Adjusting BMW Airhead Tappets
|Due to the horizontal layout
of the BMW Boxer engine, tappet adjustment is very straight forward.
The only critical parameters being that the engine is cold and that the correct crank/cam position is used.
both spark plugs were removed to allow engine rotation without
compression. The inspection bung was also removed to view the flywheel
markings and the bike was placed on the centre stand with a drip tray
With the bike in top gear the rear wheel was turned to align the 'OT' (Oberer Totpunkt) marking on the flywheel with the viewing window in the engine crankcase.
In this position both cylinders would be at their top dead centres.
Setting cylinders to top dead centre
|With the engine in this position one cylinder would be on its compression stroke and the other at the end of its exhaust stroke. The cylinder for adjustment would be the one on its compression stroke since both its valves are shut at this point in the engine cycle.|
rocker covers were removed by unscrewing the two nuts hidden within the
cylinder fins (shown here); followed by the main centre nut on the
A drip tray was used to catch the inevitable oil drips.
Location of one of the rocker cover nuts. The other is directly opposite.
|To determine which cylinder was on its compression stroke, the rocker arms were "wriggled" by hand. Only one cylinder would have no pressure on both valves (and therefore rockers which are free to move just slightly) and this was the one chosen to adjust first.|
was made by sliding the correct thickness of feeler gauge between the
rocker and the valve top and opening the lock nut to allow the
adjustment bolt to be turned. The correct adjustment would be when the
feeler gauge was a sliding fit in the gap.
Feeler gauge specification is:
Inlet = 0.1mm
Exhaust = 0.15mm
Adjusting an inlet valve (click to Enlarge)
|While the rockers
were exposed, the opportunity was taken to check end float of the
This was checked by trying to move the rocker up and down and observing the amount of free travel. If correct then there should be very little movement just enough to see the oil film where the rockers meet the mounting blocks change slightly.
Checking rocker end float
When correct then there should only be just enough movement to detect a change in the oil film circled.
|If end float is too much then
it can be adjusted by moving one of the mounting blocks towards the
To do this a woodwork clamp with some sockets or spacers can be used to apply a slight squeezing force to the mounting blocks.
This force is applied with one of the cylinder head nuts loosened, so that the block can be moved enough to set the correct amount of end float.
Set spacing so that there is essentially no clearance or free play here; but the rocker mustn't be tight.
One method for correcting rocker end float
|Once one side of the engine had been adjusted the rear wheel was turned to rotate the flywheel one whole turn and then the other side was adjusted in the same way.|
|Once end float and clearances had been set the rocker covers were replaced (reusing the gasket). The centre nut was tightened first to a maximum torque of 14ftlb and then the two m6 nuts were added behind the fins.|