Bullet: Mysteries & Myths -
Don't just love but get to know your bullet
- by B. R. Gurunandan
...also known as the Tappet
Mystery alright, because there is nothing adjustable about the tappet
at all. It is the pushrod that we actually adjust the length of.
The cams determine the opening & closing of the valves, by the
profile of their periphery. The angular movement of the camwheels is
converted to linear, reciprocating movement by the tappets which
"follow" the cam profile. The linear movement is transmitted to the
overhead valves thru the rockers, also on the head, by the pushrods.
The pushrods are a very simple part of the valve-train. But their
adjustment is quite critical !
The pushrod consists of an aluminium tube with a fixed cup on the top
end which fits onto one tip of the rocker-arm, and an "adjustable" cup
on the bottom end that fits on the top of the tappet.
The "adjustment" consists of screwing the lower-cup in or out of the
bottom of the pushrod, in effect decreasing or increasing the length of
Let us now see why it is so critical...
What happens if the "pushrod is adjusted tight" ?
It holds the valve off the valve-seat, and this is felt as low or no
compression in the engine. This results in an engine difficult to start
when cold, though it CAN be started with generous use of the choke. The
effect of the "tight" pushrod in a running engine is that the valve is
always "attacked" by the hot gases, without any opportunity to transfer
the heat thru the valve-seat because of it's poor or no contact with
it. Soon, this results in a burnt valve. Owwwww !
So, what if the "pushrod is adjusted loose" ?
Theoretically, the valve lift is reduced by the amount of slack. No big
deal, till you realise that the decrease in duration and percentage
lift is considerable ! Still no big deal if you aren't into competitive
sports. But the noise of the tappet slamming the pushrod into the
rocker every two rotations results in a racket you probably can't stand
! And slowly, over time, this results in distortion and runout of the
pushrod that is clearly visible and also interferes with proper
adjustment later on.
"We recommend 'O' clearance for the tappets to be set at cold....The
Push Rod should rotate freely without showing any up and down play" so
says THE BOOK.
Actually, the ivory tower boys for once went out of their way to think
of your comfort so that you do not need to risk burning your fingers on
a hot engine. Too bad, they overlooked the fact that the engine is
never cold when it runs !
Thermal expansion. That is the fly in the ointment. If you adjust the
tappets as per the book, you are going to have that skeletons on a
tin-roof sound as soon as the engine gets warm. Because the expansion
of the pushrod is less than the expansion of the crankcase, cylinder
and head. See
the diagram for a more accurate explain-ation and better
Adjust the pushrods when the engine is FULLY HOT ! Ya, this is fairly
harsh on your fingers, but think how gentler it is on your ears,
nerves, and conscience !
Oh, it's not THAT difficult if you approach it sensibly. First get some
practise adjusting it cold ! Become used to the whole cycle...Setting
up the engine to TDC ( comp ), opening the tappet-door, loosening the
locknut, adjusting the hex-cup, tightening the locknut...without losing
the setting !
Once you can do this swiftly, touching the engine mainly with the
spanners and very little with your fingers, ( Aw heck, why didn't we
heed the teacher who said to eat with the knife & fork ! )
THEN you can try it HOT.
Move the engine to the position where the ammeter JUST returns to zero
( points just opening )
For the A350, I guess you have to open the tappet door and park the
engine at the position where both the tappets are at their lowest
You may like to get the tappet-door-stud removed and out of the way.
The Pushrod adjustment is best done with three spanners as
Small-side of Big-spanner - Hex of pushrod
Big-side of medium-spanner- Hex of adjuster
Big-side of small-spanner - Locknut
Hold the pushrod hex and unscrew the locknut.
SLIGHTLY if you intend to adjust tighter ( Screw OUT the adjuster )
MORE if you intend to loosen ( Screw IN the adjuster )
Hold both pushrod and locknut and turn the adjuster till the tightness
Now hold both, the pushrod and adjuster, and snug-tighten the locknut.
Then check the adjustment again. It tends to get a bit tighter ( Why
?!!! ) so screw in the adjuster SLIGHTLY, without loosening the locknut,
till the pushrod is perfectly adjusted.
Now, holding the other two hex from turning relatively, tighten the
locknut fully and finally. This time the adjustment will not change.
( Why ?!! )
Speed is important. If the engine cools appreciably during your
adjustment, it won't be satisfactory at running temperature.
So is the setting ! The pushrods must NOT "spin" like the book says !
It should be possible to turn them with thumb pressure, but only just.
Check out the correctness of your adjustment by turning the pushrods
at least one full rotation, observing if the resistance changes at any
place. If so, the rod or adjuster may be out of true. You should
ideally replace that. As an interim measure, set the correct tightness
at the tightest position, NOT the loosest. ( Why ?!! )
If you feel a loss in the compression after the adjustment ( Engine HOT )
then it's too tight.
If you feel a SLIGHT loss in compression with engine COLD, you may have
to live with it, as that stage will be passed within a minute of
starting the engine.
If the loss of compression is DRASTIC when the engine cools, the
setting is too tight !
At any rate, the engine should have PROPER compression at running
temperature. Remember that there are other sources of noise in the
engine ! Do NOT tighten the pushrods TOO MUCH ! You may burn the valves.
By B. R. Gurunandan
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