900ss Cylinder head removal

Beleive it or not apart from changing oil and obvious stuff I have not so far had to carry out any major intrusive maintanance on any of my bevel headed bikes, so I’ll be asking for help quite a bit!

Question 1…

Apart from ordering some manuals, any tips that I should know about before removing the old girls heads?


It can be worthwhile lining up the dots on the cam bevel gears and the timing side gears before removing the heads although not essential. It just makes reassembly easier if you don’t rotate the crank. I’ll see if I can find some text on this to explain.

by Peter Shearman

Date of article: August 1980 Category / Type: Technical Bevel
docv.org/cgi-bin/news/newssc … ate=simple

The timing marks on a bevel drive engine are only all lined up once every six engine revolutions (three cycles). This is because of the ratio variations between the gears - for each turn of the crankshaft, the bevel drive shafts turn 2/3 of a turn, and the camshaft turns 1/2 of a turn.

HEAD BEVELS (All models)
Each turn of the crankshaft turns the lower/upper bevel gears through 3/4 of a turn, in which turns the cams at 1/2 a turn. The head timing marks are only lined up once every 4 turns of the drive shaft (three turns of the camshaft).

It can be seen from this that the drive shaft will line up with its mating tongue with the camshaft in four different positions. Only one of these is correct and that is when the timing marks are lined up on the head bevel gears. This puts the drive shaft and the camshaft in the correct position to fit onto a correctly timed lower end.

With the head drive shafts in the right position, it is possible to have the engine in two different positions where the tongues will be in line.

The only way to decide which is correct, without going to the trouble of removing the timing case side cover, is to set the rear piston at TDC with the front piston moving down. This should line up both drive tongues with their flats facing away from the engine at 180 degrees (see diagram.). The correctly timed heads should now slip straight on.

Crankshaft Revolutions 0* 1 2 3* 4 5 6*
Lower Bevel Drive Revs 0* 2/3 1 1/3 2* 2 2/3 3 1/3 4*
Camshaft Revolutions 0* 1/2 1 1 1/2 2 2 1/2 3*

  • = Timing dots