If the original setting has been lost through the camshaft or crankshaft having been accidentally turned when the cylinder head was off the engine it is first necessary to re-set the piston to top dead centre of compression stroke and at the same time make sure that the coupling slot in the bottom vertical bevel pinion is exactly in line with the two studs securing the bottom bevel bush housing to the crankcase, or in other words exactly parallel to the fore and aft centre line of the machine. As the bottom bevel pinion is cut with 23 teeth and is driven by a 22T pinion on the crankshaft the flywheels may have to be turned anything up to 21 revolutions to get the exact position.

To make certain that the piston is on the correct stroke remove the contact breaker cover from the magneto and check the position of the contact breaker cam which (if the control is fully advanced) will have mounted the cam ring and the contact points will be separated.

To set the camshaft, rotate the top bevel pinion until the coupling slot is exactly in line with the bevel bush housing studs and the mark on the bevel in line with the mark on the crown wheel on the camshaft. Again, owing to the "hunting tooth timing" obtained by meshing a 23T pinion with a 44 tooth crown wheel many turns may be needed before the timing marks will register.

Having set the crankshaft and camshaft with the vertical bevels correctly placed it will be seen that the vertical shaft, cover and couplings may be fitted and the head replaced without altering the position of the camshaft and crankshaft relative to one another and the timing will be correct.

To check, fit a timing disc to the crankshaft after removing the dynamo belt cover, see that the piston is exactly at the dead centre and attach a pointer to some fixed part of the engine and set the pointer to the zero on the disc.

It is necessary in order to check the timing accurately for the Exhaust Rocker to be removed when checking the Inlet valve timing and vice versa. The timing for the cam now standardised (marked K17 /10) is :-

Inlet opens 34 deg. before top dead centre.

Inlet closes 47 deg. after bottom dead centre.

Exhaust opens 64 deg. before bottom dead centre.

Exhaust closes 29 deg. after top dead centre. When checked with .025 clearances on both valves.

Engines manufactured before 1939 may still have their original cams (K17 /7) fitted, and in such cases the timing is :-

Inlet opens 35 deg. before top dead centre.

Inlet closes 65 deg. after bottom dead centre.

Exhaust opens 70 deg. before bottom dead centre.

Exhaust closes 30 deg. after top dead centre. When checked with .025-in. clearance on both valves.

After checking and re-setting the timing do not omit to re-set the rocker pins to give the correct running valve clearances, i.e., .006-in. inlet, .010-in. exhaust. Set cold.

The Inlet and Exhaust Cams are machined in one piece, so that should there be a marked variation in the reading from one Cam but the setting of the other be found correct the Cams and Rockers should be inspected for wear.

With new Cams and Rockers or those which are known to be in good condition it is sufficient to check only the Inlet opening and closing position, if these are right the remainder of the setting follows automatically.