air/fuel mixture. It will not help air flow as there is little surface tension anyway. What you want is a slightly rough surface to help keep the air/and fuel mixed. Many builders for larger engines will actually put bumps in the intake runner to help keep the fuel and air stay mixed in low pressure areas. Adding material to the port is not legal in WKA stock classes but small indentions are legal(hmmmm). Fuel has a tendency to collect in these lower pressure areas of the port as large droplets. Large drops of fuel do not burn as well as a small atomized mixture.

The tool for using a small square piece of scotch bright is easily made from a 4” X 3/8 bolt. Cut off the bolt head and then cut a 1” deep slot in one end big enough to put a piece of scotch bright into. Put this into a drill and have at it. You will end up with a dull looking port but who cares if it flows well! Scotch bright is easy to find in any grocery today or is available from MSC once again. You can also use a 1 inch Flex Hone available from several Kart supply shops such as EZ-Bore or order them direct from BRM.

On the exhaust side, we will not do much. The port is already too big in OD. All I do today is take out the bump in the upper right hand corner and then smooth out the first three threads on the bottom of the port counting from the valve guide. Gently smooth the other areas of the port and that’s it. Once again smaller is better on these motors.

A prerequisite for any good engine seal is a round bore. If there is any question about the bore have it line bored to the next size up by a kart shop before beginning any work yourself. Line boring ensures that the bore is 90 degrees from the crankshaft so we won’t bind the rod/piston up while operating. Have the shop leave it the standard size for the overbore as we will hone the cylinder to the correct clearance based on the piston you will use. If you are dealing with a used motor that has been run on gas absolutely go with the line boring. On a new motor I suggest running it with the standard bore until the cylinder goes out of round or is worn to the point of needed a re-bore. This will save you one cylinder size and let the bore settle down a bit. Almost all Briggs cylinders will settle(warp) a bit after being heat cycled a few times. Some will change quite a bit, while a very few will stay relatively straight until the bore is worn. These are very rare, as it seems the metal being used in the 5HP Briggs today likes to move around quite a bit. This settling happens on both styles of motors(cool bore and I/C).

After this operation the block must be honed for piston clearance and a good cross hatch developed to aid in the initial ring seal and oil retention. Only a positive pressure hone should be used in this clearancing operation. No ball or spring type hones for clearancing. Excellent hones are available from AMMCO and Sunnen for around $300, that will cover the Briggs motor up to .060 over size. You will also need a good quality 1/2” slow speed drill(300 - 850 RPM) to drive your portable hone.

The last item needed to do a professional job is a torque plate. Don’t even think of honing a Briggs without one. What the torque plate does is to simulate the head being bolted down on an assembled motor and applying the same stresses to the cylinder as does the head. With the aluminum casting on a Briggs, the stress placed upon it by tightening the 8 cylinder bolts will warp the cylinder ever so slightly, creating an out of round cylinder condition. The theory and fact is, that by using a torque plate, we will allow for this warping prior to honing the cylinder and thus have a perfectly round cylinder when the engine is assembled. Even cast iron automotive engines suffer from this same problem though not to the same degree

I like to put the torque plate on and leave it over night to let things settle. At a minimum leave it for a couple of hours prior to honing. Torque the plate to the same inch pounds you will use on the head when assembling the motor. I like to use 160 in. Lb. for a stocker. To go a bit further with this process, I will also put on the side plate with gasket and torque it down.(80 in. lbs.) Torque plates are available through most kart shops and mail order houses or you can make your own out of an old head if you have access to a mill.

My cylinder/piston clearance recommendations are as follows. On cool bore motors you can start at .007 clearance and on the I/Cs start at .006. Both of these are for the older(pre- RAPTOR III) style piston. This will give you enough clearance for heat expansion and a bit of longevity in the cylinder wear department. I have seen motors with up to .010 side wall clearance and they are fast, but they don’t last long. Too much piston rock in the bore causing scuffing on the piston. For the newer RAPTOR III pistons they are made from a unique compound that will not expand as much as the older style pistons so you will not need the same amount of cylinder clearance. We will cover these in a bit.

