Adding Crank Weights for a Big Piston (KT100)

sundog

Member
There was a good thread on the old site that had the formula for adding tungsten weights to the crank to balance out a bigger piston on a KT100. Was all that info lost?

Anyway, The (54.7 Burris) bigger piston adds 17.5 grams and I'm putting 31 grams of lead inside the crank pin to add mass. I have 4 tungsten weights 14g each. I was just going to guess and put all four 1/4" outside the stroke circle opposite the pin. Can anyone put some math to this?

Rod length 3.940"
Stroke 1.816"

Thanks,
Sundog:cool:
 

sundog

Member
It allows me to add more total weight almost 2 oz, one in the pin one for counter balance. just need to know how much to add for the extra piston weight.
 

alvin l nunley

Premium User
It allows me to add more total weight almost 2 oz, one in the pin one for counter balance. just need to know how much to add for the extra piston weight.
Sounds like a really tough math problem. Not one I can do.
I do know, (pretty sure) that there are formulas for figuring it out, or, just cut and try. There are people in karting that do this to stock KT’s. If you could find them maybe they would give you some help.
I believe your added weight of 2oz is a waste of time. Just adding weight doesn’t seem like a GOOD idea. Even if the weight is equal distance from the crank center line.
I’m pretty sure you will need to know what RPM range you will be running in and, not only do you need to know the amount of weight to add, you need to how far it needs to be from the center line of the crank.
Sounds like a fun project, good luck.
Comments, compliments, criticisms and questions always welcome.
 
What was proposed on here in the thread you are talking about was to add 75% of the extra weight of the piston, the same distance from the center that crank pin is, 180 degrees from the crank pin. Why 75% I don't know. I've read some articles that say add 100% to get back to the same balance factor as before. depending on what percentage you use for a balance factor I'm not sure you have enough tungsten to do it, your trying to counter balance 48.5 grams with 56 grams of tungsten, You have to factor in the weight of the steel you are removing to make a space for the tungsten plugs. If the steel you are removing weighs 20 grams you'd be right on the 75% factor, if it weighs more than that then your percentage is gonna be too low. I think it will be closer to 30 grams of steel. Search tungsten crank weights, there are companys that have the formula's online to figure the weight of steel vs tungsten to see how much you gain with different od's and legnth's of plugs. I'm getting ready to do a balance for a piston that weighs an ounce more than stock and it's either gonna be two 3/8 od plugs 1/2 inch long or two 7/16 plugs around 1/2 inch depending on the balance factor I decide on. Jon
 

sundog

Member
Ok the lead 31g + steel removed 24g + 75% of added piston 13g = 68g so, you're right I don't have enough tungsten 56g. That means I'll have to go outside the stroke line. Need to increase the effective weight by 21% how far should I move it?

Al, why not make the crank heavier, the K71 crank is heavier than the KT100.
 

alvin l nunley

Premium User
Al, why not make the crank heavier, the K71 crank is heavier than the KT100.
I didn’t know that. Do you know if the extra weight makes any difference in the balance factor? Maybe the cheeks are where the weight is, doing the same thing you are doing by adding more counter weight?
It would be interesting to see how much weight you would have to add to the pins to make them both balance out. That would show you how much of the extra weight is in the counter balance side of the crank.
I have heard of people doing what you’re doing in the past and it always seemed to me that they should have also done what you’re doing. More cubic inches are good, but balancing it right is even better.
Comments, compliments, criticisms and questions always welcome.
 
I can remember hearing that it's almost impossible to do a perfect balance on a single cyl. engine, because of something called, "the 5th. Harmonic"? also that it can be balanced for only 1 place in the RPM. range?
 

alvin l nunley

Premium User
I can remember hearing that it's almost impossible to do a perfect balance on a single cyl. engine, because of something called, "the 5th. Harmonic"? also that it can be balanced for only 1 place in the RPM. range?
This is 100% true. What you do is balance, as best you can, the system for the RPM range you are going to be operating in. A high RPM range engine needs more counter balance than a low RPM engine. I’m sure Yamaha engineers, when designing the crank, never intended for, or designed for, the KT100 to be operated in the range most karts run it.
When I built my open Mc’s, the first thing I did was weld on Horstman tungsten counter weights.
Comments, compliments, criticisms and questions always welcome.
 

