Expert: Dan Liddy - 3/23/2007

Question
Hi Dan, this is Bruce the hydroplane racer again.  I'm building a new small block chevy and just received my rods.They are a matched set however there is 4 grams difference between the lightest and heaviest rod. My rules do not allow the balancing of these rods, is 4 grams alot when it comes time to balance my crank?  Secondly, I am allowed to balance my pistons, should I balance them all the same or make each rod and piston assembly the same weight(therefore each piston will be different)???

Thanks

Answer
Hi Bruce:

Here's what they do when they balance a V-8.  First the rods are weighed, BOTH ends are weighe, and balanced by grinding or drillingthe heavier ones to match the weight of the lightest one.  Sometimes one rod will be heavier at the small (wrist pin) end than others, and sometimes it's the big end (crankshaft end)  that is heavier.  So they are all ground or drilled until all the small ends and all the big ends are the same. (all withiin 1/2 gram)  

Then the pistons are individually weighed and the heavy ones lightened. If you buy forged racing pistons, they are normally within a 1/2 gram of one another.   

These weights are added together and "bob weights" are attached to the crank that weigh exactly the same as the rod & piston assembly.  The crank is then spun in a dynamic balancing machine, and the heavier counter weights are drilled until the crank spins smoothly.

Now since you can't "balance" the rods, certainly you can "recondition" them.  The reconditioning process removes small amounts of metal each time and might help - - -4 grams is a lot though.  If you can reduce the weight differential some - - -like 2 grams instead of 4,  it will be a big help.  For small end weights, you can turn a little bit off the inside diameter of the wrist pin for that rod.  This is helpful expecially when you are using cheaper cast pistons.

So my answer is to try desperately to get the rods to all weigh the same, and then the pistons.   The big money boys even balance their valves and rocker arms- - -And the next time you buy rods, get them from a race shop- - -they will be a lot closer to what you need.  I recommen Summit racing (www.summitracing.com) for the big motor guys .

Good Luck

Dan Liddy
Sarasota, Florida