Bullet: Mysteries & Myths
Don't just love but get to know your bullet
- by B. R. Gurunandan
beyond Cam-gears -
"Silence is Golden"
...if that phrase frustrates rather than soothes, read
this one carefully !
you zeroed the cam-gear backlash, but the euphoria was short-lived.
Again the apple of your eyes is sounding like skeletons on a tin roof.
What COULD be the problem ? Arrrgh ! There must be a mistake UP THERE,
or sins of your past lives coming haunting now, Oooooohh, REALLY you
can't recall ANY in this life deserving THIS ! Steady, guys, steady.
There is no Black Magic at work here.
Just the laws of physics and the properties of materials and
practicalities of manufacturing processes. Directed by Murphy.
Don't be spooked by the horror stories told by your neighbourhood
mechanic or the vintage-collector websites adding half-baked "content"
in their ultimate, ulterior motive of metamorphosizing to e-commerce
The remedies are fairly simple and I had not planned to discuss them.
But then I got feedback from distressed and confused Bullet-eers,
revealing the extent of damage done by the n.m's tales and the v-c. ws.
I repeat : Dikhaave pe MAT jaao; Apni akal lagaao ! :-)
The engine-noise in a Bullet originates from one or more of the
following sources. They are classified into three categeories.
| Valve-train sources
| Power-train sources
Cyl-clamp on Crank-case
Miscellaneous sources of noise
If on the
other hand the bore is perfectly parallel, ie, ring end-gap is same at
all positions, but still the piston rocks, then you have a once-seized
piston. Just changing the piston and rings will be fine. But THIS time
run-in the engine PROPERLY ! Put your trust in Science, Logic and
Nandan, NOT in Dogma, Mumbo-jumbo and Works-Manuals !
First make sure the noise is not from this categeory ! This is as easy
to remedy as to check. Few minutes job each. Only then go on to other
One side of the head-steady is fastened to the frame at the tank
support, and the other side to a lug on the cyl-head. Make sure both
the ends are well tightened. Or you will have buzz-iness in the
handlebars, and pinking sound on acceleration.
A BANG on deceleration means an exhaust-leak, and the cause of that may
be the improper fitting of the exhaust bend. This sometimes results in
an alarming rat-a-tat-tat from the engine-bend junction on
acceleration. Dismantle the silencer-bend and re-fit properly.
Cyl-clamp on Crank-case:
This, if loose, also lets the engine vibrate and sound like missing
head-steady. Tighten, don't torque ! :-)
Tappet / PushRod adjustment:
This is a VERRRRRY common cause, all because people all too often
"adjust tappets" EXACTLY as per "manual" without understanding what
they are really doing. We will discuss this at length in "Adjustments"
Muffler elements loose:
A broken weld or corroded-out internals make the muffler "ring" when
the engine is running. Test it by striking the muffler with a piece of
rubber-hose or with your finger. If there is anything loose inside, it
will emit a metallic ring. Replace if you are sensitive to the noise,
but the muffler probably has many years life still in it.
Valve Train Sources of noise
Valve-train noises can be easily differentiated from the power-train
noises. PT noises are load-sensitive, whereas the VT ones are not.
Theoritically, there are ways to diagnose the noises by simple tests,
but conditions and combinations make it quite difficult even for
experts to be right everytime. It is best done with a partial dismantle.
We have covered this already in "Cam
Gear Backlash" chapter. At the most, the method for removal of
original spindles may be a bit doubtful, we may cover it sometime. But
some ingenuity would have taken care of that already. (Ya, I do reply
The original guides are cast-iron,
hard and unforgiving. And are mass produced, meaning doubtful
tolerances. Some may be tight, wearing unevenly during run-in, some
perfect, some too loose, wearing very fast from the rocking-impact and
abrasion. Impact ? Yes. Note how the rockers operate on them. In an
arc, not just up and down. The springs hold the valve centered with
respect to the valve-seats, and the rockers move the stems laterally
untill they are stopped by the guides, when they start a purely axial
movement. The stopping is an impact which probably squishes the oil
aside, makes oval the guides, which increases the rock & impact
& abrasion, and aggravates the problem.
