piston assembly weight difference tolerance- stock redline

[Peteinjp]

3.8 build with Mahle rsr pistons. Heaviest unit combined with the lightest pins/rings/clips came in at 612.15g and the lightest piston/heaviest pins etc at 611.48g. So .67 g difference. The rods (arrow) show a .72g delta however I have not weighed them end to end. That said using the heaviest rod with the lightest piston will get me to within .05 grams per assembly.

Is balancing by mixing and matching components an acceptable approach for a street/De engine or is there an argument for matching all the weights on the pistons and all the rod weights?

Thanks,

Pete

[dannobee]

The factory tolerance is 4 grams on piston, pin and ring assembly. You're well within that. The only reason that this is even an issue stems from the proliferation of cheap, high quality digital
scales. Back in the day, we were happy to get a 1 gram spread. Picking nits to achieve a zero balance spread will yield no appreciable benefit. In fact, some sustained high RPM engines are purposely overbalanced by up to 4% of bob weight.

Rods weights are broken down into big end and small end weights because of the relative motions of each end of the rod.

Mixing and matching parts is a common way to get all of the weights as close as possible.

[nickd]

For reference, stock 964 piston weights from my late ‘91. Pins all weighed out identically. Anything under a gram is probably way overkill, but I had some time. There’s quite a bit of surface area under the piston to carve away a few grams.

before after
1. 474.0 470.1
2. 471.8 470.0
3. 471.8 470.0
4. 469.8 469.8
5. 470.9 470.0
6. 470.0 469.8

[Peteinjp]

Thanks guys- Yes I have a digital scale which I bought for work. Until recently I used balance scale that is very accurate (.01g) but can only weigh small items.

I'll be leaving these as are (total lack of time- haha) but just out of curiosity does one typically use a die grinder and remove weight from the underside of the dome?

Pete

[Mark Henry]

I grind the inside of the pin, it plenty strong enough and steel needs way less material to take off weight.
Balancing rods end for end is a 'dog chasing it's tail' scenario, within spec is better than taking off too much material while fruitlessly trying for that perfect balance.
If you have everything within 1 gram that's pretty good.

[Trackrash]

Anything closer than + or - half a gram is splitting hairs, IMO, except perhaps on a high dollar race motor.

I was told by an old engine builder that end to end balance of the rods was worth checking however.

[Peteinjp]

Thanks- I've seen your pic before for the rod balance setup. I'll probably copy it.

Pete