Balancing Tolerances?

Looking for some more info on what is acceptable in the real world of balancing.

We are putting together a 533 combo. The crank, rods, pistons are previously balanced ran by someone else. They had a valve train failure and got a couple of pistons. So with the crank and rods all checked out good, we go buy new probe forged pistons. My local balancing guy is telling me we have to re-balance everything to "0". Which I can appreciate his efforts to give us the best he can, but this what he wont answer for me. What is an acceptable tolerance to work within? The old pistons, wrist pins & locks weigh 23 grams more than the new stuff. If I use the new pistons, old wrist pins (which all check out fine) & locks I am 5 grams less than the origional combo. Which is on the right side to be, as we won't need mallory, if we have to re-balance. But I am also learning that on a big block some people use 15 grams for oil in there calculations. And I think the factory has way more tolerance than this. So is it really necessary to spend money on 23 or 5 grams or is it just good a theory? This motor is 13:1 comp, ported dove heads, with a .640/.640 solid lift cam. I really dont see this thing making much power after 6000 - 6500 at the most 7000 rpm. Thanks in advance for any help.

5 grams light will be fine as long they are all the same making it the same on the front and rear of the crankshaft. That would probably make your assembly 1/2 of 1 percent overballanced.

not to hog your post but if you change valve locations on the pistons form std to scj would you have to reblance it ?

bluecoupe

To my knowledge valve locations, flat top or dome. Do not factor into balancing. What matters is the overall weight of the piston and its related componets in relationship to the crank and counter weights. Its the bores job to keep all the piston mass in a realtive striaght line to the crank, so the crank only has to deal with the push and pull of the total weight.

Thanks Randy for your time, thats the kind of info a guy can use!

bluecoupe wrote:not to hog your post but if you change valve locations on the pistons form std to scj would you have to reblance it ?

I think what he is asking is that when you cut scj valve reliefs in piston with std. valve reliefs, which would in fact change the weight of the piston because you are removing material from the piston, do you have to rebalance. I think the answer would be as long as you cut them all the same the weight change should be equal on all pistons and because they would be slightly lighter you should be ok.

Relative to overbalance....1% of the recipocating wt. is a very safe place to be....for a racing engine.

bluecoupe wrote:not to hog your post but if you change valve locations on the pistons form std to scj would you have to reblance it ?

NO ... an SCJ notch doesn't need to be very deep and because the original notch takes-up a lot of the area of what material should get removed if the piston had no notch at all, the difference is usually under 4 grams.

thanks for all the info