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

 - by B. R. Gurunandan


Pushrod Adjustment
...also known as the Tappet Adjustment.

Mystery alright, because there is nothing adjustable about the tappet at all. It is the pushrod that we actually adjust the length of.

The Background...
The cams determine the opening and 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
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 the pushrod.

The Adjustment
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.

The Fatwa
"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 !

The Fallacy
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 understanding.

The Solution
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.

The Newbie-Hints
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 position.

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 follows:
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 is correct.
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 ?!! )

The Precautions
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

Click here to email your queries to Nandan.