You can buy very expensive bore gauges($70 - $300) to measure this clearance OR simply use an old engine builder trick of using long(12”) feeler gauges. These gauges are available from auto supply stores locally in most thickness you will need and are surprisingly very quick and accurate to use. MSC also carries both of these items. Simply put the feeler gauge into the cylinder and ease the piston down against the gauge. A bit of practice will help you get the right feel for the proper clearance. If you choose to purchase a bore gauge follow the next procedure for the older style pistons.

With a 3” micrometer measure the selected piston at several areas to obtain the average size. Measure front to rear and side to side. This also allows you to check the piston for roundness. On occasion you will find one that has been dropped or crushed in delivery and will be more than .002 out of round. Return it. While you are measuring the piston also take your dial mics and measure the wrist pin offset. Most pistons will have the wrist pin pretty much in the center while others will exhibit quite a few thousands of offset. If you have an offset piston install it with the short side pointed toward the valves. See Chapter 6 for specific recommendations on each piston as the new Raptor III pistons are entirely different.

What I like to do is to measure the actual piston I’m going to use in a motor with the dial mic and lock it down. Then I zero my bore gauge on the micrometer. This way when I set the bore gauge into the cylinder it is easy for me to see exactly where I am verses the bore size. If I want .002 clearance then I look for .002 over zero on the gauge.

The actual honing operation is relatively straight forward. You will need several grits of hones based upon whether you are working on an aluminum or cast iron bore. For aluminum bores I suggest starting with a fairly smooth grit of 220 for you initial clearancing and then moving to a 280 grit when you are within .001 to .002 of your final target clearance. If you are using the newer Raptor III piston, then the 280 grit will do fine as we will not be opening the cylinder up very much. For I/C engines we use a 220 grit for all clearance honing. For the final .001 of clearance use a 400 grit stone on either style motor. Excellent honing oil is available from Sunnen, Goodson Supply, or you may choose to make your own. We now use a 50/50 mix of hydraulic oil mixed with transmission fluid. I have also seen a 50/50 mix of kerosene and 30 weight motor oil used with good success on cool bores. I always hone either style block with fairly large amounts of honing oil during the process.

The proper method for stroking the hone is to have the hone come out of the cylinder approximately 1/2 to 1” on both the top and bottom stroke. The speed of moving the hone up and down in the cylinder is easy to judge based on the angle of cross hatch you are developing. You are trying to develop a defined cross hatch(X) style pattern within the bore. The angle of the cross hatch should be between 35 - 40 degrees. The cross is necessary to retain oil during the engine operation. You want the rings and piston to ride on a very thin layer of oil.

Stone pressure against the cylinder wall is hard to describe, but if your drill is not dragging a bit during the operation then you need more stone pressure. As the stone removes material from the cylinder wall ,your drill speed will pick up. Too much pressure will cause the stone to load up with cylinder material and quite cutting. You can clean them by using kerosene and a flat file on the honing surface.

Once you have honed the cylinder to your target clearance, the final bore finish can be achieved by using a different set of hones or tools. Ball type hones that go by the brand name of Flex-Hones can be used for this process and are available from several karting mail order shops, as well as, local auto supply houses. EZBore has all of the Flex-Hones you will need. They come in both Silicon Carbide for cast iron bores and Aluminum Oxide for cool bores. These are available in grits from 180 to 600 . A very good job can also be done by using 400 grit sand paper or fine scotch Brite over your hone and very lightly honing the surface. Use LIGHT cylinder pressure during this operation. What the final honing process accomplishes is what you may have heard referred to as developing a plateau finish.

When a bore is honed with standard honing stone the surface will have microscopic peaks and valleys created by the stones. The rougher the grit the deeper the valleys. What you are attempting to do with the plateau finish is to knock off the peaks and leave a relatively smooth flat surface, a plateau. This would happen while the rings are seating themselves during break in, but the small pieces of metal sheared off by the rings would be circulating in your motor and ring material would be wasted in the process!!! My personal preference is to use the Flex Hones in 600 grit for both style cylinders. In either case the final plateau process should only take 25 or so cycles in the bore with the flex-hones and only a couple of cycles with scotch Brite or wet/dry sandpaper. You want to leave with a shiny finish. If you end up with a dull finish after attempting this, you have burnished the surface and need to start back with your standard hones. Too much pressure with the hone or too long a honing process will lead to this problem.