Check this out for yourself.
Remove the cylinder-head, dismantle the valves using the
valve-spring-compressor, wash out the guides and stems with kerosene /
petrol. Don't look for a micrometer and bore-gauge !!! Don't be taken
in by the expansion and tolerance theories. Just rock the valvestems in
the guides. Absolutely no lateral movement is allowable.
If you can feel any movement, then it is time to change the guides. The
cast-iron ones cost about a third of the bronze ones, and the labour is
much less too, but the theory is totally different. Make sure the
machinist knows what he is doing, or else you are in for trouble. Ulp !
Now I am in for trouble ! It needs a chapter of it's own to explain the
theory of bronze / cast-iron guides, and this one is all about sources
of noise. OK, we will take up that one subsequently. Till then take
care. BTW, I use Bronze guides.
The tappet-guides also are stressed sideways like the valve-guides.
After the cam-nose passes the tappet, there is an abrupt
force-direction reversal which tends to cause impact between the tappet
and guide, besides the gear-teeth of the cam-gears which we have
earlier seen. The temperature conditions are milder than for valves the
wear is less. Wash out with petrol & test for rock. The remedy and
theory is same as for the valve-guides, but it entails a total
dismantle of the engine to change the tappet-guides.
These are very easy to replace, and are available in two types. Cast
aluminium blocks and sintered iron blocks. It is not possible to
generalise or advise which type is better as there are several
manufacturers and the batch variation seems to be considerable too. The
best solution is to compare the available ones for fit of rocker-arms
in the blocks and shape/finish of the rocker-arms themselves. Again, no
play or movement is allowable.
It is highly un-common to bush the rockers, but it is possible and a
good engineering exercise. We will go into it one day, so don't throw
away the rockers you replace.
Power Train Sources of noise
Apart from being load sensitive, these are the result of long-term wear
or serious abuse. So your conscience is a good place to start your
investigation of these !
This is a sharp rat-tat-tat-tat
sound on mild acceleration. Comes from long-term wear, or a seizure
resulting from over-speeding with a improperly run-in engine.
( Click to read again all about "Running-in"
! Hahaha ! )
First see if the
piston looks anything like in the picture !
Wash out the bore and piston with petrol / kerosene. Insert & try
to move the piston side-to-side at various positions in the bore.
If it moves perceptibly, it is HISTORY ! But, not so fast ! Check out
bore-wear with a piston-ring. Fit the ring in the bore and push it with
the piston till it is perfectly parallel to the end of the bore. This
is important. If the ring is not perfectly parallel to top/bottom of
bore, you will get WRONG results ! Compare the "end-gap" when the ring
is located (a) at the top, (b) at the bottom, and (c) at the middle of
the bore. if (c) is much more than (a) or (b), then the bore is worn
out. You will also see "ridge" in the bore. A clear case of wear-out.
You need a re-bore and change to next oversize of piston.
Try to rock the gudgeon-pin vertically in the small-end of the
connecting-rod. Actually, this does NOT cause as much noise as we would
expect. If there is no problem with the rest of the engine (Bearings
and tappet-guides) then it is acceptable to leave this problem pending
until the future full-dismantle, at which time the con-rod small-end
can be "bushed". But of course, don't tempt fate in the mean while by
fitting a high-compression piston or by riding as though you have.
||Simple check again ! The con-rod should not rock relative to
the flywheels. Test this in half a dozen different positions. This one
is serious, though, and the slightest rock means a rebuild. Not a
matter of noise, rather about accelerating the wear of a whole lot of
If you can feel movement in the crank-shaft, HEY ! it is HIGH time the
engine was rebuilt ! Do not ride hard in this condition, and certainly
not for long. Bearing-faults are potential crankcase-distorting
problems. That has real expensive repercussions and not always a happy
Enough noise about noises :-)
By B. R. Gurunandan
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