<VV> Crank Nitriding/Engine case question Question

[AKG]

One other item regarding the 140/turbo cranks that are nitrided.  These
cranks are also heat treated for additional strength.  Ref. The Classic
Corvair pg 138 (thanks Bob).  This is something that can be done to a
"regular" crank.  Process wise, I think the heat treat would be first
followed be nitriding.

The Artful Dodger

> From: "Roger Gault" <r.gault at sbcglobal.net>
> Date: Tue, 17 May 2005 17:49:49 -0500
> To: "Corvair List" <virtualvairs at corvair.org>
> Subject: Re: <VV> Crank Nitriding/Engine case question Question
> 
> Chris,
> The early block questions have already been answered.  It's almost certainly
> cheaper to buy a block off e-bay or one of the used vendors or from one of
> the guys on this list.
> 
> As for the nitriding discussion you found, I'm not impressed.
> 
> Nitriding is not done to our cranks for wear resistance.  If you load the
> bearings enough to fail the oil film and have actual metal-to-metal contact,
> or have rocks in the oil that bridge the gap, nitriding is not going to
> help.  MAYBE the added hardness helps at startup, when there is no oil film.
> 
> Nitriding is done to put the surface in compression.  In order for a fatigue
> crack to start, it must get the surface into tension.  Bending forces on the
> crank can do that.  If the surface is nitrided, there is a built-in
> compression which must be overcome before the surface goes into tension.
> So, the nitriding reduces cracking.
> 
> By the way, "increasing the Rockwell" does not increase the stiffness of the
> crank.  It does increase the yield and fatigue strength, but at some point
> the crank becomes brittle and will fail from shock loads.  This is a fine
> line to be walking.
> 
> The best of all worlds is probably to harden the crank up to the maximum
> save level, and then nitride it.  I suspect this is pretty much what GM
> thought they were doing.
> 
> Roger
> 
> ----- Original Message -----
> From: "Chris" <joeaverage at earthlink.net>
> To: <virtualvairs at corvair.org>
> Sent: Tuesday, May 17, 2005 10:35 AM
> Subject: <VV> Crank Nitriding/Engine case question Question
> 
> 
>> Coming out of Lurk Mode for a moment...
>> 
>> Considering building a 140 HP EFI in the next year or two and have a
> couple questions. I want to use parts I already have on hand. Collecitng
> info and pieces for now.
>> 
>> 1st question: have an EM truck engine. Can I use the block? What changes
> are necessary?
>> 
>> 2nd question: what is the difference between the 110 HP and 140 HP crank
> besides nitriding? I was looking over some tech info for VWs and saw this
> article:
>> 
>> http://www.geneberg.com/article.php?ArticleID=202
>> 
>> "Now that our connection in Sweden has past, we have sought out a new
> source for crank forgings. We found a forging company in the USA that can do
> the job. We have redesigned our crank to be stronger than before. We have
> found that the forging itself was excellent, however, the Nitriding process
> of the crank created sort of an eggshell effect. The Nitriding was done to
> help with wear resistance on the journals themselves not necessarily for the
> strength of the crank. This problem showed itself with the cranks getting a
> crack in the journal radius and eventually would work itself into the center
> of the forging.
>> 
>> The first time the crank bent it would crack -- not good. So, after some
> research we found that by bringing up the core Rockwell and eliminating the
> nitride we got the best of both worlds; not a soft core with a hard shell
> covering it, but a harder core with enough hardness to prevent premature
> wear on the journals. This took some researching to get the right core
> strength without making the forging brittle, however, the wearability on the
> journals suffered slightly. Now, after 100,000 miles the journals will
> probably need regrinding unlike in the past where most of the time it just
> needed a polish."
>> 
>> Sound accurate?
>> 
>> Thanks!
>> 
>> Chris
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