The Bullet: Mysteries & Myths -
Don't just love but get to know your bullet more

- by B. R. Gurunandan


Cam-Gear Backlash

Fig 1 - Cam Drive SchematicThe 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 !
Fig 2 - Zero Backlash

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?

Fig 3 - Not Touching 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. Fig 4 - Not TouchingIn 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 tappet.
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.

Pic 1 - Original SpindlesWhat are 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 spindles.
Pic 2 - Adjustable Spindles
How to install the new adjustable spindles?
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?
Pic 3 - Adjustable SpindlesYou 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

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