Technical information

Cylinder Head - Removal, overhaul and replacement


To remove the cylinder head from the engine, firat remove the exhaust pipe, carburettor and the spark plug.

Remove the oil pipe that feeds the upper bevels and cams.  Take note of the two oil jets – the larger .044″ diameter feeds the cams whilst the smaller, .032″ diameter feeds the upper bevels.  If they have been reversed, there will have been oil starvation to the cams and rockers, usually with dire consequences.  Release the three oil drain pipes at the top and undo the jubilee clip at the crankcase.

The upper bevel cover can now be removed and the meshing of the upper bevels can be checked.  The engine should be rotated to align the timing marks on the upper bevels.  They can sometimes be difficult to see and it is not unusual to see an array of confusing additional marks.  When the timing marks are correct and in register, the piston will be at top dead centre on the compression stroke.

If the timing marks are unclear and it is the intention to fully dismantle the engine, simply rotate the engine to TDC on a compression stroke and proceed with dismantling.

In the case that the timing marks are unclear but the intention is only to remove the cylinder head for work, then the following procedure will determine the correct engine position.  Unscrew completely both gland nuts on the vertical drive shaft cover.  Allow the upper gland nut to drop and, lifting the lower gland nut, remove the circlip upwards from its groove to allow the cover to drop into the lower bevel housing.  This will allow the upper oldham coupling to become visible.

Rotate the engine until TDC on a compression stroke brings the slot in the vertical shaft in line with the crankshaft.  If the timing marks remain obscure, create new ones by painting a thin white line across the mating gears.  Note: it is important to secure the crankshaft against involuntary rotation whilst the cylinder head is removed, else the valve timing will be lost.

The cylinder head is held in place by four special nuts via four intermediate 5/16" BSC studs.  These nuts are quite difficult to access, particularly on the timing side and will require a slim 3/16" Whitworth box spanner or socket to reach them.  A 1/4" drive socket, 12mm A/F of the hexagon type, whilst not a perfect fit is adequate to remove the nuts and is small enough in outside diameter to get between the cylinder head and the bevel box.  Remove the nuts and the head will lift vertically off.

Dismantling the Cylinder Head

Before commencing, take a good look at the general condition of the head, including the condition of all threads and joint faces.  There is no reason why a cylinder head in good condition should leak any oil and any signs of leakage might indicate damage.  Observe the end of the rocker shafts, where the point of the engraved V’s should be pointing generally away from each other.  This ensures that the rocker pads are acting on the centreline of the valve.

To remove a rocker, loosen the locknut and remove the rocker spindle by rotating anti clockwise.  The eccentric spindle will cause a small compression of the valve spring at one point of its rotation.  It is recommended that a pair of simple spring compressors be made to relieve the strain and to ensure that the spindle can be freely engaged upon re-assembly.  These can be a pair of 5/16" BSW bolts long enough to screw into the bottom spring carrier with flat spacers to span the springs.

To remove a rocker, first observe that there will be a shim between the rocker and the cylinder head.  Note its position and keep with the rocker when the rocker is removed.  Repeat the process for the other spindle and rocker.  Check the condition of the cam follower pads and the pads that rub on the valve stem.

The upper bevel box can now be removed from the cylinder head.  First remove the four 3/16 BSW screws from the bearing retaining plate at the drive side end of the camshaft and remove the square plate, taking note of any shims present.  Remove the 1/4" BSF nut from the stud just below and undo the two 1/4" nuts on the bevel box flange on the timing side.  It will only be possible to unwind them so far before they contact the body of the bevel box.  Rotate the cam so that the cam lobe aligns with the cutaway in the cylinder head casting and, using a soft drift, tap the end of the camshaft. This will drive the bevel box only a little way out of its spigot in the cylinder head until the nuts permit no further movement.  Unscrew the nuts to the extent of the available movement and tap the end of the camshaft, repeating the process until the nuts come off and the bevel box is separate from the cylinder head.

Inspect the condition of the cams, and check for any undue play in the roller bearing that the camshaft runs in.  Remove the 5/8" ball race from the drive side and check its condition.

If undue play in the roller camshaft bearing is suspected, or the cams are worn, the bevels must first be separated to permit close inspection.  Undo the two 1/4" BSF nuts on the flange on the upper bevel housing.  This is a light interference fit into the upper bevel box and may need a tap with a mallet to break the seal and to loosen it.  The large bevel gear and the camshaft can then be withdrawn from the bearing outer.

Removal of the valves will require a sturdy valve spring compressor.  Grip the cylinder head in the vice by the head steady lug with the valve spring uppermost at a convenient angle.  First, note any valve caps that may be present and then compress the spring and remove the collets.  Release the spring and remove the whole assembly.  Take note of any shims that may be present under the bottom spring carrier.  Keep all of the related parts together and mark them.  Check the condition of the valve guides and valve seats.  Repeat the process for the other valve.  Check for wear on the valve stems and the seats.  Check the fit of the valves in their respective guides.

