Excellent Article. Now.... We need to get a list of shops that know how to
do this type of work (apart from ones in the U.K. , as most of us don't
want to ship an engine block across the ocean)
Hugh Barber
Santa Clara, CA
'73 TR-6
At 07:03 PM 6/12/97 +0100, Pete & Aprille Chadwell wrote:
>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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