Technical information

Overhaul  of Mkviii engine.


Dismantling and re-assembling the ‘bottom-end’


When the cylinder head, barrel and piston have been removed, along with the vertical drive shaft, outer and inner timing cover, there will remain the crankcases, flywheel assembly and the lower bevels.

Before separating the lower bevels, close observation of their mesh should be made.  The relationship between the two should be that of fully in mesh, neither gear protruding beyond the other.  Any shims should be noted when dismantling. The bevel housing is retained by two ¼" BSF nuts and is a light interference fit in the crankcase.  Check the assembly for wear in the bush and whether there is any end-float. Check the condition of the gears very carefully.

At this point, check the flywheel assembly for any roughness in rotation and for end-float on the main bearings.  The flywheels should be free to turn with no perceptible end-float.  When the assembly is rotated, using the connecting rod, there should be no evidence of the rod tracking to one-side or the other and rubbing on the flywheels.

To remove the left-hand thread nut on the crankshaft will require the crankcases to be held firmly and a suitable round mandrel placed through the small end bush (minimum ¾" diameter, approx 4" long).  The crankcase mouth will need two pieces of soft material (aluminium-minimum ½" square) for the mandrel to come to rest on and to protect the joint face from damage.  The left hand thread nut will invariably be quite tight.  With the nut and tab washer removed, the spur gear that drives the magneto/oil pump and the crankshaft bevel can also be removed.  If they are tight on the main-shaft, it might not be possible to remove them until the crankcases are separated.

To separate the crankcases, first remove the ¼" studs around the periphery.  Crankcases are often reluctant to part due to the close fit of the machined spigot and any sealing compound that has been used. Rotating one against the other will present faces that can be gently knocked apart with a mallet.  Avoid prising the crankcases apart as this risks damaging the joint faces.

In the case that the bevel is tight and still on the timing side main-shaft, it will naturally be the drive side crankcase that comes off, leaving the flywheels still in the timing side.

Supporting the timing side crankcase on suitable blocks above a wooden bench with clearance between the end of drive side main-shaft and the bench, a blow to the timing side main-shaft using a soft drift will induce the flywheel assembly to fall through the bevel gear.  The weight of the flywheel assembly will do most of the work.  Care must be taken not to damage the timing side main-shaft in so doing.  Take note of any shims behind the bevel gear.

The flywheels can now be put to one side, while, after thorough cleaning, a close observation of the crankcases should be made.  The 3/16" BSW threads around the lower bevel box and the 3/8" BSF threads for the cylinder studs are worth checking and repairing with helicoils if necessary. 

The main bearing outers should be inspected for signs of wear though it should be noted that degradation usually starts on the inner tracks.  Joint faces should be examined for any damage that would prevent sealing and any high spots carefully removed.

It is appropriate to examine the oil pump prior to removal – check for end play and that there is no roughness in rotation.  If there are good reasons to believe that there are no problems with the pump, it might be best left undisturbed.

To remove the oil pump, first remove the four screws and then the crankcase should be heated to approx 100°C.  With suitable gloves, hold the crankcase parallel to a wooden bench and bringing it sharply down onto the surface of the bench should dislodge the pump from its housing.  Note the brass shim under the pump which will need replacing if scored or worn.

Where pumps are a particularly tight fit it is possible to tap two opposite pump mounting holes 1/4" BSF and use these to draw the pump from its housing when the crankcase is hot.

In the case that the pump needs repair, that subject will be dealt with separately but in essence, the pump should have no excessive clearances between the gears and their respective bores or any perceptible end clearance.  There should be no scoring of the bores or damage to the gears.

At this point all oil galleries in the crankcases should be checked.  The gallery for returning oil to the pump from the sump has a propensity to collect debris.  Remove the 1/4" screw at its end and check that it is clear.  The 1/8" BSP plug above the oil pump housing should be removed.  This has a spring attached and a ball which, when seated, is designed to prevent oil sumping.  The seat should be checked. The oil gallery from the pump to the outer face of the lower bevel box should also be checked.  Oil is transferred through this gallery and via the inner timing cover to the big end and to the top of the engine.The lock nut on the ball assembly that controls oil pressure should be slackened and the screw, spring and ball removed.  Note the position of the lock nut so that the screw can be screwed back into its original position upon re-assembly. This will give an initial oil pressure setting when re-starting the engine.  All galleries revealed should be checked, as should the galleries which bring oil to and from the oil tank.

