Cyro crankshaft?

[Drifter27_04]

Just some thoughts on turning rod journals as I spun #1. I am really in the debating stages of having it cyroed and grinded as it has been difficult to find a good used one local. Can some of you give me your thoughts and especially anyone has done this or just turning the rod journals w/o cyro.

 

[dsm-onster]

I wouldn't worry about cryo-treating a stock crank. They have been known to handle over 1000whp frequently driven.

A discussion on cutting the crank is going to bring on 2 very devout camps. Stock cranks are nitrided for hardness at the journal. Cutting removes some of the nitriding. Nitriding is about .03" deep. Most cranks need only to be cut .01" under.

See this discussion.

And just to stir the pot, yes the manual says that you must not machine the crank to undesize. But just a page later it says that if the oil clearance is out of spec to machine the crank WTF. Click.

Read the rest of that thread there too. Good information. Nitriding is a surface treatment and does not affect the strength of the crank as a whole (horsepower potential). It is used to harden the crank in the event of bearing contact. Yet, I've never had a bearing contact issue that didn't result in the need to cut or swap in another crank, regardless of nitriding.

 

[Drifter27_04]

Thanks for the info and I have also just heard this from a well known DSM engine builder.

 

[Defiant]

Mitsubishi says their crankshafts are non-serviceable items, and are not to be machined, only replaced when out-of-spec.

They make no mention of Mr. De Bergerac.

 

[dsm-onster]

Guy, the Laser Overhaul Manual does mention for the 2.0L turbo to "replace . . . crankshaft if neccesary. Or, regrind to an undersize and replace bearing. . .", page 9-127. Yes, this is one page after it is stated, "A special surface treatment has been applied to crankshaft. Do not machine to undersize." Pics of these pages are in the second link I referenced in my last post.

Since this is a surface treatment, it's logical to conclude that the crank won't be weakened in the sense that it would be more prone to crank flex (jumproping), or snap in two. In fact nitriding makes the metal more brittle, so reports of snapped cranks are not a cause of removing nitrided surface. In fact, every single report of a broken crank happens at the number 4 journal just before the fw. This has happened to cut and uncut cranks. And, though every journal was ground on those that were serviced, it still happens on the same journal.

The real issue is getting a machinist who knows what he's doing. E.G., one may polish the crank after cutting it down; THEN flux it to see if there's any cracks. But the polish job will hide the cracks really well. Also the crank can just be cut wrong. And some shops may not magnaflux at all.

A crank that has been contacted by a bearing has had a tremendous amount of heat put on it by the friction. This causes stress fractures. Such a crank with a crack needs to be cut to be reused because when that much heat is there certainly the bearing has also cut into the surface of the journal. Magnafluxing finds these cracks. A cracked crank will break when any amount of power is achieved. You can blame removing the nitriding (which doesn't make the crank structurally weaker), or you can blame not magnafluxing the crank before polishing. As mentioned, quite a few machine shops dont flux at all.

Nitriding is to save the crank when bearing contact IS made. It keeps heat down because it just cuts the bearign up until you see that there's a problem. You have only a few seconds at idle to save your crank from being damaged even when nitrided. But the point of the bearing design is not to have the crank contact the bearing material in the first place. And I wonder how many of us have actually saved a crank from being damaged to the point that it would have to be cut when they finally realize they have an oil/bearing issue.

I've been told by reputable shops, nitriding penetrates .03" deep, + or - .01". So undercutting a crank .01" still leaves the nitriding. Many times a damaged crank needs no more than .01" undercut.

EDIT: thanx guys for letting me edit my post for 15 minutes before replying. I finally have my frame of thought properly conveyed .