Balancing pistons and rod assembly

R

RT

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Discussion starter · #1 · Jan 12, 2013

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My machine shop has already pressed my forged pistons onto my stock rods. I see that balancing is done before they are assembled. Here's my question; could I check the balance by using some sort of balance beam? I would set up a long piece of steel (pipe, bar, etc.) on a fulcrum and make sure it was balanced. Then, by hanging one piston/rod assy. at each end, find the lightest one. Once I knew that, I'd remove metal from the rods at the big end to match all the heavier ones (one at a time) to the lightest.
Is this worth doing, or would returning to the machine shop to have them disassembled and checked for balance be preferred?
Or, should I just leave it alone altogether?
Thanks,
Rich

 

G

GOSFAST

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#2 · Jan 12, 2013

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Hi Rich, the correct procedure now is to disassemble all 8 and balance the components!

Having said that, if the parts are pretty much the same, pistons, rods, pins, etc., I would take only one apart and use that as a guideline for the bobweight. It will NOT pose any running issues when the shaft is run-up. Couple grams here or there won't affect the overall performance.

Depending on the machine shop, you run a large risk of ruining some of the parts when pressing them apart, NOT all the time, but there is a risk!

Thanks, Gary in N.Y.

P.S. Come back and list all the components/brands involved, it helps to have this info!

 

Tom Mobley

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#3 · Jan 12, 2013

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If you're going to have the engine balanced take the stuff to the shop and have them do it however they want to do it.

If you're not going to have the assembly balanced (spun in a balancing machine) at a balance shop there's no point in what you're doing there.

Also, realize that the big end of the rod is rotating mass while the little end and the pistons and rings are reciprocating mass. their mass is counted differently in the formula for the bobweight.

 

R

RT

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Discussion starter · #4 · Jan 13, 2013

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OK, that all makes sense. Tom, the part about rotating and reciprocating mass is well taken.
The pistons are new forged units; SpeedPro L2399NF. The rods and crank are stock.
Thanks,
Rich