Technical information

Overhaul of Mkviii engine

Flywheel Assemblies


It is not uncommon for engines to be re-built using flywheel assemblies that have not been adequately checked.  There are few things that can make a motorcycle as unpleasant to ride than the vibrations that emanate from out of truth or out of balance flywheels.  There is often a reluctance to deal with flywheels but there is no real mystery to them.  It is impossible to ascertain the condition of a roller bearing big end without splitting the flywheels and inspecting the bearing surfaces.  Equally, it is not possible to check the truth of the mainshafts or of the assembly without presenting them to a measuring device, usually a dial test indicator.

Roller bearing big ends are, however, very robust and capable of many years of service and even a little play is not detrimental so long as the bearing surfaces are sound.  It is practice to replace rollers only when the bearing surfaces are sound, if the clearances are restored by so doing.  Oversize rollers are sometimes available, usually plus two tenths and four tenths of a ‘thou’ in diameter.

Anecdote: During the recent engine overhaul of my Mkii KSS engine, it was discovered that there was considerable play on the big end.  This engine ran sweetly and smoothly with the only indication of wear being a slight knock when the engine was running on an even throttle.  I recalled that at the last time that I had the top-end off, I had noticed perceptible play in the big end but decided that it would have to run a little longer.  The engine had seen considerable use in the intervening 20 odd years. Naturally, I fitted a new big end assembly but despite the clearances, there was no pitting or flaking to the bearing surfaces and rollers of the old big end.

Whilst there is no mystery involved in dealing with flywheels, it is not possible to do without an adequate level of facilities.

1.  For a ‘nutted-up’ assembly at least one suitably modified socket will be required to undo and tighten the nuts.  These are recesses in the flywheels and a socket of reduced outside diameter is necessary.

2.  A press of some sort or a puller capable of enough pressure to separate and then press the assembly together again.

3.  Accurate centres to check the truth of the mainshafts in the individual flywheels and then to check the truth of the assembly.

4.  A hone or a lap to get the big end outer race to be the correct fit on the crankpin and rollers.

5.  A reamer for the small end bush.

6.  A flat surface, block and measuring equipment to check that the connecting rod is straight.

Separating the flywheels: In the case that the whole assembly might not need replacing, it is better to remove the drive side flywheel first, as this leaves the alignment of the galleries in the timing side flywheel and the crankpin intact. Note: On original Velocette crankpins, there is an annular groove in the end of the crankpin, so this alignment is not crucial.  On one-piece crankpins (Alpha etc) there is a hole which lines up with the gallery in the flywheel.

Undo the nut on the drive side crankpin.  These are invariably very tight and will need a good fitting socket which has to its outside diameter reduced to allow it to go into the recess in the flywheel. It might prove necessary to shock the nut loose.

With the nut removed, the flywheel assembly is presented to a press or puller to pull the flywheel off the tapered crankpin.  As soon as there is a movement, the flywheel will come free.

Check the inner face of the drive side flywheel for any signs that the connecting rod big end eye has been rubbing against it.

Mark the connecting rod so that its orientation can be remembered and, holding the flywheel horizontally, lift off the connecting rod.  Wipe and inspect the bearing surface for any sign of pitting or degradation.  Check the fit of the unworn portion of the gudgeon pin in the small end bush.

Lift the cage and the rollers off the crankpin.  These should be 9/16" x 3/16" rollers but some replacement assemblies had two or even three short rollers.  These are not recommended.

Check the inner face of the timing side flywheel for any signs that the connecting rod big end eye has been rubbing against it.  If there is wear, the flywheels may need replacement or specialist work.

Inspect the bearing surface of the crankpin for pitting and degradation.  It is common to see an area of wear at each the edge where the aluminium cage has eroded the crankpin, mainly on one side.  This is, of itself, not of great consequence if all of the other elements are sound.

