The „Commando Tripple“ alias „Jake“ was rebuilt around an
engine I found in the spares stores of Norton Motors (1978) Ltd in the
early 1980s, when visiting there as, at the time, Germany’s biggest
Norton spares distributor. It was just a couple of cardboard boxes with
dirty engine parts which I initially mistook to be a dismantled standard
production Commando engine. Little was known about it at Norton Motors
(1978) Ltd but that it had been a complete motorcycle originally, sold by
the factory at one time, and bought back for some reason. It was then
dismantled and the engine put into a corner.
Richard Negus, then the
Spares Manager, sold it to me on behalf of Norton Motors (1978) Ltd.
25 years later that very Richard Negus, as a farewell
present to me on retiring from our jointly owned company Norton Motors
Ltd, rebuilt the engine and mounted it rigidly in a Commando chassis I had
bought and restored for it.
On the right
is the drawing how Doug Hele, who had the idea to put a balancing
device on a parallel twin, designed the prinziple originally. I
found the drawings in “British Motorcycles” by Steve Wilson.
"Hele’s balancer linkage design for smoothing out
360° parallel twin vibes, with two extra conrods going out
horizontally to two smaller links which pivot about their “small
quarter of a century later Ducati re-„invented“ exactly that
principle, and put it into their „Supermono“ racer, and today
it can be found in the BMW F800 engines.
The NVT works
R&D department tested the principle, first using a 750 Triumph
Twin engine. Under Norman Hyde’s supervision, with Jack Shemans
doing the wrenching, they used Hele’s experimental solution,
providing an extra piston, rod and a cylinder, at 90° to the
very crankshaft from the Triumph engine was put into a Commando
engine. Until I found the technical drawings in our (Andover
Norton’s) files it was a complete mystery to me why they did it. The stroke was too short,
the Triumph having only 82mm against the Norton stroke of
89mm, so Shemans put high compression pistons in the engine to get
some compression. He also- I suspect because he did not fully
understand the Commando engine- blanked off the very efficient
breather behind the “ear” of the crankcase and let the engine
breathe via the left hand main bearing into the primary chaincase,
Triumph style. He also steel-sleeved the inlet ports down from
32mm to 30mm, probably because nobody told him 850 heads with 30mm
inlets existed off the shelf, and did his usual spiel on the valve
guides, slimming them down and cutting them back, racer style.
still on the low side, and, probably because the moving masses are
so heavy- steel conrod and piston in the balancer- the bike does
not really want to rev much over 5000 rpm.
I wondered for
many years: Why did they take the trouble to build that crank into a Commando
They changed fundamental criteria in that the stroke and cylinder
head were totally non-standard and, normally, when one tests one
particular modification one leaves all other components standard.
their test rides with the Triumph they must have known the system works-
so why take the trouble to put it into a Commando?
Now that I
have seen the engineering drawings I know why they did it, and
that we made a fundamental error when we rebuilt that motorcycle.
We mounted the engine rigidly in the frame, which works, because
vibration is nearly gone, assuming this was how it was tested.
also makes the bike probably the best-handling Commando I ever
rode, with hairline steering that I rate better even than that of
my featherbed Manx.
the drawings I saw they mounted that engine IN ISOLASTICS,
probably to get rid of the vibrations one still feels in low revs,
even though they are not significant. This would explain why they
built a Commando with that crankshaft, which otherwise was pretty
found the drawings three years after the bike was finished and
after I found it rather pleasant to ride. I am not sure yet if I
want to take the trouble to rebuild it to the factory experimental
special feature of the engine is the timing cover, which appears
to be a pre-production prototype of the Mk3 timing cover with the
oil non-return valve. This puts it into the 1973/74 era. The
balancer conrod drawing is dated Sept.73 (On Triumph/BSA paper),
all the Norton drawings are undated, which is unusual, but on
Norton/Wolverhampton drawing paper.