Question on balancing rotating assembly.

[BlackRat]

I bought a SCAT9000 crank new that is already balanced. I am looking at a set of EAGLE rods that are already balanced. So if this is the case should you still take all of the peices to the machine shop and have them re balanced or just things like the pistons, balancer, fly wheel etc? The machine shop also asks for one complete main and rod bearing as well as one piston with the ring installed. Would one of you clear this up a little for me.

Thanks
Wade

 

[BLACK MOON]

Always balance the complete assembly. Variances in piston weight can make a huge difference. Your flywheel and balancer are also involved. I just did a rebuild and didn't rebalance since it was done earlier and only new rings and bearings were used but I wished I had. They always seem to run smoother. Just my opinion.

 

[AKRAY4PLAY]

take all the entire rotating assembly in for balance. the pistons, rods, rod pin, rings, crank, flywheel and harmonic balancer are a rotating assembly. they should be balanced as such and you should be provided a bob sheet for proof. a good balance job will let you spin an extra 800-1000 rpm without hurting anything, Brian

 

[Jack]

take all the entire rotating assembly in for balance. the pistons, rods, rod pin, rings, crank, flywheel and harmonic balancer are a rotating assembly. they should be balanced as such and you should be provided a bob sheet for proof. a good balance job will let you spin an extra 800-1000 rpm without hurting anything, Brian

Brian is so right ... a crank is balanced to a specific bobweight ... the bobweight is calculated to match (but not EQUAL to) the overall mass of piston, pin, rings, rod bearings & surface oil. Some of the lesser scat & other bargain cranks can require a lotta mallory metal (added metal mass) to get them to balance. Around here, a typical balance job costs about $125-$150 if NO mallory is needed. If you haven't already bought pistons & rods ... ya might wanna have your machine shop check out what the crank's present bobweight is ... and possibly use that as a guideline when choosing pistons/rods ... mass becomes a factor in choice ... by doing this you might avoid installing much/any mallory metal that otherwise might be required for your crank.
JACK:gap