Before performing any work on the valve seats themselves we need to check both the intake and exhaust guides for proper valve clearance. Proper clearance is around .002. This is difficult to measure without some quite expensive tools, so I would suggest you use the simple rule that a new valve should drop freely down onto the seat from approximately 2” and the valve should not move around excessively when it is pulled up to around 2”. Also watch the valve when it’s installed with the spring as it closes on the seat. It should drop straight down and not shift as it hits the seat. Most motors will not need any work on the intake guide as it seems the cool methanol charge minimizes any wear on the intake side. If you do find a used motor that has quite a bit of slop between the guide hole and a new intake valve using the standard Briggs guide will work nicely and last forever. The exhaust side is quite a different story indeed.

Do to the extreme heat experienced by the exhaust valve, guide wear is a constant problem. Almost any re-build or work on an old motor will mean replacing the valve guides on this side. The best guides available today are from CV Products and EZ-Bore. They make guides for all types of race engines and theirs seem to last quite a bit longer than any other guide I have used. The small standard Briggs guide (part # 555139) does a good job but will not last much beyond 1/4 season of racing. This guide is the standard guide used by the Briggs guide installation/replacement tool set and has a .283 outside diameter. CV has a guide that is a direct replacement for this guide and is of much better quality. The Briggs tool set does a fairly good job of installing and removing valve guides. The only issue with this set is that it will not keep the guide as true to the original guide bore as some of the after market kits available from places such as EZ-Bore or ARC. The EZ-Bore kit is the best on the market(IMO) and does an excellent job of keeping the guide concentric to the original guide bore and seat. Their kit also uses a bit thicker guide that seems to wear very well(actually a CV guide I believe). If you have a new RAPTOR series block I would not worry about replacing the guide until the first re-build. It’s clearance is a bit loose for my taste, but will work just fine for the first time around.

If you have an existing guide in your motor, the Briggs kit contains a tap and 7mm screw to remove older valve guides other than the STEEL guide used on older motors or the dual bearing block seen today. If you have a Steel guide leave it in place and using the reamer contained in the kit, cut it for installation of the brass guide. Steel guides simply wear valves out too soon and don’t belong in a race motor. Be careful here as there are several outside diameter guides available for the Briggs and if you remove the guide and don’t have the proper size replacement, well you’re off to the shop looking for a replacement. After removing any old guide you can clean the hole with air pressure or carb cleaner and a Q-tip. If the motor has a steel guide ream the guide with the appropriate reamer in the kit to prepare for the new guide. A simple method for cutting to the proper depth is to place the reamer into the centering tool contained in the kit and let it center itself in the existing guide or hole. Next place the replacement guide on the top of the centering piece and mark it’s height on the reamer shank with a magic marker. This will be your depth limit for reaming.

To install the new guide, I like to taper one end just a bit to help with guide it into the hole. A bit of emery cloth will do the trick, just be sure everything is clean prior to installation. To install the guide, simply place it into the hole and with the guide driver, contained in the kit, drive the guide into the hole until it is flush with the bottom of the port. After driving the guide in place you will have most likely peened the hole shut a bit and will need to ream the hole out to proper size. The reamer for doing this contained in the Briggs kit is TOO BIG! OK I guess, for a tiller motor, but simply too big for a racing one. It is a .250 reamer and most valves are around .246 in diameter. Obtain a .2475 and .2485 reamer from MSC for this operation. I will use both in two separate operations. Once you have opened the guide up to .2485 try a new valve for fit and clearance. If it seems a bit tight still then move to a honing operation with Flex-Hones available for this process. You simply want one around 180 grit in 1/4” or 6.5mm size. This are available from the same sources as previously mentioned. I always hone my guide for final fit and finish. Again Q-tips are great for clean-up. If you choose to purchase the Ez-Bore guide kit everything you need will be included. We use the Ezbore in the shop.

