I bought a crank at a swap meet a while back. It was a good price for a new factory forged crank at $50. But I recently checked it with a dial indicator for straightness and it's .005" out.
This crank is a 3.25" stroke LJ factory forged piece.
I have some questions on how this should be fixed.
I read somewhere that nitriding can cause a bent crank.

Is it possible that this crank has been nitrided?

If so how could I tell?

How deep is nitriding?

Would machining the crank .010" undersize be a good way to true it, or should it be sent to a shop that specializes in straightening?

Thank you for your help

 

Do you have a set of true "V" blocks and a calibrated dial indicator? I'd get a machine shop to have a look at it and see what they say about truing the crank (if it really needs it).

Not doubting your ability to measure the crank, but without a good set of known to be true "V" blocks and a calibrated dial indicator you are shooting in the dark. It may be out of true less or not at all...or it may be worse than you measured.

 

I didn't use V blocks. I just spun it in the block, supported by the 1 and 4 bearings.
I'm sure that there are more professional ways of measuring. But other cranks I've measured using that method usually show no more than about .0005" runout, about the limit of my dial indicators accuracy. Anyhow, I'm certain it will need to be corrected.
I'm still curious whether, being .005" out, it could be turned true. Or if .010" undersize would remove the nitriding, if there is any.
Or if possibly, I should have it nitrided.

 

don't think grinding it undersized will straighten it, but ask a shop and see. i've seen a fixture for straighting cranks but never seen it used nor know how effective it was. all it amounted to was sometihng to hold each end of the crank and a hook sort of fixture to hold the bend spot and hydrolics to move the crank upward while it was restrained by the hook fixture. it had a makers name plate on it but it looked crude to me.

 

I've seen it done with a fixture and a hydraulic press. The hardest part, it determining EXACTLY where it's bent, and applying the restoring force to EXACTLY that spot. Beyond that, it's pretty straightforward.

A good crankshaft shop usually has the tools and experience to handle it.

Not sure about how to check nitride treating. I'd think tat anybody that could straighten one, might know how.

 

Thanks for the replies guys, I'll have to start doing a Google search for crank specialty shops. No one nearby wants to tackle it.

 

It may end up costing more than a new forged crank by that time.

 

Yeah, This 327 project just found itself a new position on the back burner.
Oh well... one less engine to build.

 

Is it possible that this crank has been nitrided?

If so how could I tell?

How deep is nitriding?

Would machining the crank .010" undersize be a good way to true it, or should it be sent to a shop that specializes in straightening?

Thank you for your help [/B][/QUOTE]

IIRC, nitriding is usually .010" to .015" deep penetration. So when machining undersize, it usually removes the majority of it. A decent shop will redo it, or use a different hardening process.

Not to sure on the other stuff, but like the other guys said any good shop that deals with that stuff should be able to answer your q's.