SSTAT

So, Ive been racing on this motor for about 5 years, NASA TT but only about 3 weekends a year. Shes been bulletproof. I set a track record at Beaverun in may then the next session the (stock) crank snout broke off. I sent oil for analysis and no metal in it, cylinder pressures are great, sent the balancer back to ATI and its fine, and I had the whole assembly balanced before I put it together, so I figure I'll just put a new crank in it and move on. But I am reading that LT1 cranks are balanced differently than standard 350 SBCs. So my question is, if I know my ATI damper and fidanza flywheel are zero balanced, and the whole assembly was balanced, can I assume then that the old crank was zero balanced and that then I can just get any zero balanced SBC 350 crank and drop it in?

raroz28

That was my understanding when I replaced my crank and when I replaced it with another stock LT1 crank it was fine.

MachinistOne

You would need to have the replacement crank balanced to the same bobweight that your old crankshaft was balanced to. Any stock crank that you get as a replacement is going to be heavier on the counterweights because they're all overbalanced from the factory, additionally, full round rear main seal crankshafts are zero balanced on the front, but externally balanced on the rear so if you're right in saying that your flywheel is zero balanced, then there must be slugs of mallory in the real counterweight on your broken crank.

End result, no, you cannot just drop in a replacement crankshaft without having it matched to the old one since you said you had it balanced and are somewhat unclear on what exactly was done, and yes, the bobweight for an LT1 is lighter than other full round rear main cranks.

87Aerocoupe

In my experience if you don't ask the machine shop to internally balance your rotating assembly then they will balance it to whatever is stock. The stock crank is internally balanced at the front and externally balanced at the rear so if your flywheel is zero balanced then you are going to have a big problem.

SSTAT

I'm sorry guys, I feel like an idiot here. I read somewhere that the cranks for the LT1 were balanced differently even than the 86' + SBC because of lighter weight pistons? It said that the part numbers were the same but there was still a difference. A search on summit racing for stock stroke 1pc RMS SBC cranks brings up completely different cranks than the same search for gen II LT1 cranks. These would suggest they are not the same, but it seems you guys think they are. What am I missing?

MachinistOne

Exact same casting, just has extra drill holes or notches in the counterweight to account for the lighter weight pistons. That is the only reason why they should not be interchanged.


Something is wrong with their site then, when you buy aftermarket, the crankshafts for an '86+ full round rear main motor will work in either a standard Gen I full round block or a Gen II full round block.

SSTAT

OK Im still an idiot. To me it sounds like in the top you say an LT1 crank should not be interchanged with an 86+SBC because its balanced differently, but in the bottom part you say any aftermarket 86+SBC crank will work fine in an LT1. Maybe you could write the explanation in crayon and attach a pic so I can understand ... Do you mean any will work so long as you have the assembly balanced?

MachinistOne

An aftermarket crank will work because it must be balanced before being used in every case. You can't just buy a new crank and put it in either motor, in both cases it has to be drilled to lighten it to the final bobweight for your specific combination.

Yes - exactly. I could take a regular '86+ crank and put it in the LT1 as long as it's been balanced by subtracting weight depending upon what the balancer computer tells me.

SSTAT

Yea! I'm not an idiot anymore! OK I am, just not about this...

ACE1252

Hehe.