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· Fast with class Datsun?
160 Posts
Discussion Starter · #1 ·
My buddy is into outboard motors (Mercury thumb_up ) and I was watching how he balanced the connecting rods and the pistons. For the connecting rods, he used different abrasive wheels untill the rod was almost near the correct wieght. He then started to polish the sides with different grits untill the rod was right at the correct weight (almost a mirror finish). For the pistons, he used a bench press and a drill bit that looked to be about 1/4" dia., and drilled a few holes at varying depths around where the pin slides through to reach the correct weight. He was using one of those gram scales.

My 3.8 that's going in my 240Z is the first engine I have ever rebuilt, so this is all a learning experience. I was told that the rotating assembly on the GN's came balanced from the factory. Is this correct? If not, can I balance my connecting rods and pistons the same way as my buddy did on his outboard motor? I figure, as long as it's out, and I have a long winter ahead of me, why not putz around with this stuff so I can learn from it.

Oh, one more thing, what about the crank? How is that balanced? What would I have to do to the harmonic balancer along with the crank? What about "knife edge"'ing the crank to free up some drag?

I did try searching for some answers here, but came up pretty much empty.



· Registered
2,098 Posts
The Buick was factory balanced. As far as I know, all factory engines are balanced. I don't know how close the factory tolerances are, they don't actually check the individual rods and pistons, they use an average weight, and balance the crank based on that average. If you want to "blueprint" your engine, and make sure the pistons/rods are matched, you can surely do that. The crank is externally balanced, it is balanced along with the flex plate. Balancing a crank takes a special machine, so you would have to get someone to do it for you, along with the flex plate. Not sure about the harmonic damper, some engines use that as part of the balance, too. The Buick engine is not going to be high rpm engine, so stuff like knife-edging and polishing the crank won't gain anything much.
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