340 question?

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hosehead

hosehead

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So I scored a 70 a body 340 today (200 bucks). Its standard bore but the crank is turned 20/20. Any idea why a standard bore 340(w/ original pistons, oil pan, dipstick, balancer, etc) would have a crank turned that much? Also, it looks like the crank will polish, but what is the most you can turn a steel 340 crank? Thanks guys.
 
I'd be curious about the date stamped into the bearing shells. I'm not sure on the absolutes, but the factory may have turned a scratched crank rather than scrap it. Otherwise, a previous owner may have lost oil pressure such that it munched parts without hurting the bore. There are a lot of variables with a 40-year-old machine.
 
As a general rule, a crank can go 0.030 under without too much trouble. Detonation will tend to be something to avoid even more with the additional soft material within the lines of force transmission.
 
I've had a half dozen or so original 340s and have seen this before. They appear to be shot peened and Nitrided. I can only guess that perhaps the crank may have "moved" a bit and it took more to clean up the journals. I think they are surface hardened so it is not deep. If you cut it you may be running a softer journal.
 
It will safely turn to whatever undersize bearings are available. I think that's .030", but there might be even smaller available. I've never seen them though. My guess is it spun a bearing. Oh, and rehardening parts nowadys ain't very expensive. Lots of companies offer different processes.
 
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