On Turning Stock Cranks

[FORSFED]

As a couple of you know, I have a 92 TSi AWD sitting in the garage right now because after I built it last summer and ran it for 300mi I fried an eagle rod (Steve, you remember talking to me about this correct?). I was unfortunate to have the same luck as old man Hill did with his same setup where a rod was poorly cut too small on the bearing end.

Moving on to the subject, I originally had the crank turned .010" under due to a small amount of wear on the journals from the stock bearings. I've heard a lot of mixed opinions on turning dsm cranks, mostly of the negative sort, but I wanted to get some good, strong facts from those that have any experience here. I can't believe taking that small of an amount off could affect much but maybe I'm wrong. I just want to know if I'd be alright re-using this crank again or if I should find one in good shape and have it micro-polished when the motor is ready to go back together.

[1ViciousGSX]

My understanding on this (somebody correct me if I'm wrong) is that the DSM cranks are Nitrided from the factory. If they are, then turning them removes the hardening process and makes them more prone to failure.

[FORSFED]

If that's in fact the case I could see there being a longevity problem there...

Obviously turning the crank would remove the nitride assuming it's not a very thick layer. Wouldn't some of it also get removed with polishing or not necessarily since you aren't affecting the size of the journals?

[Matt D.]

I read somewhere, possibly even in a DSM tech manual of how thick the nitride coating really is. It recommended against machining the crank at all. I've also read that if you do turn the crank you can get it coated somewhere and it'll be good as new.

[Raptor]

My .02

Yes Mitsu cranks are nitrided and turning them will remove it depending on how far you go. This isn't necessarily a bad think considering that by reducing the diameter of the journal you are slowing the speed of contact at the bearing surface. You can have the crank re-nitrided, not sure exactly on the cost, last one I did was a 440 crank and it was around $140 about 18 years ago. It may be worth checking into, the price has probably gone down some since then due to the cost of technology etc.

Reducing bearing surface speed improves reliability at higher RPM and the journals are plenty large enough to handle it without sacrificing any strength.

As far as only going down .010" I don't think that will remove the factory nitride. Somewhere I remember reading it was only in danger at around .020" but don't quote me on that, I definately could be wrong, I am getting old you know.

[Jacek]

does the manual have any dimensional specs on the crack like out of round and diameters for the pins and mains? it would probably say in those specs or have some footnote saying something.

[LightningGSX]

Wouldn't reducing the journal diameter increase contact speed? I would also think reducing diameter will also decrease bearing surface area, therefore increasing overall friction.

[Jacek]

wouldnt it just fill up with a thicker layer of oil?

[JET]

You have to use the correct bearing size Jacek.

Raptor, I agree with Lightning, you are going to increase the pressure on the oil between the bearings if you decrease the diameter of the crank. Just like smaller diameter tires have a smaller contact patch. I would think that is a bigger possibility of squeezing the oil out than the extra speed.

[1ViciousGSX]

Originally posted by Raptor+Feb 12 2004, 07:29 PM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Raptor @ Feb 12 2004, 07:29 PM)</td></tr><tr><td id='QUOTE'>My .02
This isn't necessarily a bad think considering that by reducing the diameter of the journal you are slowing the speed of contact at the bearing surface.

Reducing bearing surface speed improves reliability at higher RPM and the journals are plenty large enough to handle it without sacrificing any strength.&nbsp;

I definately could be wrong, I am getting old you know.[/b]

Originally posted by LightningGSX@Feb 12 2004, 09:02 PM
Wouldn't reducing the journal diameter increase contact speed? I would also think reducing diameter will also decrease bearing surface area, therefore increasing overall friction.

Originally posted by JET@Feb 12 2004, 09:19 PM
Raptor, I agree with Lightning, you are going to increase the pressure on the oil between the bearings if you decrease the diameter of the crank.&nbsp; Just like smaller diameter tires have a smaller contact patch.&nbsp; I would think that is a bigger possibility of squeezing the oil out than the extra speed.

The difference is not going to be noticeable on such a small change in diameter. Yes in theory it makes sense, but without a bigger change in size (like cutting down a 400cid sb chevy crank to fit in a 350cid block) it's not even really worth thinking about. Small block Ford guys would try and use smaller rods for that reason also.

<!--QuoteBegin-Jacek@Feb 12 2004, 09:11 PM
wouldnt it just fill up with a thicker layer of oil?[/quote]
You would use thicker bearings to go with that thinner crank.

And yes Raptor, we know you're getting OLD. :bounce: