When changing to a lighter piston (400 engine) while using the stock rods, crank, etc, what balancing issues are there? Someone on a different website told me that as long as the lighter pistons all weigh the same, rebalancing isn't necessary. I don't buy that since the entire bobweight will be lighter.
Lighter pistons...rebalancing?
- Thread starter polkat
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lilcuda
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The person who told you that is 100% wrong. You must rebalance a v8 when the pistons are substantially lighter or heavier than stock. If it was a slant 6, then it wouldn't matter because the pistons counterbalance each other since they are all in a straight line. In a v8, the crank counterweights help to balance the pistons.
daredevil
Well-Known Member
lighter is better on rebalancing price as you drill the crank counterweights instead of adding Mallory metal which is expensive. Lil cuda i could see that in a horizontally opposed motor but why do you balance cranks in 6s? Horizontally opposed i could see how the crank would weigh the same on both sides. why does the crank have counterweights? Not saying youre wrong just need a little explanation so i understand. i,m not a machinist so i like to know why things work.
ramcharger
Dismember
If the new pistons weigh any more or less than what it had, it needs to be rebalanced.
74fldart
Well-Known Member
i had a 383 in a car, i wanted a 440 so i rebuilt a cast crank 440 using the same crank, rods but changed the pistons to a set of kb (much lighter than stock) that had been used before and were all balanced (just the pistons) to the same wt. when i fired up that motor i could feel that it wasnt as smooth as the 383 was the 440 did have a 274 cam in it so it was going to be a little rough idling anyway. none the less it ran good and i never had a problem w it and turned it to 5500 many times till i eventually sold the car.
lilcuda
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In a straight or horizontally opposed 4 or 6, they always have the same number of pistons "up" as they do "down" at any given time. The crank counterweights are there to balance the crank itself.
In a v engine (4, 6 or 8 ), the counterweights have to counterbalance the crank, big end of the rods and a part of the reciprocating weight (which includes the pistons, pins, wrist pin clips, and the small end of the rod) also. Therefore, they must be balanced when the pistons change weight significantly.
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