ttownwideglider

New member
crank weight

I have done the crank weight adjustment and did the original posting. The 75% # came from george Claussen or go fast George, cant remember which. This is a reference point used by most engine builedrs to achieve a ballance point in the upper range in the rpm band of a kt or 2stroke engine running around 12,000- 14,500 . I installed a 54.5mm Ausie piston in my kt that weighed 28grahms more than a 52.70 piston. I installed two 3/8 by 1/2 tungsten weights at 14 grahms each. These were 180 degrees from the wrist pin the location the same distance in on the wheels as the wrist pin one in each wheel. The steel removed from the holes in the wheels weighed around 5 grahms to 5-1/4 grahms,that equals 10-1/2 grams. The tungsten weights were 14 grams each or one ounce 28 minus 10.5 equals 17.5grahms of added weight or about 75% of the added piston weight. This worked extremly well for my kt it balanced it out very well in the upper range. I was vibrating so bad before doing this that my engine was throwing the anti-vibrating rubbers in the head and cylinder. It also seems to have increased the acceralition, but i installed a new pipe and carb which may account for some of this feel. Never had a chance to dyno this build or race it much becuse I been on the road with my job, havent even been on bobs in a long time. You could experiment with the 75%# and maybe increase it if you plan on running a higher rpm or even to see where it moves the balance point but Iam very happy with mine. Do not use lead it is to soft and doesnt have near the weight difference as the tungsten. I bored the holes about .004 undersize and heated the crank to about 450F and froze the tungsten dowels in liquid nitrogen and they almost fell in. I also bored them about .010deeper than the dowel length and fit them flush to the outside of the flywheel. Good luck hope this helps
 

alvin l nunley

Premium User
I have done the crank weight adjustment and did the original posting. The 75% # came from george Claussen or go fast George, cant remember which.
No doubt in my mind you know how to do it, but there are people doing KT cranks with stock pistons and getting very good results with, I think, about the same weight addtion. You would have to talk to Louie Figone about the size of the inserts used. Jeff Nelson lost a first place enduro Nationals trophy because his cranks had lead in them and one came lose. They found it in tech. I think that was about 1986-87. Jeff didn’t build the engines, a guy from Ohio did them.
I think I know why the TKM was so fast; it had inserts in the crank. From my Mc experience, I know what extra balance in the crank will do.
I don’t know, but my guess is those Horstman tungsten weights added more than what you’re talking about. It would be interesting to find out for sure.
Comments, compliments, criticisms and questions always welcome.
 

arc100

Member
I have done the crank weight adjustment and did the original posting. The 75% # came from george Claussen or go fast George, cant remember which. This is a reference point used by most engine builedrs to achieve a ballance point in the upper range in the rpm band of a kt or 2stroke engine running around 12,000- 14,500 . I installed a 54.5mm Ausie piston in my kt that weighed 28grahms more than a 52.70 piston. I installed two 3/8 by 1/2 tungsten weights at 14 grahms each. These were 180 degrees from the wrist pin the location the same distance in on the wheels as the wrist pin one in each wheel. The steel removed from the holes in the wheels weighed around 5 grahms to 5-1/4 grahms,that equals 10-1/2 grams. The tungsten weights were 14 grams each or one ounce 28 minus 10.5 equals 17.5grahms of added weight or about 75% of the added piston weight. This worked extremly well for my kt it balanced it out very well in the upper range. I was vibrating so bad before doing this that my engine was throwing the anti-vibrating rubbers in the head and cylinder. It also seems to have increased the acceralition, but i installed a new pipe and carb which may account for some of this feel. Never had a chance to dyno this build or race it much becuse I been on the road with my job, havent even been on bobs in a long time. You could experiment with the 75%# and maybe increase it if you plan on running a higher rpm or even to see where it moves the balance point but Iam very happy with mine. Do not use lead it is to soft and doesnt have near the weight difference as the tungsten. I bored the holes about .004 undersize and heated the crank to about 450F and froze the tungsten dowels in liquid nitrogen and they almost fell in. I also bored them about .010deeper than the dowel length and fit them flush to the outside of the flywheel. Good luck hope this helps

