Balance Question

[Scott Clarke]

I'm finally ready to install my pistons, and want to insure that I place the wrist pins and pistons in order to achieve the best possible balance. The rods are installed, with the heaviest pair closest to the flywheel. I had all of the components logically placed, but then discovered that my pistons needed machining (after the rods were installed). I have re-weighed the pistons. Of course, they are not the same weight they were before machining.

I can achieve near perfect balance between each opposing cylinder (within .05 grams), but the result is a discrepancy between pairs of up to 1.5 grams. Alternatively, I can get a condition where each pair is within .35 grams, and the variation between pairs is about .75 grams. Which is more important? The balance of each pair, or the overall balance of all cylinders?
-Scott

[afterburn 549]

I think there is ( I know ) enough meat on them to carve enough off to make them all weigh the same, thats what I did, end of problem

[Eagledriver]

I think matching the opposing pistons is most important. They move exactly opposite of each other. The others move out of phase from the pairs.

-Andy

[911pcars]

I would machine them so they all weighed the same, including the con rods (both big and small ends), but that's just me. If you have the means to do it, why not?

Sherwood

[Scott Clarke]

Thanks for your responses. Yes, the ideal solution is to make the components in "perfect" balance. Unfortunately, as the rods are installed and case sealed, I'm stuck with the rods as they are without a lot of work and expense. As I mentioned in the first post, everything was worked out (within .3 grams) prior to the pistons being re-machined. Lesson learned: verify piston dome volume and CR prior to sealing case! My gut tells me that Eagledriver is right, and that I should match the pairs and not worry about the 1.5 gram discrepancy from heaviest to lightest pair. Unfortunately, I can't have access to the scale I was previously using. Of course, if this is a significant thing I can find another one and, I guess, remove material from the pistons in order to achieve total balance. I'm wondering if it is worth the effort for a street engine that, one day, may see 7300 RPM.
-Scott

[afterburn 549]

A 3 beam gram scale is pretty cheap these days ............. later it will never bother you

[Scott Clarke]

Where should material be removed from the piston? The bottom of the crown?
-Scott

[afterburn 549]

took mine off the skirts..15,000 miles ago so far so good, and I took alot off

[Henry Schmidt]

Truth is that the engine would never know the difference but while your in there why not get it as close as you can.

The two best places to machine the piston are the wrist pin boss above the pin and the center of the inside of the dome.

That said we remove material from the pin. It requires a lathe but we find it the easiest clean way of removing even a very large amount of weight.