Marty,
If our engines were designed like Ed Kalfus' .90 cu in boat engine we could use Sir Harry Ricardo's formula for balancing single cylinder engines. Ed used 1/2 the total reciprocating weight plus the total rotating weight on the crank counter balance. His engines ran very smooth in all RPM ranges from 8,000 to 26,000. This doesn't work in our engines because there is no hanger piston & no windows in the piston to make the reciprocating weight very low.
I had to also add 2, 5/16 dia, carbide slugs to my crank counter balance to give 1/3 of the total weight of the piston & connecting rod assembly on the crank counter balance. My engines run very smoothly in the 15,000 to 35,000 RPM range. My engines also have a very long connecting rod. This greatly reduces any piston slap as the piston rocks from side to side in the bore. Ed's engine had the longest connecting rod possible because of the hanger piston.
Jim Allen
Jim:
So, what you are saying is that you make a Bobb Weight to place on the crank pin that is 33% of the total of Rod, Wrist Pin, Clips and Piston?
Marty,Jim.....
This is what I have used for over 30 years with great success........
I weigh the piston & wristpin ........I then weigh the connecting rod and divide the weight of the rod by 2.......I do all this on a triple beam lab gram scale.....
Add together the weight of the piston + wristpin and 1/2 the weight of the rod and multiply that total weight times .6 [or 60%] for the weight of the bob weight....
IMPORTANT>>>>>>Make sure you don't make the bobb weight any larger in diameter than the O.D. of the bottom of the rod......
The bob weight can be as long as you want just no larger in diameter than the bottom of the rod.....
I have tried several multipliers I.E. .....55 & 60 & 65% and haven't seen any difference in performance.....
Generally what happens is I have needed to lighten the piston to obtain the correct balance......
Since I am not a big fan of skirting pistons,I generally remove material from the piston crown from underneath the crown with a endmill up between the pin bosses.....I don't go any thinner than .060" piston crown...
I turn a bobb weight , put it on the crankpin and and put the crank on a fixture very similar to a hi-point balancer.......Depending on what it needs I either add tungsten to the counterbalance or remove piston crown material...
I am not saying this is the only way to balance but this is the way I was taught and it has always worked very well for me......
PS:I looked for my balancing fixture to post a photo,but I think I may have given it to Truex......If he has it maybe he can take a photo of it when he gets home from Pocono and post it if you want to see it.....
Rod Geraghty