desmoto Article

BEVEL DRIVE CAMSHAFT TIMING GEARS AND ALIGNMENT MARKS

August 1980

By Peter Shearman


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. The point's drive cam also turns at half engine speed on the round case models.

HEAD BEVELS (All Models)

Each turn of the drive shaft turns the camshaft through 3/4 of 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.

LOWER END SQUARE CASE 750/860/900)

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.

860 MODELS TIMING CHART.

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

LOWER END ROUND CASE 750's)

With the head drive shafts in the right position, it is possible to have the engine in four 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 coming up almost at TDC (10 BTDC) with the front piston moving down. This should line up both drive tongues with their flats facing to the rear of the engine at 90 degrees (See diagram).

The point's cam must also be checked and should be just about to open the points for the vertical cylinder (The points closest to the condensers). If this is not the case then rotate the crankshaft three times and all the above should line up again. The correctly timed heads should now slip straight on.

NOTE 1 A correction courtesy of Damian Birch: There is just 1 tiny detail not correct on this page. When all the dots line up on a roundcase motor, the crank is between the 2 cylinder, in line with the distributor, which is 45 degree BTDC, not 10 degrees. If you look at the pistons they look like they are both close to the top but it is actually 45 crank degrees.

NOTE 2 If you are lining up the lower timing marks in the timing chest of a round case motor you will notice that the central bevel gear, which drives the two cam drive shaft gears, has two timing dots 180 apart which match a single timing dot on each cam drive shaft gear. When the bike left the factory these two dots on the central gear were painted either red or green and matching paint was applied to the single dots on the mating gears. It is probable that this paint will be long gone when you come to do the timing so be aware that you can have all the timing marks lined up but still have the point's cam 180 out! Read the previous paragraph on how to check this and, if necessary, fix the situation.

NOTE 3 The points drive has a set of 2 to 1 reduction gears under the point's body. There is a timing dot on the small drive gear that matches up with a painted mark on the large gear. As mentioned previously this paint may be long gone so if you pull the points drive out be sure to mark the large gear with an engraver, paint or similar to aid correct re-assembly.

750 MODELS TIMING CHART.

Crankshaft Revolutions 0* 1* 2* 3* 4* 5* 6*
Crankshaft Bevels 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*
Points Cam Revs 0* 1/2 1* 1 1/2 2* 2 1/2 3*

* = Timing Dots