To check the truth of the mainshafts in the flywheels, first check the condition of the centres on the mainshafts. If in good condition, the flywheel and shaft can be placed between centres and checked with a dial test indicator.  If there is discrepancy, then new and probably oversize mainshafts might need to be fitted.  This is a specialist job.  Please note that it is not uncommon to find damage to the ends of mainshafts.  If the damage is severe, and enough to compromise readings ‘between centres’ it is possible to hold the flywheel in a four jaw chuck and test the shaft for run-out.  This is a skilful operation which requires patience.  Damage to the centres will also make truing of the assembly difficult.

A simple test for the straightness of the connecting rod is to press the outer race from the rod and to hold the machined face of the big end eye onto an accurate block.  Note : the outer race cannot be relied upon to be square in its bore, so must be removed. A dial test indicator is zeroed onto the rod at the small end (not the bush).  If the rod is turned over and the dial test indicator returns to zero, the rod is straight. When a rod gets bent, it is usually to a noticeable degree and not a matter of a couple of 'thou'.

If the rod is bent or a twist is suspected, sophisticated mandrels and other equipment may be required.  This is again specialist work and is detailed in Paul Irving’s ‘Tuning for Speed’.

The fit of the crankpin into the flywheel needs to be checked.  To check the roundness of the tapered holes in the flywheel, place the crankpin in to where it stops.  Try rocking left to right and then up and down.  Any rock shows that the hole is not round and will need specialist attention.  For the crankpin to be a secure fit in the flywheels and not to come loose in use, the distance at which the crankpin comes to rest needs measuring.  Ideally the crankpin stops at half its length of taper- i.e. 7/32" (5.6mm) but will be safe if it goes 2/3 of that distance- 5/32" (4mm).

Provided that everything looks ok, the process of preparing to re-assemble can begin.

If the bearing surfaces on the crankpin and the outer race are sound, then it is prudent to at least replace the rollers.  If there is wear on the crankpin where the aluminium cage has been rubbing, then the cage should also be replaced.  The cage wears the crankpin because small particles of hard bearing material become sintered into its surface, creating a lap.

Check that the oil galleries through the timing side mainshaft, flywheel and crankpin are clear by using an oil feeder can to pressure oil through the mainshaft and out through the hole in the bearing surface of the crankpin.

If the small end bush shows wear, a new one should be pressed in.  It is fair practice to push the old bush out with the new one, ensuring that the oil holes are correctly aligned and the bush projects equally each side of the connecting rod.  The bore of the bush will be too small for the gudgeon pin and will require boring or reaming to fit.  Boring is absolutely the best way to do this job as it creates a smooth bore, square to the connecting rod. It is possible to do this job manually using great care to keep an adjustable reamer square to rod whilst reaming.  Small incremental cuts should be made, keeping the reamer rotating clockwise throughout and passing fully through. Without adjusting the reamer, pass through again in the opposite direction, this keeps the bore parallel. Too many passes can cause the development of chatter marks in the bore, as can too large increments.  The gudgeon pin should be a sliding fit with no perceptible play

In the case of re-using the existing big end assembly, the old outer race is pressed back into the connecting rod.  Great care should be taken to ensure that it is pressed in square and that an equal amount projects either side of the connecting rod.  Squareness of the outer race can be checked using the same method described for checking straightness of the rod.

Holding the flywheels horizontal so that the crankpin is uppermost and vertical, place the cage over the crankpin and insert the new rollers.  It is useful to hold them in place with a rubber band at their lower end.

When the connecting rod is placed over the cage and rollers, it should do so freely and when rotated briskly, in both directions, show no tendency to track up or down the crankpin.