Living in Australia I've used the big Strike pistons you mentioned. I hate em because of the balance problem. Yes, I found on the dyno I actually broke low and high speed needles off the carb within 5 pulls. You definitely know you've got a problem when the dyno room floor is giving you a foot massage. Instead I cut on the piston and reduced the weight back to almost stock weight for a 52.8 piston. This was possible because of the extremely thick wall in the piston. Save yourself the time with the KT and just get a Burris 54.7 piston. They are just a few grams heavier than a Strike 52.80.

Almost forgot to mention the added piston weight kills the power and the rpm range where the power is made. So as mentioned unless you correct the balance for the bigger capacity from the heavier parts its all just a waste of time unless of course your running low rpm, the you may just get away with it.

Anyway guys, great to have this balance info back up on Bob's. Everyone cut and paste before the next big crash.
 

ttownwideglider

New member
I forgtot to mention that I hoged out the Aussie piston and removed about 4-5 grahms of weight on it so that put me a little over 75% of the total added piston weight . I dont know a exact formula for the total weight that can be added, but was afraid of adding to much and getting out of balance in the other direction. I do know that there is about 10grahms of weight difference in a 52mm to a 52.80mm piston and the kt seems to do alright in compensating for this at the stock crank weight. Although I could see a balance problem starting when I reached this point in boring it. When building 350 hot rods I always tried to balance eack rotating member to within a grahm of each other. But these engines only rapped to 8,000 rpm tops and I used a fluid harmonic balancer . So I reliese that balancing is very important. I saw where Jon Brogden said he had read about using 100% # weight to compensate for added piston weight and thats where I was going to start but George said he never went that high on any of his builds and he had tremendious results. Hope someone tries more weight and posts the results and findings. I really like the Strike pistons super hard and can be reused in a new cylinder later. The Burris pistons are still heavy enough in a 54.5mm that they will require a crank balancing just not as much added weight. Havent weighed one yet so not sure as to how much. A 52.70mm Yammi piston weighs about 108 grahms and a Strike 54.5 weighs 134 grahms but can easily be cut to 129 grahms maybe a little less. ANOTHER QUESTION IS WHERE DO YOU START ? THE INITAL POINT OF REFERENCE, FROM STOCK PISTON WEIGHT AT 52MM WHICH IS ABOUT 100 GRAHMS IF THIS IS CORRECT THEN A 54.5MM STRIKE PISTON IS 34 GRAHMS HEAVIER BEFORE SKIRTING AND HOGING. THIS WOULD PUT MY BUILD BACK AT JUST UNDER 75% ADDED WEIGHT. SOMETHING TO CONSIDER.
 

alvin l nunley

Premium User
I think there is another youtube lecture on the same subject but with better English. I've watched these lectures before but I find him very hard to understand.
I found him very hard to listen to. I'm old and my eyes are bad and my hearing, (to much karting) is bad.
 

Burris Racing

Premium User
Here's a little info and statistics to help with the discussion/argument! lol To begin with, several years back Yamaha had a redesign of the OEM piston because of cracking and breaking in the wrist pin bosses so weight went from (approx) 108 gr up to 112gr. Then about a little over a year ago they reinforced the skirt and the weight went to 117gr. These being in stock engines to my knowledge no one has changed anything in the crank to compensate for it and I've not heard of any problems. (Years back when we ran KT's on a regular basis we ran the heavier con rods and never noticed a difference in vibration OR lap times).
Our new 52mm cast KT100 stock piston with the reinforced skirt and pin bosses weighs 110gr and our forged 55mm is 120gr. IMO, its not that big of a deal that you have to spend time modifying the crank for this little of a difference especially since 2 cycle single cylinder crankshafts have got quite a bit heavier in the evolution of the engines since the 60's. These numbers are not exact to all the pistons since slight differences in manufacturing and the minimal added weight for an oversize affects the numbers.

Mike
 
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