Before attempting to replace the oil pump, a new brass shim should be to hand for the pump to sit on in its housing.  Four 3/16" guides should be made to ensure that the pump goes to its correct location.  The guides need to be made from 3/16" diameter steel rod three inches long, threaded 3/16" BSW at one end for 3/8" and with a screwdriver slot in the other. Lightly screw the guides into the crankcase leaving them slightly loose so that they can wobble a little and not restrict the free movement of the pump over them.

Fit the guides, heat the crankcase and grip securely in the vertical plane.  Slide a shim in place along the guides and then slide the pump into position.  Use a hammer and soft drift to ensure that the pump is fully home.  It is better to do this job with crankcase vertical as it prevents the loose scavenge gear from falling out of the pump body during insertion, out as it may do if the crankcase is horizontal.  The pump can only be fitted in two positions and is orientated so that the drive is closest to the crankshaft. Remove the guides and replace the four countersunk 3/16" BSW screws.  When cool, check that the pump rotates freely.

The main bearings are removed by the same process of heating the crankcase and bringing it sharply down onto a flat wooden surface to jar out the outer races.  Note should be taken of any shims that are between the bearing outer and the crankcase.

It is not uncommon to see drive side crankcases that have had two 1/8" holes drilled from the outside of the main bearing boss to enable the bearing outer to be removed with a drift.  Since this option is not available for the timing side and it is always possible to remove these outer races in the conventional manner, it is questionable whether it is desirable to drill holes.  Furthermore, in the case that the bearing outers have to be removed during the adjustment of shimming, using a drift damages the shims beyond use.

The flywheel assembly can now be inspected.  In general terms it is not possible to make an accurate assessment of the big end assembly without separating the flywheels.  The separation of flywheels and subsequent re-assembly is dealt with separately.  The fit of the main bearings on the main shafts and the general condition of the shafts needs to be ascertained.

The same applies to main bearings – no accurate visual assessment can be made without removing rollers to look at the inner track.  Even so, bearings have a finite life and unless there is degradation of the surface, it is difficult to see how a visual inspection will determine how much longer a bearing will last.  The main bearings and the main-shafts have a ‘one thou per inch’ taper, so have effectively a half a thou taper in the bore.  It makes them easy to fit but difficult to remove without a proper bearing puller.  The large end of the bearing is identified by either a larger internal radius in the case of original bearings or by being engraved in the case of new replacement bearings from the Vintage Bearing company.

It is worth noting that the same bearing is used on both sides - a lipped roller 50 mm OD x 20mm ID x 17 mm wide, tapered internal bore. If a ball race has been fitted to the drive side, it will need replacement, whatever its condition.

If the main bearing  has become loose in the crankcase, oversize bearings can be supplied by the Vintage Bearing Company, up to 2" or 50.8 mm diameter.



With the oil pump in place and operating satisfactorily, attention can be turned to reassembling the flywheels into the crankcase.

The accuracy of the flywheels is crucial for a smooth running motor.  When measured at the main shafts where the main bearings will be fitted, the maximum permissible run-out is .001" per shaft.  In practice it’s quite possible to achieve half of this - the more accurate, the better.  The balance factor is 70%.  How to dismantle and reassemble flywheels is dealt with separately and is also covered at length in ‘Tuning for speed’ by Phil Irving.

There is no easy way to achieve accurate end clearance on the main bearings.  If good measuring equipment is available, it is possible to place the main bearings onto the main shafts, complete with the outer races and measure the overall width of the bearings.  It is then possible to measure from the mating face of each crankcase to the machined bearing face.  If these are added together (they should be similar) and the width of the bearings on the flywheel assembly is deducted, then what remains is the amount of shimming required.

Alternatively, the notes taken of existing shims are a good start point.  If a new big end assembly has been fitted, there will invariably be a small difference in width of the assembly.

