How many grams can the reciprocating weight change before you have to balance the crank again? Pistons are going to lose a little weight...

I'm running Icon .040" pistons in place of original balanced motor with the same .030 pistons and didn't rebalance. Can't be for sure on this, but I think the weight difference was something like 16 - 19 grams heavier for the .040 pistons. Seems to work fine

I would advise all high priests of the balancing religion to go somewhere else. I am an unbeliever and will go through the thought process.

Consider the fact that the popular formula for determining bobweight mass is just a finely-tuned guess. It is not derived from a mathematical analysis of the crankshaft and piston motion. It is not written down in the Bible, Koran or even the Kama Sutra. It is empirically derived.

Consider the fact that race engine builders routinely underbalance or overbalance engines. They change bobweight mass by a percentage they find has worked in a particular application. Look it up. The magic formula isn't the last word.

Consider that piston mass is multiplied by 0.5 for its contribution to the cylinder's bobweight. So a gram difference turns into a half-gram difference in the bobweight.

Now for a real life example. A board member reports that he changed 591 gram 318 pistons with 507 gram KB pistons with no noticeable change in how the engine ran. There was no attempt to rebalance the engine.

I'd suggest you calculate your old bobweight and new bobweight and if they are within five percent of each other you do not rebalance. If you must add weight consider pressing a copper sleeve in the wristpin to add the mass. But my gut feeling is you don't need to rebalance.

R.

The amount that is going to be removed is probably only going to be a few grams.

But the high rpms is what I'm worried about. 9400+

You better rebalance.. and get it close... some of these guys that
dont balance dont give a damn or just have junk... I balance every
engine... yes Chrysler was the worst when it came to balancing and
in a lot of cases it showed.. unless it was just stock junk... I
normally turned 8200 but I did turn 9600 on a few runs.. smooth as
silk... see what happens at 9500+ with a crap balance... you sure
wont like it

I went with a piston that is 14 grams lighter when I converted to pump gas. No re-balance and ran just fine on the dyno. No issues on the street either. I don't turn 9400 RPM On the dyno we went to 7200.

Should be no problem at all. It is in the right direction. I have ran engines with 70 gram too low on bobweight to 7000 + rpms without noticing vibrations. And my old Hemi I took up to 9000 rpms with nascar rods on an unknown Kellog crank, could not tell if anything was wrong. But I installed a crank balance for alu rods in a Hemi with stell rods, and it shaked violently, had to exchange the crank!!
I have my own balancing machine now, so I try to correct for such issues, but in your case I would not bother....If you overbalance by one % or two it is appx 10-20 grams, so then you can just fiddle with the formula to get it correct!!!

stock application no problem. high rpm race motor balance it, shouldn't even be a conversation about it.

I just want everyone to know we're talking maybe 3-4 grams

In that case, there really shouldn't even be a question about it - at any rpm.

Vizard's latest BBC book strongly suggests that closely matching all reciprocating weight (1 gram) is far more important than making a specific percentage number.

All the weights will be the same. Taking same amount off all pistons.

the amount of oil on the rotating assembly could vary more than that, i think you'll be fine.

how many 9600 rpm motors have you tested that theory on?

None of course. Don't think there are too many out there spinning that #. Mine has lived 16 -19 grams piston heavy @ 7K for some time now - which is the extent of my experience. I pretty much relied on my engine machinist for the decision not to rebalance. Could blow up tomorrow .

The engine is probably overbalance already. Small changes in pistons are tolerable even pins. You change rods that a different weight on big end, rattle bad.

Some people won't notice a subtle vibration, others will.

3-4 grams is noticable in the balancing machine, but hardly in the engine. If you calculate it with the forces this gives, even at 10000 rpms it is negligable compared to the internal mass forces in the engine from the piston reversing direction. Beside there is an oil factor which is estimated only, at a value more than the 3-4 grams. So you keep the pieces identical which keeps the center off gravity exactly identical for all 8 cylinders and you are good to go with your R5 engine!

Most NASCAR engines have been built with OVER BALANCE since the 1980's. I wouldn't sweat if it is near 2% O.B. I highly doubt 2-3 grams will move you into dangerous territory. (Do the math) I think you will have more trouble if you ZERO BALANCE. I wouldn't rely on this site for exotic-build data/info...alot of good sources no longer regularly visit here unfortunately.

3 grams = 0.106 oz. and 4 grams = 0.141 oz., a penny weighs about 2.5 grams.