How far can one grind a 390 crank. I just picked up a crank and it is already .020 rod and .030 Main?

 

.030/.030 seems to be reasonable on a 428 crank. You really have to watch the fillets.

 

Peck, the smallest undersize bearings I have seen offered for an FE crank were .060 undersize. But before you regrind the crank to the next smaller size.....does it really need it ? Many many FE cranks don't need to be reground, a size check and a polish are plenty good. Does you crank really need to be ground, or is your machine shop guy looking for something to do ? DinosaurFan

 

As long as it is on size and round to within .0002 on each journal with no belly or hump in the middle from over polishing and the fillets or radii are correct you should be in good shape. Have it Magnafluxed before you start spending serious money on it.

 

I have my 428 crank ground 40-40. I'm not happy about it, but so far it works fine. I bought it used and freshly ground at 20-20. Had it in for balancing and the Machine Shop (who I trust) found all the journals had quite a bit of taper. So down to 30-30. A few years later I had a head gasket leak and the crank became scored. Now at 40-40.

Although the stress is concentrated on the outside surface of the journals, and therefore larger diameter is better for strength, for a 40 grind, you are only losing 0.020" per side of a journal diameter about 2.75 inches. It's not really that much, however the bearings are certainly noticeably thicker when you take them out of the box.

From my limited experience, 40-40 is OK, but I'd be very suspicious about going to 60 if it's for a hopped-up engine.

Another option, pricey but nice, is to have the journals hard-chromed back up to their original diameter. I"m not sure if this is done like hydraulic cylinders or not. I think they may flash-spray the chrome with some sort of electro-static gun, thicker than required, and then grind it, but am not sure. Apparently it makes a very nice crank. Maybe someone else here knows more about it.

ps: At 40 thou, I noticed the chamfer of the crank oiling holes was gone in some places. Had the Machine Shop fix that up. Their only mistake in all the years of doing business with them and they fixed it for free.

 

A good shop that has good equipment could weld the journals and then grind back to standard size. The journals would then be harder than original and wear longer. The chrome process is good if done by an experienced shop as long if it is a real plating job, dipped not sprayed, the spray chrome has a tendency of flaking off. But again that may be an operator/machinest fault. I think the best, after welding would be to have an aircraft crank rebuilder chrome and grind the crank. It will be pricey.
I have ground chrome printing press rolls and crankshafts and those machinests earn their money.