A thorough inspection of the bare cylinder head should now be made, taking note of the condition of joint faces, the threads on the rocker cover studs and particularly of the 1/8" BSP tapped thread where the oil jet to the cams screws in.  Be prepared to replace any studs that look worn or stretched and to use helicoils to repair the tapped threads.

The threads for the exhaust nut are susceptible to damage and wear over the years and need careful inspection.  If damaged, they will require specialist attention.  The valve seats should be inspected for wear.  If slightly pocketed, they can be re-cut but if excessively pocketed, they will need replacing.  This again is a specialist job, outside the scope of the average home workshop.

Overhauling the camshaft roller bearing

Using a puller, remove the bearing and then the cams.  The cams are generally very tight and it can be difficult to find something suitable to do the job.  Remove the woodruff key and then the aluminium spacer that goes between the cams and the roller bearing.  It will now be possible to remove the cage and inspect the rollers, the inner track on the camshaft and the outer track.

At this point, check the fit of the spigot on the rear of the bevel box into the cylinder head.  It is the spigot that gives its accurate location.  If loose, it will require replacement or specialist attention.

Inspect the bare bevel housing carefully for damage to the joint faces and signs of stretching or wear on the threads of the studs.  If it is suspected that the joint faces are not flat or it they are damaged, it will be well to remove the studs, drawfile the surfaces and lap on ‘wet and dry’ on a flat surface.  The same applies to the bevel box cover.

The rollers run directly onto the hardened camshaft.  If there is no degradation to the track and the outer race is sound, new rollers can be fitted.  These are the same rollers used in the big end assembly 9/16" x 3/16" and used to be available also in oversizes of +.0002" and +.0004" but are now difficult to find.  A moderate amount of play in the assembly might be eradicated by fitting new standard rollers.  If more wear is present on the otherwise sound tracks, oversize rollers can be fitted and the outer race lapped to size.

If the bearing area on the camshaft shows wear or degradation, it should be replaced, as should the outer race which is an interference fit in the bevel housing.  The outer race also has notches ground into the outside diameter, into which material of the housing is peened to secure it against movement.

Should there be wear on the camshaft, a satisfactory outcome can be achieved by pressing on a hardened sleeve 5/8" inside diameter, 7/8" outside diameter and 1" long.  Theses are ‘off the shelf’ from bearing stockists and can be used in conjunction with a heavy duty caged roller 13/8" outside diameter and 3/4" wide – they will need a sleeve making to fit into the bevel housing.

When the roller bearings have been replaced, the spacer, woodruff key and the cams can be pressed back into place.  Note that the inlet cam goes on first – it can be easily identified in that the keyway is central to the inlet cam lobe.  The end of the camshaft where the bearing fits is reduced in diameter by .002".  The cam slides comfortably onto this section and can be rotated so that the keyway accurately lines up with the key on the shaft.  Positioning of the cam is best achieved by using the bearing as a spacer – this prevents the cam being pressed on too far.  Press the cam onto the shaft until the bearing is flush with the end of the camshaft.

The bevel box can now be re-united with the upper bevel housing.  Assemble, using a paper gasket of the same thickness as previously used and a thin coat of Wellseal and stand aside until it is ready to be re-fitted to the cylinder head.


The rockers should be inspected for wear and damage to the pads.  Small roughnesses can be stoned to an acceptable finish but severe damage can be repaired by building up with ‘Stellite’’ and re-grinding to profile.  If this course of action is being undertaken, it may be well to increase the radius of the cam follower pads from the original 1/2" to 5/8".  This is, of course, a specialist job.

The bushes in the rockers are floating and must be checked for excessive wear and replaced if necessary, though this seems to be rare.

Valves and Guides

The original valve material was EN52 for the inlet valves and KE965 for the exhausts, with the inlet valve having 5/16" stems and the exhaust valves 3/8" stems.

Valves are now available in 21-4n with nitrided stems.  This material exceeds the qualities of the original material and is suitable for both inlet and exhaust.  Moreover, because of its superior strength, the exhaust valve can also have a 5/16" stem.  The weight of the valve is reduced by 20% as a result – this makes life easier for the valve gear.  It is not anticipated that the valve caps will be used, so the valves have the additional length in the stems.

New valve guides are available in Colsibro – a modern equivalent of the original aluminium bronze.

Subject to there being no excessive wear on the stems and that any re-facing of the valve seats leaves sufficient backing material, the valves can be re-used.

