On Turning Stock Cranks

FORSFED · 02-12-2004

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** · 02-12-2004

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 · 02-12-2004

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. · 02-12-2004

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 · 02-12-2004

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 · 02-12-2004

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 · 02-12-2004

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 · 02-12-2004

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

**JET** · 02-12-2004

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** · 02-12-2004

Quote:

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. 

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

Quote:

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.

Quote:

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.  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.

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:

Raptor · 02-12-2004

Quote:

Originally posted by JET@Feb 12 2004, 09:19 PM
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.

Actually, this isn't something that is just me guessing, it is proven theory. Decreasing diameter does reduce bearing surface speed. Consider it like this, in the same revolution, a larger bearing journal is going to have a larger circumfrence to cover which means higher speed on the bearing surface causing more friction at higher RPM.

As far as squeezing oil out, there would be no issues if the correct oil viscosity, preasure and bearing clearances are used.

As Vicious pointed out, on this small of a reduction in diameter, the benefit would be negligible. My main point on that was that it isn't necessarily a bad thing if the nitriding isn't affected.

And yes I am getting old, but you are still older Mike and from what I hear, Shane too! :razz2:

**1ViciousGSX** · 02-12-2004

Quote:

Originally posted by Raptor@Feb 12 2004, 10:06 PM
And yes I am getting old, but you are still older Mike and from what I hear, Shane too! :razz2:

Yeha I heard Shane's been holding out on the old age :razz2:

FORSFED · 02-12-2004

As long as my physics are up to par, I'm about 99.9% sure that you are correct Mike. After taking all of this information in, I'm thinking that the main issue on turning dsm cranks is the nitride layer. I need to find out how thick it is from the factory to determine whether or not it's still there or not. If it's there or I have the crank re-nitrided, either way I can't really see that there could be anything bad coming from reducing the crank journals by .010"...

**JET** · 02-12-2004

Quote:

Originally posted by Raptor@Feb 12 2004, 10:06 PM
Actually, this isn't something that is just me guessing, it is proven theory. Decreasing diameter does reduce bearing surface speed. Consider it like this, in the same revolution, a larger bearing journal is going to have a larger circumfrence to cover which means higher speed on the bearing surface causing more friction at higher RPM.

As far as squeezing oil out, there would be no issues if the correct oil viscosity, preasure and bearing clearances are used.

I agree with the speed being lower, that is simple physics. I just don't think it outweighs the smaller contact patch. I too agree that it would be negligable with this small change.

The nitride thickness on a DSM is supposed to be .012", so turning one .010" is asking for trouble if you ask me.

LightningGSX · 02-12-2004

Forget I said that(the speed thing), I was thinking about it the wrong way.I did a little math and now I feel dumb.

FORSFED · 02-12-2004

Hey JET, are you pretty sure about that thickness then? If that's the case I may have to do some rethinking.

Raptor · 02-12-2004

The physics on the small contact patch are accurate, there is less area to absorb load, but still not a problem. The formula 1 circuit and bikes should be adequate proof of that. As with anything else, if you arn't turning extremely high RPMs there is no reason to ever consider reducing journal diameter other than getting past blemishes on the journals.

If the nitriding is only .012" I definately would have it re-nitrided.

Last note on the journal size thing, with a large shot of N20, I would probably opt for the larger journal just due to the more violent immediate load on them that nitrous can cause which would make the contact patch theory more important.

Any other thoughts or comments on this? I love this kind of thread just for the mental exercise it causes. At my age that sort of thing is good for ya.

**1ViciousGSX** · 02-12-2004

:idea: Here's one for you Raptor. :idea: With the power levels the 4g63/64 are reaching these days, the blocks are starting to twist. How would you go about trying to stop it from twisting?

Time for bed, I'll check with you in the morning.

Raptor · 02-12-2004

Hahahaha, Hey Mike, we already have that covered. If you want to find out how, send your motor back to Magnus, and we will hook ya up

Jacek · 02-13-2004

Quote:

you have to use the correct bearing size Jacek.

sorry i should have known that im dumb this week.. it started on monday when i killed my finger on a cutting wheel on a handheld grinder at work :headache: