Technical information

Valve Timing An Engine From Scratch

June 2012

The engine shall have the cylinder barrel and piston fitted and the lower bevels meshed. To set the flywheels into the correct position, first either install the crank-cases into the frame or devise a method to hold them securely on the bench in such a manner that a timing disc can be attached to the drive side main-shaft and the engine can be rotated, with a clear view of both sides of the engine.

Attach the timing disc to the drive side main-shaft and arrange an adjustable pointer from the crank-case.

An accurate method of setting the timing disc is by ‘positive stop’. This requires setting a stop for the piston to come up to, some 20/30° BTDC. By rotating the flywheels backwards and forwards between the stop positions, the pointer can be set equidistant each side of the TDC mark, or zero. When the pointer gives the same reading each side of TDC, it will be accurately set. It is well to clearly mark the opening and closing times for each valve event on the disc and also the ignition timing.

Rotate the flywheels forward (clockwise when looking at the timing side of the engine) until the slot in the lower vertical bevel is ‘fore and aft’, i.e. in line with the flywheels. This might take several revolutions until the slot is exactly in line with the two ¼” studs that fasten the bevel housing to the crank-case. Secure the flywheels against accidental rotation.

To prepare the cylinder head for valve timing, the valves, springs, rockers etc., must be installed, ready as for use and the valve clearances set to the checking clearances (.025” on a K17/8cam and .030” on a K17/11 cam). Note – when setting valve clearances, ensure that the arrows on the end of the rocker shafts point away from each other. If there are clear timing marks on the upper bevel and crown-wheel, rotate the camshaft until the timing marks are in alignment. With the marks in alignment, the slot for the Oldham coupling in the upper bevel will be ‘fore and aft’. (If the timing marks are aligned and the slot in the bevel is not ‘fore and aft’, it is likely that there is a problem with the timing marks. It will be well to treat the engine as if it had no timing marks and follow the procedure for this case). Fit a cylinder head gasket, the Oldham couplings and vertical drive shaft and fit the cylinder head, engaging the upper bevel onto the upper Oldham coupling. Slightly rotate the cylinder head on its spigot to ensure that it is seated and lightly screw down the cylinder head nuts. Check that the vertical clearance in the drive shaft is between .012” and .025”.

If there are clear timing marks on the upper bevels that are known to be accurate and the vertical drive shaft and Oldham couplings are also known to be correct, the vertical drive shaft, Oldham couplings and vertical drive shaft cover can now be fitted, taking care to pack the lower gland.

If there is doubt about any of the above, it will be well to ‘dry’ assemble the engine without the vertical shaft cover and to check the timing, prior to dismantling for final assembly. Fitting temporary light springs to the valves at this point will make the procedure easier.

By rotating the engine forward, the exhaust valve opening will be the first event and the clearance should take up at the point on the timing disc marked 75° BBDC.

If the timing figure is close to that specified, continue to rotate the engine forward and note the other timing figures: inlet opening 55° BTDC, exhaust closing 45° ATDC and inlet closing 65° ABDC (60° on a K17/11 cam). The point being determined by clearance being just taken up when a valve is opening or when clearance first appears when the valve is closing.

If the figures achieved are in accord with the timing figures for the cam, the operation has been successful. If the figures are consistent to the disc but are a few degrees advanced or retarded (less than 5°) final adjustment can be made on the crown-wheel which has slotted holes under the four ¼” nuts.

If the figures differ more widely, it is possible to correct by calculating how far advanced or retarded, lifting the cylinder head and rotating the engine forwards or backwards and then re-fitting the head and re-checking. However, it is probably as well to treat the engine as if it had no timing marks and adopt the following procedure:

Set the flywheels so that the slot in the lower bevel is ‘fore and aft’. Rotate the engine backwards one revolution to TDC. The slot in the lower bevel will now be slightly out of alignment – move the flywheels off TDC to align the slot in the bevel ‘fore and aft’.

Holding the cylinder head assembly firmly on the bench and engaging a flat tool into the upper bevel slot, rotate the camshaft until both valves are lifted an equal amount from their respective seats. The slot in the upper bevel should be ‘fore and aft’. If it is not, continue to rotate the camshaft until the valves are lifted equally from their seats and the slot is ‘fore and aft’.

Fit the cylinder head, vertical shaft and Oldham couplings, to the engine and lightly tighten the cylinder holding down nuts. Rotate the engine forward one revolution to TDC and make a temporary mark on the crown-wheel and upper bevel. Continue to rotate the engine forwards, checking the timing figures. Note the variation, which will be very small (less the 5°) and make any final adjustment using the slots in the crown-wheel. Rotate the engine backwards until the temporary timing marks are aligned again and make them permanent. (A thin white painted line is very effective).

Remove the cylinder head whilst the timing marks are in alignment and secure the flywheels against rotation. Fit the vertical drive shaft cover, shaft and Oldham couplings. Pack the lower gland nut and tighten. Re-fit the cylinder head, tighten down. Pack and tighten upper gland.


John Hannis was kind enough to proof-read the procedure described and has drawn my attention to the works procedure of only fitting one rocker at a time and checking the timing of that cam lobe. This is to prevent pressure on the other cam lobe taking up backlash in the cam drive and introducing inaccuracy.

My experience has been that if the bevels and drive train are set up correctly, there will be no undue backlash. Not enough to skew the results anyway. This is particularly true if the practice of fitting light springs to the valves is adopted. It also makes the whole procedure physically easier.