In case that a new big end assembly is being used the procedure will start by removing the timing side nut and pressing out the crankpin.  The hole should be checked for roundness by inserting a crankpin and rocking, as previously described.    The new crankpin is pressed in place, taking great care to align the oil hole in the crankpin with the drilling in the flywheel. Upon this depends the free flow of oil to the big end. The crankpin can be pressed home to its shoulder and the nut tightened. If using the Veloce type two piece crankpin, there is not such crucial alignment but the pin must be orientated so that the oil holes in the crankpin bearing surface point rearwards at approximately 9 o'clock, looking at the outside of the timing side flywheel with the crankpin at 12 o'clock. The crankpin is pressed home until the shoulder just meets the inner face of the flywheel and then the galleries should be checked by using an oil feeder can to pressure oil through the mainshaft and out through the hole in the bearing surface of the crankpin.Care must be taken not to press the crankpin beyond the point where the shoulder meets the inner face of the flywheel as it is possible to press the crankpin through the bearing ring and not leave enough taper for the drive side flywheel. Tighten the timing side nut when the drive side flywheel has been fitted and its nut tightened.

The outer race of the big end assembly is pressed into the big end eye of the connecting rod with equal amounts projecting each side.  Check for squareness using a block and dial test indicator, as previously described.

The flywheel is held as previously described and the cage and rollers fitted.  It is expected that the outer race will not fit easily over the cage and rollers, if at all.  The outer race will have been manufactured so as to require lapping to fit.  This is because the variable fit between the outer race and the big eye in the connecting rod influences the internal bore of the outer race.

A Delapena honing machine is the purpose built piece of equipment for honing out the bore to a correct fit over the cage and rollers.  If one is not available, a lap can be made to do the job.  A piece of softish material (aluminium is ideal) is turned to be an easy sliding fit into the outer race. Set the lathe at a very low rotation speed, coat the lap with fine grinding paste, slide on the rod and holding small end, keeping the rod square to the lap, start the lathe.  Move the connecting rod back and forth along the lap.  Grinding paste will sinter into the surface and will need regular replenishment.  Reverse the direction of rotation regularly and reverse the connecting rod on the lap.  Stop regularly, clean thoroughly and offer to the cage and rollers.  Metal is removed slowly and patience is required-nothing happens dramatically!

Eventually the outer race will slip over the cage and rollers but enough clearance is required to ensure that the rod does not track to one end of the crankpin or the other when being rotated.  An oil film requires some space, so it requires that when dry, there will be a perceptible small movement in the big end.  The connecting rod will not exhibit any side to side rock.

When a satisfactory fit has been achieved, thoroughly clean all of the rollers, crankpin, cage and connecting rod.  Apply a thin film of clean engine oil and fit together.

Place the drive side flywheel onto the crankpin and press together lightly, ensuring that the flywheels are concentric to each other.  Use a straight edge to position them accurately ‘by-eye’ and then press the flywheels together until the shoulder on the crankpin comes up hard against the flywheel.  Fit the nut and tighten.

Truing is done between centres with dial test indicators measuring deflection.  Measurements are taken where the main bearings sit (or if the main bearings are still on the shafts, as close to them as possible).  These are excellent illustrations in ‘Tuning for Speed’ of the variations of ‘Out of Truth’ that can be present and how to correct them.  It is difficult to describe in words only and would require lengthy explanations that would inevitably be subject to semantic interpretation. Illustrations here are worth a thousand words. Maximum permissible run-out is .001" per shaft but half of that is usually achievable on Velocette flywheels. When truing has achieved a satisfactory accuracy, both of the crankpin nuts need tightening. Real tightness needs to be achieved and tightening one against the other using two sockets with long tommy bars is a good way. Put the assembly between centres for final checking and truing.

Trueing can also be done with the mainshfts supported upon knife edge rollers, at the place where the bearings fit. Deflections are measured at the end of the shafts, where run-out is magnified but in the opposite plane. Innacurate readings which might accrue where there is damage to the centres on the mainshafts are avoided by this method.

Where it is deemed prudent not to undertake this work and to pass it on to a specialist, some care here also must be taken.  It is well to ask other Velo owners about their experience with local engineers.

I have worked with Max Nightingale of Alpha Bearings and supplied him with copies of ‘works’ drawings to help him to provide a complete service, including new mainshafts. Max can offer a two piece crankpin of the type that Veloce used.  I have always been pleased with Max’s work. 


Rob Drury   November 2006

Thanks to Meter Miles for proof reading and claifications and corrections.