Piston Balance

[stevesingo]

I'm in the middle of putting an engine together and in that process I weighed the pistons. I found that the difference between heaviest and lightest was 1.1gram. Is this worth doing something about?

If so, where should I take the material from?

I was thinking of taking the shrp edges from where the machining gas been done.






1g of 2618 alloy is about 1/3 of a cm3, so not a lot really.

Edit to add, I do have old pistons to try first, so not much in the deep end.

Any thoughts...

Steve

Edited by stevesingo on Monday 22 November 09:25

[Pumaracing]

I wouldn't bother. That's close enough. You might just weigh the rods and put the lightest pistons on the heaviest rods if you like.

[stevesingo]

Thanks. That was the plan also. I'll weigh the rods (with bearings tonight at work and post back.

I did some calcs and the peak piston acceleration was just short of 4000g, so am I correct in saying that my 1.1g multiplied by 4000=4.4kg or a force of about 45newtons?

Bore 95
stroke 87
rod 144
Max speed 8000rpm

Steve

[Pumaracing]

Peak piston acceleration is 4054g so the force is indeed about 4.5 kg.

[stevesingo]

Thanks for confirming that for me. I'll weight the rods tonight. The rods are from Arrow, and are supposed to be balanced to within a gram, so I should be able to balance it out.

Steve

[Pumaracing]

Strictly speaking it's only the little end mass of the rod which reciprocates and to measure that you need a special jig which weighs the rods end over end which is gilding the tulip a bit although I do it that way myself being absurdly pedantic.

You might try weighing the pins separately and see if they account for much of the difference.

[stevesingo]

When I weighd the pistons I also weighed the pins and my 1.1gram difference is including the pins. I have weighd the rods and they are withing 0.2gram. So putting the heaviest rods to the lightest pistons, I have a 0.9gram spread accross all four. Given the risks involved of taking a dremmel to the pistons, I think I will live with that.

edit to add, the factory tolerance is +/- 4gram, which seems huge!

Thanks for the input.

Steve

Edited by stevesingo on Monday 22 November 22:13