I got a kit that balance with Pistons that r .030 over but I need to replace them with .040 over of the same brand. Which that would make the piston 5 grams heavier do I have to balance the crank again. Trying to safe some money but want to do it right.

 

Unless you could take 5 grams off each piston. I would consider that, but the piston design plays a part. You can't remove what isn't there to begin with.

The real danger is in the exact weight of each piston. If the crank was balanced without weight matching your parts, you are probably gonna have to have it all rebalanced. If your pistons were weight matched and your rods were weight matched, you could literally swap pistons one at a time for a new one weight matched to that hole.

make sense?

 

IMHO, all the engine rotating assembly should be rebalanced any time any part is replaced.....

 

Thanks for the info

 

The bottom line and common sense is that if your new pistons are the exact same weight as your old ones then your still at the same balance you were!!! Ask your machinist! He will tell you the same, if not he just wants another balance fee and he`s a crook!

 

Also, BALANCE is a relative term. Ask how tightly it was balanced.

A stock assembly might have anything from 40-140 grams of imbalance. These run fine. They just don't live long in extreme conditions.

A good street assembly should be under 5 grams of imbalance.
A decent race assembly on a low rpm motor under 1 gram is good.

Nascar builders aim for less than .1 gram of imbalance.

It takes a lot of time on the balance machine to get that tight. A $200 or $300 balance job should get you under 5 grams. Nacar specs come with a hefty price tag.

 

Here at our local shop its 0.000 for 200$ they believe if you pay for a balance job you should get it done right! My crank had 6 holes drilled in front and 5 in the back to obtain 0.000 looked like i lost an extra 2-3lbs off the weight!

Too about 3.5 hrs on mine i watched and i have watched many others be done also!

My crank prolly made five miles of turns before done lol!

Most places just get you to about 10grams out and let it ride!

 

Like stated, check the weight spread of the old pistons and go from there. If they are all within a couple grams, re-balancing isnt needed on a street deal. Just bobweight placement can change the weight by more than 5 grams.

Is this a stock 50oz crank? Do you have the original balance sheet?

 

Well the pistons should have been moded to weight the same, as for the rods also, then theres no guessing about the replacement weights! Match your piston weights and your good but who knows what you balance job was held to far as specs! The Sheet would be good!

 

You are right, they "should" have. Some shops only weigh 1 piston, 1 rod, etc. This is alright if the parts used are quality parts with a small weight spread.

It really just depends; balancing is easy to do, and easier to make a mistake.