Balancing Question

[Andrew]

Recently had my rotating assembly balanced but now I'm changing pistons. The new pistons are 12 grams lighter than the old ones. What effect if any will this have?

 

[Supershafts]

Pull the crank and have it balanced.

Unless all you're doing is A to B driving

 

[Garys1969RR]

I believe it will now be out of balance. Have the entire rotating assembly re balanced. Maybe the guy who balanced it before will give you a break, or at least some free advice. Probably needs some weight removed from the counterweights. Good luck.

 

[Andrew]

I've read some articles about intentionally overbalancing a crankshaft, but I'm not sure it applies to my build, or how overbalanced my set up will be. its a roughly 550 hp 496" mainly street use. so if the only benefit is in the 5000 rpm range, I wont be living up there too often.

 

[Meep-Meep]

The basic formula for calculating bob weight, once everything is equalized, is: One half the reciprocating weight + the full rotating weight. For a V8 you add up the weight of both pistons, ring pack, pins, clips and small end of the rods and divide by two. Then add that to the big end of the rod and the bearings and a little for oil. Or you can use the weight of one reciprocating assy and add it to the total of two big ends. So your 12 grams per piston comes in at 12 grams added to each bob weight (not 24 grams) but I will need the total weight of the rotating assembly to get a tolerance in Oz-In or Gm-In.

 

[rev.ronnie]

In my experience, when I re-balance a crank for lighter pistons, the amount of correction is usually real close to what the difference is between the two piston weights. That said, you are close to 1/2 oz. out of balance and probably should have it done.

Another options is, find out from the manufacturer what the weight is of the next thickest wrist pin is for that piston. I'll bet .010" thicker is about dead on. One or two grams I wouldn't get worried about.

 

[RustyRatRod]

Rebalancing is cheaper than throwing a torn up engine in the garbage can.