<VV> Hardened, (or Nitrided) Crankshafts

Mon Feb 12 22:28:18 EST 2007

Nitriding cranks is much mis-understood.  Here's what I know.

As stated, the 140 and Turbo cranks were nitrided at the factory and marked
with a "&".

The puropse of the nitriding is to increase fatigue life, not to harden the
surface.  Yes, it does harden the surface, but except for startup that
doesn't matter.  Unless you have giant bolders running around in your oil,
nothing hard contacts the crank surface once the engine is running.  A nice
oil film keeps the rod bearings away.

What the nitriding does is put the surface in compression.  Cranks crack
because some bending load puts the surface in tension - usually around some
stress concentration like the fillet between the bearing surface and the
crank throw.  If there is a scratch or other imperfection in the surface,
the tensile stress will be concentrated at the tip and the metal will tear a
little at that point.  Each time the stress cycles, the crank grows a little
until the crank fails.  The important point here is that the crack has to be
in tension.  Something has to pull it apart.  Nitriding makes the surface
"larger" like you were trying to cram nitrogen atoms in where there isn't
any room.  Since this "larger" skin has no place to go, it puts the surface
in compression.  Any tiny cracks are forced shut by this compression.  A
bending load on the crank has to be large enough to stretch the inner
material enough to first overcome (relax) the compression and then put the
surface in tension.

This only becomes important if you spend a lot of time pulling a large
amount of power out of the engine.  While Corvair cranks do fail, it's not
exactly an epidemic problem.  Unless you're going to race it, and live up
around the power peak, I wouldn't worry about it.  There are LOTS of 110s
running around with 140 heads.

If you're bent on spending money on the crank, have it X-rayed.  That's
cheaper than nitriding it and more likely to save you grief.

Roger

----- Original Message ----- 
From: "R. Stalder" <stalder at cox.net>
To: <MoVair at aol.com>; <virtualvairs at corvair.org>
Sent: Monday, February 12, 2007 3:59 PM
Subject: Re: <VV> Hardened, (or Nitrided) Crankshafts

> I'm new to corvairs so this is my first response to a question on VV.  But
I
> recently learned some stuff about cranks while building my late model
turbo
> engine.  So this is what I know, hope it helps.  If your crank was
> originally nitrided from the factory, it will have "&" type symbol stamped
> on the flywheel end.  If your crank has been ground it will need to be
> nitrided again.  Nitriding hardens the surface of the crank.  Grinding
> removes the hardened surface.
>
> However, I was told not to worry about nitriding my crank unless I was
> racing or planned to due a lot of hard driving.  A non-nitrided late model
> crank would be fine for normal daily driving.
>
> That's all I know,
> Russ Stalder
> Mesa, AZ
>
> ----- Original Message ----- 
> From: <MoVair at aol.com>
> To: <virtualvairs at corvair.org>
> Sent: Monday, February 12, 2007 1:44 PM
> Subject: <VV> Hardened, (or Nitrided) Crankshafts
>
>
> > Hello folks,
> >  Recently, I was having a phone conversation with "Big John" Kirkman
> > about
> > an engine I plan to build. It started as a 65, 110 engine with PG.  The
> > engine
> > block has been assembled, using the stock 8409 crank. The cam has  been
> > upgraded to Clark's OT-20. Plans are to run .040 pistons.
> >  Now for my predicament. I have a set of 65, 140 heads in the machine
> > shop
> > that will probably be ready this week. Although I still have a few  more
> > ducks
> > to get in a row, I thought this would be another upgrade I could do.
> > Problem
> > is, the crankshaft is not hardened. Remember, the engine is a 110.
> >  Since the crank uses the same part number, regardless if it is
hardened
> > or
> > not, is there a visual inspection I can do to determine a Nitrided  from
a
> > non-Nitrided crank? If not, is it possible to get a crankshaft
hardened?
> >  At this time, the only other late heads I have are for a 95 HP  and I
> > don't
> > want to use them. I'd get a set of 110 heads first.
> >  In case you're wondering, no, I don't have car to put it in  yet. That
is
> > a
> > bit further down the road. Right now, I just want to do the  engine.
> >  Any info that anyone can provide will be of great help.  THANKS!!!
> >
> >
> >    Kelly Parker
> >    Oxford, GA. 30054
> >    _MoVair at aol.com_ (mailto:MoVair at aol.com)
> >    62 700 4-dr
> >    62 Greenbrier Deluxe
> >    64 Monza Convertible
> >    65 Monza Coupe 140/PG
> > _______________________________________________
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