Bullet: Mysteries & Myths -
Don't just love but get to know your bullet
- by B. R. Gurunandan
crankshaft has a 20-tooth pinion gear which drives a 40-tooth gear having the
exhaust-cam and this gear drives another 40-tooth gear having the inlet
cam, (Fig 1). Thus the cams are driven at half the engine-speed. If
these gears mesh without "play" i.e. backlash,(Fig2), all is well. But
if there is any backlash between the crank-gear and the
exhaust-cam-gear, or between the exhaust-cam-gear and inlet-cam-gear,
then there are problems !
Somewhat like a slack chain, in that the situation keeps deteriorating,
but worse, it does that ever so noisily ! It would be a very, very,
heavy-duty, high-precision machine that consistently drills and bores
an aluminium casting with a zero-tolerance in centre-to-centre
distance. From the observation that there are hardly any Bullets
without cam-gear backlash, we infer that the machine employed for the
job is not exactly as required.
How does this cam-gear backlash cause problems?
Imagine first, the spur-gear of the exhaust-cam being driven by the
crankshaft-gear. This situation persists until the tip of the cam
reaches the tappet, which is under pressure from the springs of the
fully open valve. As soon as the cam-tip passes the tappet, the entire
spring pressure which was resisting the rotation of gear, suddenly
starts assisting the rotation. In case of backlash, there is
instantaneous acceleration of the exhaust-cam-gear to a driving instead
of the former driven position relative to the crankshaft-gear. The
gears take up the new positions with an impact. Apart from the noise,
this also wears the same two teeth each time the cam-tip passes the
The same story may be told with the exhaust and inlet gears in the roles
of crankshaft and exhaust gears, in case there is backlash between them
also, which is equally common.
What can be done about it?
The root of the problem is wrong centre-to-centre distance between the
gears. If the distance is exactly the sum of the pitch-circle radii of
the meshing gears, there will be no backlash. So the problem boils down
to correcting the centre-to-centre distance.
The original cam-spindles have to be replaced with adjustable ones. ( An
easy operation, provided you do NOT follow the Enfield Workshop Manual
method of removing the cam-spindles ) And the spindles must be adjusted
for correct centre-to-centre distance systematically. Carelessness here
is asking for trouble. Serious trouble.
these adjustable spindles?
The diagrams show the two types of spindles. The originals ( Pic1 ) are
simple rods, with a collar & lube-groove. The adjustables are
similar at the end fitting in the crankcase but with a smaller dia on
the exposed part ( Pic2, Pic3 ). They end with threads, a matching nut
and spring washer are included. The sleeve that is to be tightened on
the smaller dia is the key to the functioning: it is an eccentric. It's
wall thickness varies along its circumference.
How to remove the original spindles?
For starters, NOT as they tell in the manual ! It can be done with the
engine very much in the frame. In about 15 minutes if you have the
required "puller". If not, it becomes a bit complex ! It is time to go
to a specialist or borrow a welding-set. With a welding-set, you
fabricate a "inertial-hammer" in-situ, and slam OUT the original
How to install the new adjustable
There is a precaution! The part of the spindles embedded in the case is
NOT always of a standard diameter ! You have to measure the ones you
removed and buy identical, ( or buy several with the agreement that all
but two will be returned. Remember what I said about being nice to the
parts-dealer ? )
You also need a tube for using as a drift to hammer in the new spindles
without damaging the threads. Note that the flats on the collar must be
aligned to the flat in the case.
How are the spindles to be adjusted?
You have already changed the
spindles. Now remove the push-rods. Yes, that means the tank,
rocker-covers, rockers too. With only the crankshaft-gear and
exhaust-cam in place, turn the sleeve. It will not turn thru 360
degrees. At the two positions it stops, the backlash between these
gears is zero. Slightly tighten the nut on one of these positions and
add the inlet-cam. It's sleeve should behave similarly. In case the
gear refuses to mesh with exhaust-cam-gear, use the other zero-backlash
position. Now add the idler. It should mesh easily at atleast one of
the zero-backlash positions of both cam-gears, but the backlash between
inlet-cam-gear and idler is not important. ( Why ? )
NOW comes the important part!!!
1. Realize that the spindles are in an aluminium case
which is going to expand in operation, and the distances between them
are going to increase, ever-so-SLIGHTLY, but increase they will. The
cam-gears being steel will expand slightly LESS. Their mesh will tend
to slacken at operating temperature.
This you can compensate, but how much ? Excess will cause severe strain.
The correct setting is where there is absolutely no backlash, but the
cam-gears will still slide in & out on the spindles. Test this, as
usual, in several different mesh-positions.
2. If you have used the cams with much backlash for long,
say 5000km, then it is a safe bet that the gear-tooth-profile of the
few critical teeth has changed, and zero-backlash elsewhere will still
NOT make the backlash zero at the critical point where the cam-tip
passes the tappet!
Do not think of zeroing the backlash with cam-tip at tappet position !
The severe interference at other positions will spall the gears within
a hundred km! The only solution, if you have struck this problem, is to
replace the cams. Yes, technically speaking, the gears can be re-ground
to a slightly smaller pitch-circle, but I haven't found anyone
undertaking this on a batch of two pieces.
3. After you tighten the nut, ( you didn't forget the
spring-washer, did you! it is critical here ! ) check that the adjustment
has not "drifted". One cause of the "drift" is that the sleeve faces
are not square. Put a spanner to the sleeve and check that it can not
rotate. That would be fatal ! Check also the tightness of the nut. And
to be on the safe side, use a dab of locktite on these threads.
4. At the next oil-change, open the timing-case and
inspect these points once again. Look for drift, wear, loosening. If
all is OK, it will remain so indefinitely. Or else you are assured of
sleepless nights and oily fingers for some time to come! But don't
worry, this rarely happens.
By B. R. Gurunandan
Click here to email
your queries to Nandan.