To remove worn guides, a pair of simple stepped drifts will be required, with an outside diameter slightly less than the outside diameter of the guide.  Since the coefficient of expansion of the Aluminium bronze valve guides is the same as the cylinder head, little is achieved by heating the cylinder head up. This is not true, of course, when replacing them.  The valve guides are shouldered and can only be drifted out into the valve spring chamber.  The guides will be a robust interference fit into the cylinder head and will require some force to remove them.

When replacing, keep the guides as cold as possible and heat the cylinder head evenly to approx 100 degrees C.  Drift the guides in until the shoulder makes firm contact with the head.

It is likely that the valve seats will require a light re-cut when new guides are fitted and then the valves lapping onto their seats with valve grinding compound.  Great care must be taken to clear all traces of the compound when the job is completed.



Reassembling the valves and springs

First, inspect the valve springs for signs of wear and any cracking in the area of the loops.  Wear will be most evident where the upper spring collar rests on the springs.  If any significant wear is present, the springs should be replaced.

Holding the cylinder head in a vice, as previously described, oil the valve stem and insert the valve into the guide.  Fit the springs into the lower collar and assemble onto the valve guide, along with any shims that were present when dismantling.  Assemble the upper spring collar and upper cap and place on the upper spring loops.  Using a valve spring compressor, compress the springs until it is possible to put the collets in place.  Slowly release the spring compressor and make sure that the collets are firmly seated and that the gap between them is equal at each side.  A sharp tap on the valve stern with a mallet will ensure that the collets are seated.

To check that the seating pressure is correct, measure between the springs at the upper and lower carriers.  The distance should be .562" and may require to be adjusted by using greater or fewer shims below the lower carrier.  Repeat the process for the other valve.

Re-fitting the bevel box to the cylinder head

Great care must be taken to obtain a good seal between the bevel box and the cylinder head as a leak here will require a good deal of work dismantling and reassembling to cure.

The bevel box mating face will have already been checked, so gently draw-file the mating face on the cylinder head to remove any high-spots and to ensure flatness.

Whilst Wellseal is a good general sealant, a thin film of silicone sealant here is worth considering.  The spigot will prevent the sealant from squeezing inside the engine. Particles of silicone sealant that come loose inside the engine inevitably find their way into jets and cause serious damage by oil starvation and its use is generally to be discouraged.  There are, however a few circumstances where its superior sealing qualities can be used with safety.

Rotating the cam to align with the cutaway in the cylinder head, enter the long stud and push home until the spigot starts to engage.  At about this point, the two ¼ nuts can be started on the short studs on the timing side as well as the nut on the long stud on the drive side.  Draw the bevel box into place by tightening three nuts in sequence.  Fit a bearing onto the drive side end of the camshaft and test the camshaft for free rotation.  Unless the bearing is a particularly good fit into the cylinder head, there will be a fair degree of lateral movement of the camshaft.

Fit the square bearing retaining plate to the cylinder head, complete with any shims that were present when dismantling.  Use jointing compound on the mating faces and tighten the four 3/16 screws.  Using a soft drift on the centre of the camwheel, drive the camshaft over to the drive side of the engine.  Check the meshing of the bevel gears throughout a full revolution.  At the tightest point, there should be a small perceptible clearance but no excessive backlash.  The gears will be in full mesh across the teeth.  Provided that the basic mesh was correct upon dis-assembly, any small adjustment can now be made via shims under the bearing retaining plate.

Re-fitting the cylinder head

It is well to leave the rockers out when initially re-fitting the cylinder head because even if the timing marks were clear on dismantling, it is wise to check the valve timing using a timing disc before contemplating re-starting the engine.

It might be worth considering replacing the cylinder retaining studs and nuts with more accessible cap head screws.  I have developed a system of cap-head screws and recessed washers which will be described separately.

With the cylinder barrel and vertical drive shaft in place, the cylinder head gasket can be re-used but will require annealing.  Heat the copper ring to a cherry red and allow to cool.  It is not necessary to the annealing process of copper to quench.

Push the copper ring into the cylinder head recess and, if using the standard intermediary studs, screw them into place.  If using cap head screws, and recessed washers, it is easier to drop them into place whilst the head is on the bench.  Likewise for the standard cylinder head nuts on the timing side.

Place the cylinder head onto the cylinder barrel spigot and engage the cap head screws or nuts.  Check that the valve timing marks are in register and engage the gland nut onto the upper bevel housing.  The cylinder head fixings can now be run down and lightly tightened.  Do not fully tighten at this stage as if the valve timing is proved to be wrong, they will have to be undone and the gasket re-annealed.

Checking the valve timing

Fix a timing disc securely to the engine mainshaft and a pointer to somewhere suitable on the engine.  The set up should be robust enough not to come loose during subsequent operations and it is worth spending some time to do this part of the job well.