My personal preference for valve seats is to cut them quite a bit, thus lowering the valve on both the intake and exhaust side. This will increase your installed height(lower pressure). This seems to help air flow on the intake side at the higher RPM ranges and also helps to dissipate heat on the exhaust side. The exception would be a rookie motors, where I don’t cut the intake seat much at all. This seems to help low lift air flow. A very nice tool for this operation is the NeWay valve seat cuter available from most kart shops or Goodson Supply for around $150- $170 at this time. This tool consists of a guide and a combination cuter having both 31 and 46 degree carbide cutter for the standard Briggs motor. The intake seat is set at 30 degrees by Briggs and the exhaust at 45. The one thousand difference will be taken care of when we lap the valves onto the seat. If you care not to invest in a seat cutting tool, your local kart shop can perform this operation for you at a minimal cost. After you have cut the seats, it is time to lap you valves so they seat perfectly onto the seats. This is a very important process to obtaining the best seal you possibly can in one of these motors. The more I work with these motors, the more I have come to believe that seal(valve and piston) is everything in that better engine.

You can get a small engine "Valve-Lapping Kit" from most auto parts stores. All it consists of is two containers of lapping compound, one course, and one fine, and a stick with small suction cups on each end, usually one smaller than the other. It is essential that you have clearance between the lifter and the valve BEFORE you begin lapping the valves. It may even be best to remove the cam and lifters for the novice in order to reduce confusion at this juncture. Valve "lapping" is a process used to mechanically "mesh" the fit between the valve and the seat. This basically allows a more perfect fit between the valve and seat, sealing the valves against compression leakage. The valve and seat have a difference in angle of 1 degree Briggs calls the "interference angle". What we are trying to do is resolve the 1 degree difference in angles. Facing a valve in a valve machine is now WKA legal and will help to insure the seat is concentric to the stem. Remember the intake is 30 degrees and the exhaust 45. We “blueprint” all our valves these days on a valve machine.

To lap the valve, first establish the fact that the valve is sitting solidly on the valve seat and not being held up by the lifters. Then take some "coarse" valve lapping compound and spread a small amount around the seat face of the valve. Place the valve back into the engine. Using the suction cup tool, place the suction cup firmly down onto the valve in its center. Flatten your hands out and place them together with the vertical shaft of the suction cup tool between them. Then, while placing slight downward pressure against the valve, begin rubbing your hands back and forth to rotate the stick back and forth.(Similar to the old method of starting a fire with a stick and a rock) Also don't forget to periodically lift the valve and turn slightly while lapping. You will hear the valve "gritting" against the valve seat, "sanding" a path of conformity and unity against each other. After about 1-2 minutes of this, you will hear the "sound" change to a softer tone, indicating the compound has done its job. Take the valve out, wipe both seat and valve thoroughly clean with a clean rag or paper towel. Inspect the valve for a uniform "gray" line around the face circumference. Inspect the valve seat for the same. If at any place around the valve face or seat the line is not the same width, or if there is a place on the valve or seat where the abrasion doesn't exist, then the process must be repeated until you see a uniform area of abrasion around the valve face and seat circumference.

Once you are finished with the course compound , proceed with the same procedure using the "fine" lapping compound. Again, when the valve begins to "seat" well, you will hear a noticeable difference in the grinding sound. You may have to repeat the compound application more than once to get the valve to truly achieve a good seat. One word of WARNING. Never leave any valve lapping compound in or on the valve, seat, or port area. This compound can and will destroy an engine if it gets into the cylinder, but we’ll talk about cleaning the motor later. WD40 actually works very well for the final cleaning of the sealing surface!

To verify that you have a good seal you can use one of several methods. The easiest one is to replace the valve and try and turn it in it’s seat by placing your thumb on top. If you have done a good job, it will be very difficult to rotate. A better method is to place some thin liquid such as 3-1 or Marvel Mystery oil on top of the valve once you've cleaned everything up and re-installed the valves, springs and retainers. Put light air pressure (aprox 8-10 lb.)into the port. Any valve not thoroughly seated will have little air bubbles leaking up around it. Yes I have seen people blown into the port and this will work just fine. You can also make an adapter for your air supply that will cover the surface of the port and have an air connector in it.

