<VV> Crank Nitriding/Engine case question Question

[Roger Gault]

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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