Successful Crankshaft Balance on 66cc GT-5

Tim Bailey

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Over the weekend I followed Jag's instructional on crankshaft/piston balancing. I did not take the engine off the bike, I simply pulled the jug and piston off. I clamped the crank at top dead center and marked/drilled one hole on each side of the rod into the crank. The holes were .330" diameter as this was the only drill I had with a reduced shank which was required for my 90 deg drill motor. The holes were 14-15mm deep. I also lightened the wrist pin. I did this by drilling the pin out which was very hard and a carbide drill with lathe was required. Besides drilling the pin I also shortened the pin by .200". To keep the length correct I made two aluminum end caps, (.100" each) one for each end to make up the difference. The cap idea was put into place for two reasons, one to lighten the pin (Take steel away, replace it with aluminum) and the second was to turn the pin into a solid which I believe reduces crankcase volume by a teeny bit. In summary, I did no scientific measuring, just poked a few holes in the crank and got rid of a little weight on the piston side. The results are significant in regards to reducing vibration. One of my better mods.
 
Over the weekend I followed Jag's instructional on crankshaft/piston balancing. I did not take the engine off the bike, I simply pulled the jug and piston off. I clamped the crank at top dead center and marked/drilled one hole on each side of the rod into the crank. The holes were .330" diameter as this was the only drill I had with a reduced shank which was required for my 90 deg drill motor. The holes were 14-15mm deep. I also lightened the wrist pin. I did this by drilling the pin out which was very hard and a carbide drill with lathe was required. Besides drilling the pin I also shortened the pin by .200". To keep the length correct I made two aluminum end caps, (.100" each) one for each end to make up the difference. The cap idea was put into place for two reasons, one to lighten the pin (Take steel away, replace it with aluminum) and the second was to turn the pin into a solid which I believe reduces crankcase volume by a teeny bit. In summary, I did no scientific measuring, just poked a few holes in the crank and got rid of a little weight on the piston side. The results are significant in regards to reducing vibration. One of my better mods.

I should give this a try too perhaps.
 
I'm glad it's better but it can be "the best" if you take the time to take the crank out and measure its holes and weigh the parts and use my spreadsheet calculator to know exactly what to do for the absolute best balance.
 
I'm glad it's better but it can be "the best" if you take the time to take the crank out and measure its holes and weigh the parts and use my spreadsheet calculator to know exactly what to do for the absolute best balance.
The only problem is that I come to some kinda brain block when I try to use your calculator . Not only that it's not super convenient to tear a motor open all willy nilly. My engine is a real real pain to pull out from my bike as is. Let alone tearing down which involves new crank seals and all other not fun things, then actually measuring and drilling and yadi Yada.

I like that an improvement was seen without having to take the motor out, only issue with my bike is there's not even room for a drill with the cylinder out.
 
The only problem is that I come to some kinda brain block when I try to use your calculator . Not only that it's not super convenient to tear a motor open all willy nilly. My engine is a real real pain to pull out from my bike as is. Let alone tearing down which involves new crank seals and all other not fun things, then actually measuring and drilling and yadi Yada.

I like that an improvement was seen without having to take the motor out, only issue with my bike is there's not even room for a drill with the cylinder out.
The OP had the same issue. That's why he used the 90° drill adapter. :)
 
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