Piston weight difference

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mhuppertz

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I am prepping my parts for bob weight calculation to balance my forged crank.
Is 1 gram max difference between pistons acceptable for a 440 that probably won't see more than 6k rpm or so, or do I need to zero them out?
 
A paper clip weighs 1 gram. A 5 cent nickel weighs 5 grams. Just as a reference.
 
Doesn't your machine shop do that for you?
 
What happens when one piston is under weight? Do you add weight? Or take weight off the other 7?
 
Unless you have a crank balancer why bother. How are you going to balance that? (against the bob weight)
 
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Find someone with a paint scale. Measure all the pistons and find the lightest one. Get on the inside of the pin boss with a grinder and dress them until each piston weighs the same. It's not hard. Yeah it's "just" a gram, but how much is one gram throwin around at 6000 RPM? I don't know. If I had them all apart and in my hand, I'd get them dead perfect.
 
I had 7 pistons at x, and 1 piston and x-2g. I hit the inside of the pin with a Mig wire and built up the 2g there. The pins were floaters so EZ to slide out and weigh. I scrubbed the inside with a brush to make sure no slag was left in there. I took weight out of the pins in another case where I had 3 pistons that were >+5g out and there was not 5g in he piston to take out. Strange lot of pistons there.
 
mhuppertz said:
I am prepping my parts for bob weight calculation to balance my forged crank.
Is 1 gram max difference between pistons acceptable for a 440 that probably won't see more than 6k rpm or so, or do I need to zero them out?
Click to expand...


Out of balance of 1 gram then add the dynamics of 2,000 rpm, that actual force squares at 4,000 rpm, then that force squares at 6,000 rpm.
Your seat of the pants wont notice it, but in the motorsport theory of everything counts...maybe its worth it.

I had a 71 340 engine with 150,000 mile on it and one rod was 42grams lighter than the heavy one, must have been down on power fighting off physics like that.
 
I bought 18 SBC LT1 PM rods when we were gathering parts for my kids dirt modified.

The weights vary by 15 grams. These were nearly new rods that came out of marine engines that froze during the winter.

These rods are used by hundreds of guys in their dirt cars... Most engine shops screw em together and simply balance the cranks.. new pistons are pretty close on weight
Anyway that 15 grams doesn't seem to make much difference in these motors running 50 or so laps per week at 6500-7000 rpm.

My son's engine runs smooth and strong.
 
You guys are creating stress risers in aluminum parts for nothing. And MIGing on heat treated wrist pins.....FFS. S/F....Ken M
 
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