Crankshaft hardening specification

[Mike Wattam <101714.1343@domain.elided>]

Not many people know this.

The correct specification for the Stag is a 'soft' crankshaft, that is
unhardened in any way.  This is used with Vandervell type bearings, these
are hard bearing shells with a dark grey surface.  There is no real
weakness with this setup, indeed my own Stag has done c100k miles, most
used very hard, and not a murmur.  Mind you, I have changed the oil always
between 3k and 6k miles (or every year).

Most suppliers supply 'Glacier' bearings, probably because they are
cheaper.  These have an almost silvery white bearing surface appearance,
and are real soft.  Needless to say, these were not recommended by Triumph.

An engine specialist would now tell you the soft Glacier bearings should be
fitted with a hardened crank, and the hard Vandervell bearings should be
used with a soft crank.  So, if you have a hardening process done on your
crank, make sure you have what I suggest are the 'right' bearings.

Crankshaft Tuftride hardening is a highly skilled process.   It needs to be
done deep enough and at the right gas concentration to prevent surface
break-up, while if the penetration is too deep it can cause crank fatigue
failure.   Similarly, get the oven too hot and the crank may well distort
(heating it may cause it to distort anyway if there is residual stress from
manufacture present).  This can only be overcome by reverse bending the
crank in a heavy press - a horrific sight by the way - until it runs true
again.  It can be necessary to re-grind to get the crank running true,
destroying most of the hardness.

As for polishing the crank after heat treatment, I have seen both nitrided
and tuftrided cranks which have blown out the bearings because the
hardening bloom was not thoroughly cleaned off, or because the crank 'grew'
during hardening so it was tight in the bearings.

Therefore, I suggest that if you insist on a hardened crank, that you use
the much more durable and deeper penetrating nitriding process, and have
the hardness depth made sufficient to enable the crank to be re-ground to
size and true AFTER nitriding.

Mike Wattam
Trumph Stag Register