One of the neatest tools I’ve used for years is a rotating valve lapping tool I got from Sears. With this one you simply rotate the hand crank and it will work the valve back and forth and around continuously. Very nice old tool for around $15. Some folks will use a drill fixture to rotate the valve via a portable battery drill. This also works

On single bearing motors, the flywheel side bushing needs one small modification that is more a reliability issue than performance. All new RAPTOR blocks come with a Teflon impregnated sleeve bearing referred to as a DU(Depleted Uranium. Don’t worry it’s not radioactive) bushing. This bushing is a great improvement over the older steel bushings or any of the older tiller motors that only had a hole in the side for a bushing. Aluminum and the iron crankshaft really don’t mix well when there is any lack of lubricant.

The one modification that I do on any single bearing motor is to add a 1/8” oil hole in the sleeve bearing. I do this even on the DU bushings. EZ-Bore manufactures a tool to facilitate this modification and I would suggest that you only attempt this with one of these guides. Their tool will permit you to add this hole on either older style blocks or the newer RAPTOR style as there is a bit of casting difference between the two. This modification is WKA legal.


On any older tiller block you will most likely have to close up two holes left in the block from the removal of the governor and any points. On the governor hole left from removal of the arm, simply tap the hole from the outside for a 1/4” X 20 screw and then use a new allen head screw to plug the hole . Many people will put LockTite on the threads, but what I like to do is to only tap the hole about half of the screw’s length and then thread the bolt completely into the hole. This will force the bolt to self tread itself for the last half and eliminate the bolt backing out. Tight fit.

On the hole left by removal of the points caming rod, I simply fill it with silicon seal. This will keep any dirt out of the motor.

Decking the block for piston pop-up is an operation I would generally suggest you to leave to a kart or machine shop that has a mill with a flywheel cutter. What we are doing is cutting the top deck down so we have a piston only sticking out of the block at Top Dead Center(TDC) the maximum amount. In WKA this maximum is .005. For IKF the maximum is .015. This process help raise compression a bit and if done at an angle will also help open the eyebrow area around the valves to help with air flow.

To do this properly you must first gauge the amount of pop-up you currently have or determine how much you can cut off if the piston is below the top of the deck at TDC(most common). The method used by WKA in teching for piston pop-up is good starting point. With the crankshaft installed with bearings and the side plate drop the piston and rod you will be using in the motor down into the bore. Be careful to have the crankshaft journal at it’s uppermost position so the rod will not get hung up in the cylinder. Now install your Dial micrometer positioned over the center of the piston. Rotate the piston back a bit, so it will not be a TDC and place a 4” long piece of bar stock across the block in line with the wrist pin. This bar stock can be from 1/2” to 1” square. Now zero your micrometer on the bar stock. Depending upon your micrometer mounting tool you may have to raise the mic up a bit to allow for the bar stock height. I have in the past used an small socket underneath the tool combined with a longer 5/16” bolt to raise it enough for this measurement.
Gently rotate the motor forward and see if the piston raises the bar stock at TDC. If it does it cannot go beyond the .005 max. If it is over the max be sure the rod and piston are fully seated on the crankshaft before you throw the block out. On a new block, most likely the piston will not hit the bar stock and it will be “in the hole” several thousands. To measure the amount it is in the hole use a 2” piece of the same bar stock. Place this piece over the center of the piston and raise it to it’s most upward point(TDC) and zero your dial indicator. Now remove the piece and gently place your original 4” piece over the block again centering on the wrist pin area. Your mic should give you the exact difference and thus the amount to cut the block to have zero pop-up. You can give this measurement to the machine shop as their limit on removing material.

My personal recommendation is to not go over .002 pop-up on any WKA stock class motor. Nothing wrong with having zero pop-up. This will allow you some variance with different length rods and wrist pin locations on pistons which may cause you problems at a later rebuild. There is no real HP advantage to getting close to the .005 max.

The most common method for removing material from the top of the deck down to your pop-up limit is to angle cut the block with a mill and flycutter. What this means is you will have the back of the block elevated for the cut, thus cutting more material off the valve side up to the point on the block where we are measuring pop-up. Most builders will cut up to this point, then stop once they have cut enough of the block to reach their initial pop-up height. I suggest shimming the rear approximately .025. This method is currently WKA legal. IKF motors must not be milled at a single angle! They can be cut but must be a flat cut across the entire deck.

