Peter, your reply was directly to me but I'm replying to both T-Type lists
instead of just to you. Hope you don't mind, but the whole point of
Internet mailing lists is to share information with a group, and this is a
thread others seem to be interested in.
On Fri, 23 Jun 2000, Peter Pleitner wrote to Chip Old:
> I agree with Chip on all opinions except one, that being the negative
> on Nitride hardening of an XPAG crank. I had mine Nitride hardened
> and balanced, and have put over 7K miles of hard use on it. And my
> oil pressure is still at the upper limit of comfort, e.g. 90 psi when
> cold, and no less than 65 psi otherwise.
There were two different factory crankshafts, the primary difference being
that the later ones (from late TD production on) were forged from a more
durable grade of steel. However, according to the Nitriding experts of 25
years ago (last time I looked into it) neither grade of steel is suitable
for Nitriding. I don't remember all the details (sorry, another brain
leak), but as I recall Nitriding makes original XPAG/XPEG cranks very
brittle. In practical terms, this means a Nitrided original crank will
resist wear far better than an untreated crank, but will be far more
likely to break.
> I've heard of surface crazing twice due to Tufftriding, but this on
> 1950s vintage Alfa cranks. This was traced the first time to a cold
> winter draft on a hot crank, and the second time due to Nitriding a
> previously Tufftrided crank.
I know that can happen, but I don't know what conditions cause it. A
Tufftrided crank should always be micropolished anyway, because
Tufftriding leave a slightly rough surface. That removes the crazing if
not too deep, but if not you have to regrind and re-Tufftride.
> On a related expensive and labor intensive subject, I've concluded
> that a possible cause for rebuilt crank failure which I've recently
> become aware of is incompetent dressing of the grind stone at the
> crank grinder. In the past several years I've learned of a rash
> inexplicable crank failures traced to an extreme reduction in the
> radius between the bearing surface and the throws of the crank. A
> narrow, or worst of all, sharp edge at either end of the bearing
> surface creates stress risers and should retire the crank to door stop
> duty. I wonder if this rebuilt condition could not explain some of
> our reported XPAG crank failures. How many on this list have seen an
> XPAG crank break at either end of a bearing surface?
Yes, reduction of the radius at the edges of the journals by incompetant
machinists does increase the risk of breakage, and is a common problem.
Choose your machinist with care, and make sure he understands that the
radius must be maintained on all journals. In fact, if your machinist is
willing, have him measure the *installed* main and rod bearings, including
width from radius to radius as well as inside diameter, and grind the
journals to suit. It is sometimes possible to grind the journals with a
greater than standard radius, which increases crank strength.
However, incorrect grinding isn't the only cause of breakage. Even brand
new unground XPAG/XPEG cranks were known to break. It just isn't a very
strong design to begin with.
--
Chip Old 1948 M.G. TC TC6710 XPAG7430 NEMGTR #2271
Cub Hill, Maryland 1962 Triumph TR4 CT3154LO CT3479E
fold@bcpl.net
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