To find Top Dead Centre, ‘positive stop method’ is the best.  Typically an old spark plug has the insulator knocked out and the body threaded 3/8" or 10mm.  A piece of studding is threaded through and a locknut is positioned on top.  A hole will need to be drilled through the centre of the studding to prevent a compression as the piston rises.  This device is screwed into the cylinder head and the crank carefully rotated until the piston comes up to the stop.  A useful position will be 30 to 40 degrees before or after TDC.  Remove the stop, rotate the engine past TDC and replace the stop.  Rotate the engine in the opposite direction until the piston again comes up to the stop.  Position the pointer to a similar position on the disc and repeat the process until the exact number of degrees is achieved on the stop at either side of TDC.  By this method, TDC can be relied upon to accurate.


Fit the exhaust rocker, shim and spindle using the spring compressing screws described earlier.  Rotate the engine until the cam follower pad is resting on the heel of the cam i.e. opposite the cam lobe.  Using the eccentric adjusters rotate the rocker spindle until V is pointing towards the front of the engine.  Set the tappet clearance for .020" for M17/8 cams and .025" for M17/11 cams.  Tighten the lock nut moderately.

Rotate the engine forwards until the clearance at the valve stem is just taken up.  The pointer on the timing disc should register 75 degrees BBDC.  Make a note of the actual reading and continue to rotate forwards compressing the valve spring until the valve rises and clearance is achieved again.  The pointer should register 45 degrees ATDC.  These figures are for either K17/8 or K17/11 cam.  Make a note of the reading.

Rotate the engine back to TDC, compression stoke, checking that the timing marks are in register.

Remove the exhaust rocker and insert the inlet rocker.  Repeat the process earlier described with the poimt of the V pointing towards the rear of the engine this time.

Rotate the engine backwards until the clearance is taken up.  The pointer should be indicating to 65 degrees BTDC and upon continuing the rotation, clearance will be achieved again at 60 degrees ABDC in the case of a M17/8 cam and 65 degrees in the case of a M17/11 cam.  Note any actual variation.

If the variation in the actual timing figures is within a few degrees it is possible to correct the timing by using the adjustment offered by the slots in the crownwheel.  The actual amount of adjustment either way is dependant upon where the studs are in relation to the slots when the engine was previously assembled.

Observe that the crownwheel rotates anti clockwise when the engine is rotated forwards.  Loosen the four ¼" nuts and using the available radial movement in the slots, adjust the position of the camshaft in relation to the crownwheel in the appropriate direction.  Lightly tighten one or two nuts and re-check the timing figures.  When correct firmly tighten all four nuts.

The cylinder head bots or nuts can now be fully tightened in sequence.  Tighten the gland nut on the upper bevel housing and clean off any excess Wellseal that squeezes out.

In the case that the discrepancy in the timing figures is greater that can be accommodated by the adjustment in the crownwheel, another approach will need to be adopted.  This will be dealt with in a seperate section, as if in the case that the engine has no reliable timing marks or has been built from scratch.The procedure will start from a different point.

When satisfactory valve timing figures have been achieved and whilst the timing disc is still set-up, the ignition timing can be set. This procedure is described in a separate article.  Lubricate the chain with a little grease and re-fit the timing cover.

The running valve clearances can now be set using the same procedure described for setting the checking clearances.  The original information sheets state .012" clearance for the inlet valve and .025" for the exhaust.  These clearances were for valve materials with higher expansion rates and also to cope with long, high speed races, such as the TT.  It has been my experience that with modern valves and for short circuit racing, clearances of .008" for the inlet and .012" for the exhaust are perfectly adequate. The amount of mechanical noise is reduced as a concequence. Tighten the lock nuts and check the clearance again, as tightening has the effect of increasing the exhaust clearance and decreasing the inlet.

The rocker covers can now be replaced.  Clean both joint faces thoroughly, paint a thin film of Wellseal on each and allow the solvent to evaporate.  Assemble, using a paper gasket and tighten the ¼ nuts in sequence to pull the cover down evenly.  Fix the bevel box cover in the same manner.

The oil drain pipes should now be fitted, using paper gaskets and Wellseal at the top.  Care must be taken not to overtighten the 3/16" BSF nuts.  Fix the lower end with a jubilee clip to the tube in fixed in the crankcase.

The oil feed pipe to the rockers can now be fitted, taking care that the larger of the two jets is uppermost, feeding the cams.  Fit the banjos using soft aluminium washers.  Do not fit the end of the pipe to the union at the timing cover at this point, as it is adviseable to see oil rising to this point as a first stage in the preparation to start the engine

After fitting the carburetter, exhaust system, fuel tank etc, the engine will be ready for the pre-starting procedure.

Rob Drury 20 december 2006