You can do this yourself if you have access to a 4 or 6” belt sander that can be moved to a vertical position. This takes quite a bit of practice and should be done first on old junk blocks to get a feel for the process. Believe me it can be done. You simply hold the block tightly and let the sander take off more on the valve side as you work toward your pop-up measurement area. Go very lightly here. If you cut too much off you’ve just ruined a good block.! You can cut a bit at a time and then re-measure your pop-up. Also be very careful around the valve seat area as it’s illegal to touch the seats during this operation. I would suggest for your first motor to have a shop do the operation and then examine closely what and how they did the decking before trying this on your own. You can also mount sandpaper on a flat surface and work the block over it to remove material.(very very slow)

One last item on piston pop-up. If you find yourself with a combination that is just too tall to be legal you can attempt one of the following:

First try and find the shortest rod and piston combination possible. All I can say here is that you will have to hunt through parts looking for that short rod or short piston. Very tough at times. There are also kart shops around that have the shortest length rod in stock. (3.125”). ARC sells short billet rods these days to address this situation.

The second thing is that if you within .001 of the max you can possibly lightly sand the top of the piston with 600 wet/dry paper and solvent. This will not scratch the piston(illegal) and will remove a surface blemishes that may be lifting up the tech bar. This is a bit common on oversize pistons due to the size(010,020,030) being stamped on the top of the piston in the very spot we use to tech pop-up.

The absolutely last way to eliminate excessive pop-up is by compressing the piston. This will sound horrible but it actually works with no ill affects that I’ve ever seen. Simply install you piston, rod and crank in the motor and rotate it to Bottom Dead Center(BDC). Now take an old piston and turn it upside down on top of your new piston. Take a wooden dowel of appropriate side and with a hammer knock the *&&#$ out of it once or twice. Then go back and measure you pop-up-up. Like I said, this is the last resort to save a good running block! Never had a problem with this procedure on several engines over the years. I have not tried this method with the newer Raptor III piston. Because it has a rather large wrist pin offset turning the piston in different directions will quite possibly help with any pop-up problems.

With many of the newer Slapper style cams( Dyno 95-3, 95-5, 99-3, 7X, 96-3,00-X, 04-3 and the PC145, 150 and 2K2) we have now encountered problems with the intake lifter bore being compressed by the lifter banging into it at high RPMs. If you look closely at the lifter in the area where the stem meets the base you can see a bit of taper. It is this taper that will peen the lifter bore and sometimes crack it!!

We can eliminate most broken parts here by counter sinking the intake lifter bore where the lifter would butt up into it. Most kart shops and mail order catalogs have a tool specially made for this procedure. They are also available from MSC and EZ-Bore. These are great items if you are doing lots of block but for the home builder a hand held counter sink took will do quite nicely. Many builders are also welding up the lifter bore area(WKA legal) to help eliminate any cracking. This cracking is always seen on the exhaust side as it is thinner and somewhat un-supported compared to the intake lift boss. Cams such as Dyno’s 95-3 and 99-3 or Precision’s PC145 have this problem. The chamfer cut should not be so deep as to extend out to the side of the lifter boss itself as this will get you tossed in the tech barn! Tech is a maximum size of .500 on the counter sink size.

One new area on the lifter boss area is that WKA now allows welding of this area to strengthen it. Many of the more aggressive “slapper” cams will have a tendency to crack the exhaust lifter boss due to it being somewhat unsupported when compared to the intake side. Heli-arc welding will eliminate this problem and should be done prior to running these types of cams. I suggest using an old lifter placed up into the exhaust lifter bore when welding. This will help keep it straight under the heat from the welding process. You may have to drive the lifter out afterwards. A .2485- .249 reamer will size the hole for proper use afterwards. This operation runs around $25-30.

A smaller base circle cam will also help in this area. You can order these as an option on any Dyno or PC cam. Try something around a .720 base circle. You will have to use new valves with this type of cam but it will give you more room between the running height of the lifter and the lifter boss. The normal base circle is around .760.