However the amount of initial shimming is determined, it is unlikely not to require fine adjustment and patience is required. The shimming should not only be calculated to give the correct pre-load but also to centralise the flywheels in the crankcases. It has been my experience of measuring crankcases that the distance from the joint face to the bearing face of both cases is usually equal and accurate to within a couple of 'thou' but this should not be relied upon. Without the benefit of accurate measuring equipment, aligning the connecting rod on the crankcase centreline will be fine.

Heat the crank cases and fit the main bearing outers placing the appropriate shims in first.  When installed, place a weight on the bearing to resist creeping out as the crankcase cools.  Once cool, it is well to use a soft drift and hammer to ensure that the bearings are absolutely ‘home’.

When both crankcases have cooled, the flywheel assembly can be fitted to the crankcases.  With the timing side crankcase on the bench, bevel box down, the main shaft will not protrude, giving a safe working platform.  Fit the flywheel assembly, timing side down and then offer up the drive side crankcase. The spigot usually offers some resistance to assembly and may require a mallet to ease into place.  Align the crankcase mouth accurately and fit the ¼" BSF studs but do not tighten at this stage.

Check for perceptible end float and gradually take up the ¼″ studs in sequence, keeping the joint faces parallel.  If there is end float when they have pulled the mating faces together, then more shimming is required. Insert the 3\8" engine mounting bolts and tighten the nuts. A Dial Test Indicator at the end of one main-shaft should provide an accurate enough measurement to get things right in one more step.* Using a lever between the flywheels and the crankcase wall will ensure a true side to side movement is measured. It is generally easier to work from the position of some end-float than the opposite situation where pre-load exists.

If the required amount of additional shim is small, say 002" or 003" it is not necessary to remove both main bearing outers to equalize the shims, replacing the shim under one bearing outer will be adequate.

If no end float is present and the crank is noticeably stiff, shims may need to be removed.  To ascertain how much, loosen the studs, separate the crank cases and re-tighten in sequence.  Using a feeler gauge between the mating faces, determine the point at which end-float disappears. The measurement between the mating faces is basically the amount of shim that needs removing.*  If a large amount, remove both bearing outers and equalize the amount of shims after the deduction has been made.

It would be worth noting at this point that it would be well not to rely upon the 1\4" studs to hold the crankcases together during the final stages of  this operation. Using the 3\8" bolts will ensure that the crankcases are operating under the same conditions that they will be when the engine is installed.

When all the adjustments have been made and the crankcases are bolted together using the ¼" studs and the 3\8" bolts, the flywheels should have no end-float and be almost free to turn.  This is, however, beyond physical measurement and needs the application of ‘feel’.  *Furthermore, for the flywheels to exhibit no end-float at running temperature there will need to be some resistance to rotation when cold.  This means that there is a small pre-load when cold of .001" to .002" and this will be added / subtracted to the shims. Though roller bearings are not intended for axial loading, they are capable of resisting some and this is only until the crankcases have warmed up enough to relieve them.

A proven method of determining that the correct set-up has been achieved is that there will be enough resistance to rotation to allow the flywheels to remain static with the crankpin at 3 o'clock or 9 o'clock.  Warming the crankcases a little will see the crankshaft spontaneously rotate to its balance position.  Standing the assembly in front of a gas fire is usually enough to do this.

The crankcases are now ready for final assembly, separate them again and clean all surfaces paying particular attention to the mating faces, which must be thoroughly de-greased.  A thin coat of ‘Wellseal’ should be applied to both faces with a small paintbrush and time allowed for the solvent to evaporate (see the maker’s instructions). * Fresh oil should be applied to the big-end and main bearings and the flywheel assembly placed into the crankcases, the timing side first as previously described.  Put the drive side in place, accurately align the crankcase mouth, fit the ¼″ studs and tighten in sequence. Re-fit the ball and spring anti-sumping device and the oil pressure assembly to the position that was noted during dismantling.

The engine is now ready to have the bottom bevels meshed. This is quite a complex proceedure and has been made the subject of a seperate article - 'Meshing the bottom bevels'


Rob Drury – October 2006


* As an alternative to Wellseal, which can be difficult to obtain nowadays, Dai Gibbison recommends 'Threebond 1104' . This product can be checked out on -

Thanks to John Hannis for proof reading and offering corrections.