Dave928S

Your kids will be able to imagine they're swimming somewhere in the Gulf of Mexico next time they use that inflatable pool. Excellent use of a summer product off season

I'll bet you've put parts in the dishwasher when no ones been looking too ....

gbgastowers

I already have 7 of my alternate pistons installed. I put them in the original bore locations and am using 1 piston that I was able to get the rings out of in the original location so it should be balanced.

gbgastowers

Not the dishwasher but the bathtub.

Imo000

I don't think that's how ballancing works.

Lizard928

You need to use the original pistons in the original bores.

gbgastowers

JP Rodkey wrote:"I remember reading somewhere that the engine short block is internally balanced through tolerance control, as one should expect from a high performance precision machine. Within engine family, all pistons and rotating parts are interchangeable and will not affect balance. What I read stated that new factory pistons are ready-to-go and require no balancing because there is, purposely, very little variance including weight(s).

From the WSM, page 13-24a: Since 1980 models pistons and piston pins are paired according to weight. Pistons are weighed with components (piston pins, piston rings, circips).

Piston pins must always remain in the corresponding pistons and must not be mixed up within a set of pistons for one engine. This is important when disassembling and assembing an engine and pistons must be marked if necessary.

If pistons and pins are mixed up, they must be rearranged by checking the total weight.

Weight = 772 g
Permissible tolerance = +/- 4g

In other words, so long as the piston assembly total weight is within that spec range, you are good to go. "


Based on this I think I'm ok-I hope.

AO

What's the worst that could happen? j/k

But that's a great find in the WSM. I need to fwd that to a friend whose working on a motor. Very timely.

namasgt

I wouldn't worry about the balance, they should weight the same, I'm just worried about the coating wear on the piston walls. The piston has a coating. If your pistons have most of the coating left, you should be okay.

Lizard928

The pistons and parts were weighed and all put together. The crank was then balanced to that set.
This was my understanding of it when I read that section in the factory manual.

namasgt

Colin is right,
I didn't realize you were using rods and pistons from another engine on a different crank. Remember that every thing is weighted, balanced and then the crank is balanced. If you change the weight it will alter the crank balance, but I don't think the difference is going to be a large number.

blown 87

From what I have seen 928 engines are not really balanced, they are just close and built with a lot of hope.

get a good scale (0.1 gram or better) and weight the pistons and rods, you can make your own mind up on this.

Roy928tt

It doesn't matter one way or the other, one 928 piston and rod assembly will exchange for another especially 30 years after the fact, there will be no discernable difference. I once had a Mini that had an odd exhaust beat, on dissassembly it had 2 flat top pistons and 2 dished pistons ! It still drove and ran fine.

It is only an engine for goodness sake! and it will do just perfectly after the previous one.

Finally looks like you should be on the home straight Gunar, Good Luck

Cheers Roy

gbgastowers

Thanks Roy,
I have the piston back from the machinist and I will put this last one in tonight. I cleaned the pan of the black stuff again. Wonder where that stuff comes from.

928 at last

I've always used a really accurate scale to match piston weights......but what do I know.

Lizard928

You need to ensure that your pistons, and rod end weights are all roughly the same as the other pistons otherwise you do risk having a shake in the motor, and if too far off could throw everything out the side....