Article from "Your Classic" magazine, August 1992
"Triumph Crankshaft"
'Six-cylinder Triumph engines are notorious for failure of their poorly
designed crankshaft thrust bearings, but Chris Horton shows in full detail
how the problem can b overcome once and for all'
IN THE VAST MAJORITY of engines the longitudinal movement of the crankshaft
in the cylinder block is determined and then limited by a number of
so-called thrust bearings (usually four crescent-shaped washers) set at
right-angles to the main crankshaft bearing journals. These thrust
bearings have a small surface area, but they take only an intermittent load
and even when the rest of any given engine is totally and utterly knackered
it's rare to find similarly worn-out thrust bearings.
Unless you own a six-pot Triumph, that is. In its wisdom the company
decided to retain some means of controlling the backward movement of the
crankshaft as well as the forward movement, but at the same time it halved
the working area of each thrust bearing surface by dispensing with those
normally found on each side of the main-bearing cap.
This meant it was now the single thrust bearing in the cylinder block at
the rear of the crankshaft which did all the work, and because of its
necessarily limited working area it tend to wear very quickly. And because
the thrust bearing sits in a very shallow recess in the block, it doesn't
take too much wear to reduce the thickness to the point where, assisted by
the rotation of the crankshaft, it simply slides out past the bearing cap
(which is its only retaining device) and drops into the sump.
In the longer term this means that the crankshaft end-thrust is now taken
by the sharp edges of the bearing housing in the main bearing cap (leading
to rapid wear of both the crankshaft and the housing), but of more concern
is what happens to the forward thrust bearing.
Freed from its equally shallow recess in the front of the bearing housing
by the extra forward movement of the crankshaft, it soon follows its
partner into the depths of the sump, and suddenly you have a crankshaft
which can now move back wards and forwards by up to a quarter of an inch.
It's easy to tell if your engine is affected. Quite apart from obvious
grinding noises when the clutch is depressed, and the possible difficulty
of engaging gears because the clutch won't release that is, the
longitudinal movement of the crankshaft is often large enough to be
measured with a ruler. The greater accuracy of a dial-test indicator can
be useful in determining your exact course of action if the problem isn't
yet too severe.
Assuming you're using the dial-test indicator ( they can be hired if you
don't have one), attach it to some convenient point at the front of the
engine, near the crankshaft pulley, so that its measuring tip is actually
touching the front face of the pulley.
Lever the crank as far back as it will go, reset the gauge, and move the
crank as far forward as you can, at the same time observing the result on
the gauge.
If the thrust bearings are in good condition you can expect to see no more
than about 0.006 to 0.008in of movement, but it's not unusual to see up to
0.015 to 0.020in on an averagely poor six-cylinder engine.
If there's more than 0.025in of movement it'll probably jump straight to as
much as 0.25in suggesting immediately that at least one and possibly both
of the thrust bearings are sitting in the bottom of the sump.
Once the rear thrust bearing has dropped out, the crankshaft will move
forward to rub against the rear face of the thrust bearing housing in the
main bearing cap each time the clutch pedal is depressed, and
intermittently during normal operation.
But because the detachable bearing cap is wider than the matching housing
in the block (that's what holds the thrust bearing in place), it's the
latter which takes the stain first. Only once the edge of the cap has been
worn away does the crank shaft turn its attentions to the edge of the
bearing housing in the block.
There are various ways of repairing the damage. If the problem is caught
early enough (before the end-float has exceeded about 0.010in) and the
crankshaft isn't damaged, you can simply remove the sump, unbolt the
bearing cap and, with suitable modifications, fit both it and the block
with new standard-type thrust bearings (available in several over-size
thicknesses) to restore the end float to between 0.006 and 0.008in.
Your options also depend, to a certain extent, on the exact engine in
question. On the so called 'CP' engine from the TR5 and earlier TR6, f or
example, you can either machine the thrust face of the crankshaft back to a
good, flat surface, perpendicular to the bearing, and then have thicker
thrust bearings made up in phosphor bronze-or you can use one of the new
standard-size 'CR' crankshafts which are currently readily available for a
very reasonable 90 pounds or so.
The later 'CR' engines can be tackled in exactly the same way although in
this case, of course you can fit a brand-new crankshaft without going to
the trouble of finding a matching flywheel. In all cases any damage to the
engine block and the crankshaft bearing caps is usually repairable, and you
are unlikely to have to try finding replacements (as a matched set of
course) unless they really have been trashed.
So how is it done? With the block upside down on the bench, a new or good
second hand crankshaft is Iaid into position in a new set of shell bearings
and a pair of standard thrust bearings slid into their housing. Then,
without adding the bearing cap, the end-float is measured and recorded.
Let's say for the sake of argument that it was 0.015in. What you must now
do is subtract the required end-float, in this case 0.007in, from the
actual figure (0.015in) to determine the extra thickness of the thrust
washer needed to return the end-float to its specified limits. Thus here we
need an extra 0.008in, and we can achieve that by selectively installing
one or more over sized thrust bearings. Now comes the really clever bit
which should prevent the problem from recurring. Once you have selected the
thrust washers to sit in the block it's a good idea to have them and the
block itself drilled front and back to accept tiny locating pins. Made of
brass, these sit just below the surface of the bearing material and are
themselves held in position by the crankshaft's thrust bearing face.
Next you have the rear face of the bearing cap machined to take a
standard-type thrust bearing of the appropriate thickness (again easily
worked out once you know the depth of the housing) which, by sharing the
load with the one already in the block, will considerably extend its life
expectancy. The idea is not to go too far into the cap, simply clean up
any damage on the edges and allow the fitting of a thrust bearing of
suitable thickness from the range of 'standard' over-sizes.
If the engine has been running without the standard thrust bearings for any
length of time, you'll probably find that the crankshaft has gouged its way
into the edge of the block and bearing cap, and in this case rather more
serious engine modifications are required.
What you have to do is strip the block bare apart from the main bearing
caps, and then have this assembly line-bored to eradicate the damaged area
and restore a circular groove deep enough to allow the fitting, top and
bottom, of specially made (and much thicker) phosphor bronze thrust
bearings. Again these are pinned into place and, being much harder-wearing
than the standard type, should last more or less indefinitely. Simple when
you know how, isn't it?
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