GROUP 10:- Engine mountings
ENGINE MOUNTINGS: This is the part of the bike which sets the Commando apart from other machines,
for although rubber engine mounts are used in all cars and bikes made by BMW, Sunbeam, Suzuki, MZ,
Villiers and many others, nobody else had gone to such lengths to ensure that the swinging arm is
mounted on the engine plates, thus avoiding the chain pull problem. The first two firms mentioned
cheated rather by using a transmission which was not affected by rubber mounting the engine, and most
of the others have not got the enormous out of balance forces from a long stroke large parallel twin.
Isolastic mounting was developed as a way of producing a lighter, smoother motorcycle by spending as
little as possible on research and development. Once the development had reached a stage where the
system worked it was put into production, and from 1968 to the electric start models we have all been
stuck with an under-engineered system, along with all the other bits and pieces like valve clearances,
points clearances, ignition timing, primary chain tension, cam chain tension, rear chain tension. clutch
clearance, front brake adjustment and rear brake adjustment The problem is further complicated by the
strangeness of the system-the average dealer still does not know how it works-and the feeble way in
which it was constructed. A .010in clearance has to be maintained in an assembly of unplated steel
washers and two bits of soft plastic situated directly behind the front wheel, and covered with a loose-
fitting plastic tube so that the rain and grit which gets in cannot get out again.
Let's have a look at what there is.
The head steady up to the 850 was a single piece pressing with an S bend in the middle. This bend was
strengthened by a tip each side which was stretched in the pressing operation and had cracks in it from
new. Cracks and alternating stress mean fatigue, and so the head steady breaks right across the middle.
The head steady supports the engine when the back wheel is twisted either way, when cornering, for
instance. Please, all of you, fit the later box section 850 head steady; it does not break, and it resists the
twisting much better. Part no: 064179 or 065459.
Never mind what the factory fits, the three socket screws and the six }in A.F. nuts should each be fitted
with one plain flat washer only. "Shakeproof" washers are an abomination because they allow water down
the threads so that the screws seize into the head.
While the head steady is off, check the tightness of the rubbers in the frame using, if necessary a pair of
slip-joint pliers on the steel part of the rubber mounting nearest the head.
The best for the front mounting is to take it right off and fit the Mark III unit which has threaded adjustment
rather than shims, but as this would be expensive you can get reasonable life by using bronze
impregnated PTFE mounting washers instead of the plain yellow polyurethane ones. The early bikes (pre
1973) had buffer rubbers which were not located on the centre sleeve, and which always tended to work
over to one side, affecting the way in which the engine moves in the frame. If this happens and the rubber
becomes very loose on the centre tube they must be replaced, but if they are still reasonably tight you
can keep them apart by using a bit of garden hose or by binding the centre tube with insulating tape to
form shoulders to locate them. More recently the buffers were located by circlips and do not therefore
wander. You have to take the end rubbers out to see what goes on inside; to do this hold the mounting
assembly in one hand, and insert the centre bolt about 1 in. Use the bolt to prize the end rubber out-go
on, you won't hurt it, but watch it does not finish up on next door but one's pigeon loft, it can be persuaded
back in without the recommended special tool by lubricating the outside of the rubber with brake fluid,
Castrol R or rubber grease, and prodding it with a suitable blunt instrument until the rubber is about 1/16in
(1.5mm) below the edge of the outer case. Smother the shims and so on with rubber grease or silicone
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