This is an easy job that should be part of every service (Haynes manual recommends checking and adjusting the valve clearances every 12-15000kms) however despite this there are few things to understand and a few different ways to do the job.
What are we actually doing when we “adjust the valves” or “set the valve clearances”
Basically we are setting the gap between the end of the rocker arm and the top of the valve stem when the piston is at TDC on the compression stroke. The rocker arm is activated by the pushrod and pushes the valve open. There is normally a small gap or “play” in this chain. For the exhaust valve the clearance between rocker arm and valve stem is 0.20mm and for the intake valve it’s 0.10mm.
The reason to do this is to keep engine noise (valve train clatter) to a minimum whilst still allowing a little room for the expansion of these metal parts at operating temperature.
This is achieved by using two 12mm spanners- one loosens the outer locking nut and the other moves the adjuster nut which is against the pushrod. After loosening the locknut a feeler gauge can be placed into the clearance between the rocker arm and the valve stem. Then the adjust nut is moved out until the rocker arm bites on the feeler. When the feeler gauge is snug but still moves one spanner keeps the adjuster still while the other tightens the locknut. This takes a little trial and error but you start to feel it pretty quickly. I err on the side of fractionally too tight rather than too loose.
Hold Up! What’s this TDC on the compression stroke business??
This might require a refresher on the basics of the 4 stroke boxer twin cylinder engine. In a four stroke each piston goes through 4 “strokes” per spark/combustion cycle:
- Intake stroke- piston move from top (dead centre- TDC) to bottom (out to in on flat boxer engine) with the intake valve open- sucking in airfuel mixture
- Compression stroke- piston move from bottom to top (in to out) compressing the mixture- squeeze. At the top of this stroke is TDC Compression!
- Combustion stroke- spark ignites mixture and piston moves from TDC to BDC (bottom dead centre) again- bang
- Exhaust stroke- piston moves from BDC to TDC again with exhaust valve open, blowing out the exhaust gas
This give the so called suck, squeeze, bang, blow aide-memoire. You’ll notice the piston goes through TDC and BDC twice as shown in this little animation.
Even though the animation is a an upright cylinder with an overhead cam it demonstrates the sequence nicely. You will also notice that on the compression stroke the valves are closed around TDC (end of compression stroke and beginning of combustion stroke). This is why this point is chosen to adjust the valve clearance because at this point the valves are closed and the valve train should be on slack (with the small clearance between the rocker arm and the valve stem). At other points of the cycle one or both of the valves are being opened or closed at so of course you would expect no clearance.
Of note, the boxer flat twin has the two pistons in phase- that is both pistons move out together and both move in together. This creates good engine balance and less vibration. However they are not both at the same point in the stroke cycle- when one cylinder is at TDC on the compression stroke the other is at TDC on the exhaust stroke.
So how do I find TDC on the compression stroke?!
There are a number of ways to do this but they all basically involve turning the crank over and looking through the window in the crankcase to see the flywheel markings. The marking OT with a line under it indicates TDC on the BMW twin. Some people mark this with white paint to make it easier to see.
- Remove the rocker (valve) covers- two 10mm nuts between the cooling fins and the larger central 13mm nut
- Remove the spark plugs
- Put the bike in the 5th gear
- Put the bike on the centre stand so the rear wheel is off the ground
- Remove the dust plug from the flywheel viewing window and shine a light in there!
After the above have been done you can use the rear wheel to bump the engine forward and watch for the markings in the window. An alternative way of turning the crank over is to remove the timing (front) cover of the engine and use a 6mm allen key in the alternator spindle.
But how do I know WHICH cylinder is at TDC when OT is in the window?
There are a couple of ways to do this. From our understanding of what the valves are doing at TDC on the compression stroke you should now be able to work this out from observing the rockers and valves. When OT is in the window use the wheel to rock the crank back and forth and observe the valves. One cylinder will have the exhaust valve closing and the intake valve opening; that cylinder is at TDC on the exhaust stroke. The other will have no valve movement. That is the one at TDC on the compression stroke and the one to adjust the clearances on. An alternative way to check this is to seal the spark plug hole with your thumb as you come up to OT- on the compression stroke the piston will be pushing up towards closed valves and generating compression which you can feel with your thumb and release as a hissing sound.
Here’s a video going over the process in a little more detail! I hope this helps you if you are trying to adjust your valve clearances for the first time.
