Clutches, Primary Drive & Gearbox

Clutches fitted to the 500 and 1,000 c.c. models differ radically as do the primary drive arrangements. The single-cylinder models have a Burman gearbox and clutch of orthodox multi-plate design, movable to provide adjustment for the single-row 64-pitch ½-in. by 5/16-in. primary chain. Full oil-bath construction is used, though the Ferodo-insert clutch is shielded from oil to some extent by guards in the casting.

The twins carry a Vincent gearbox which is driven by a servo clutch and an endless, triple-row, primary chain, adjusted by a slipper tensioner on the bottom run. The engine-gearbox construction is of a unit type, with the gearbox shell cast as part of the crankcase instead of being bolted up. Only the primary chain is in the oil bath, the clutch being a fully dry component housed beyond the chaincase.

Another difference between the two is that the singles are fitted with a chaincase drain plug whereas the twins are not; both have level plugs - a ¼-in. B.S.F. set-screw with a copper washer under the head.

Five insert plates are used in the Burman clutch and it is recommended that, when the material is worn or badly oiled or glazed, the owner fits service-exchange plates rather than attempt to reline them himself.

Burman Clutch Renovation

Glazed linings can be roughed-up temporarily with coarse emery paper and oiled ones washed in petrol; residual fuel is burnt off by applying a match to the plates suspended indivually from a wire. This treatment is drastic and only to be regarded as a palliative until new plates can be obtained.

If the tongues on the periphery of a plate are badly worn through 'chattering' in the clutch drum, the only practical remedy is to fit a new component. The springs, held in cups in the cast light-alloy pressure plate, tire with service and should be renewed whenever the the clutch shows signs of slip, or other parts are found to be deffective. The springs may be adjusted individually so that the plates run true when the clutch lever is lifted and the kickstarter operated. Tilting plates results in clutch-drag.

Fitting wrong inserts, or over-thick ones, may result in the outer plate 'popping out' of the drum and remaining suspended even when the handlebar lever is released. It is also possible to have clearance at the lever yet find a tight push-rod causing slip. Make certain, therefore, that the operating lever within the gearbox does not foul the cover when the handlebar lever is released. A further adjuster is provided in the preesure plate to overcome this condition.

Primary chains require little attention other than adjustment and periodic replacement - say every 40,000 miles. There should be about ½in. up-and-down play with the chain at its tightest point. A chain with tight and slack spots is excessively worn and should be discarded. The shock-absorber is mounted on the engine shaft; it is of the multi-spring type and is dealt with later in this section.

It is important to note that the Burman gearbox pivots on a through fixing bolt. When the adjusters are operated to set the primary chain tension, therefore, this bolt must first be slackened. Failure to do results in the gearbox shell cracking. Adjustment is effected by push-bolts; obviously one is slackened as the other is taken up.

Vincent Clutches

The servo-clutch is, in effect, two units. One is an orthodox plate clutch, consisting of two steel driven plates and one friction-material driving plate. Two forms of driving plate are available. Inboard is a servo component consisting of a pair of shoes expanded towards a drum by the torque action of the plate clutch. The drum is a driving component mounted on the clutch chain wheel by six high-tensile screwed dowels provided with screwdriver-slot heads; under each head there should be an internally-toothed shake-proof washer.

The shoes are mounted on a carrier secured to the gearbox input shaft and therefore are driven components. Each shoe pivots on the carrier and is additionally held by a link; the link is "over-centre," so that the more power the clutch is asked to transmit, the more firmly the driven shoes are pressed outwards against the driving drum. Centrifugal force plays little part until about 5,000 engine r.p.m. is reached with the clutch fully home. With the engine revving and the back wheel stationary (as in a sprint take-off) only the drum rotates and the shoes are stationary. Until full clutch engagement occurs, centrifugal force plays no part.

Two friction pads are attached to each shoe. The major ones accept driving torque. It is the task of the minor ones to deal with reverse loadings as, for example, when the machine is on the over-run or being kickstarted.

Dismantling Work

Before dismantling this clutch it is well to arm oneself with an assortment of screwdrivers including, if possible, one of the ratchet-spiral type - invaluable for dealing rapidly with the nine 2-BA countersunk screws that retain the pressure plate.

Six springs with conventional cups are used. In each of the nine holes in the pressure plate is a distance piece; all are of the same length. No individual spring-pressure adjustment is permitted, and all nine screws are run right home and the the slots are centre-popped for security. It is useful to slacken all the screws one turn, remove the six spring-loaded and finally the three remaining ones, so relieving at least the first six screws from the pressure-plate friction.

Detach the primary clutch plates (C23, C24AS, C21) along with the special spring (C45). Prise carefully to free it. Signs of rubbing by the primary clutch may be noticeable at several points. If seen inside the clutch dome on the chaincase, probably a pressure-plate retaining screw has backed off. It may be found at three points at the back of the inboard driven plate. If these marks are blue, then that is evidence of heat - and heat may have caused distortion. So check the steel driven plates. The marks are caused by the clutch carrier pins rubbing the plate and this will have created friction on the secondary, or servo-clutch, and possibly impeded its correct operation. A cure is made by slipping a 5/16in. plain washer over each of the nine pins before offering up the inboard steel clutch plate; this holds the entire clutch plate mechanism slightly away from the shoe unit.

Badly scored or distorted steel clutch plates should be renewed. So should the friction-insert plate if the tongues are too badly battered through 'chattering'. If inserts (C25) are renewed, fresh ones should be pushed into position from alternate sides. Careful examination will show that they are slightly tapered on the edges. If every second one is pushed in from one side, narrow face first, and the remainder from the other side, there will be no danger of them 'creeping' in use.

Clutch Failure

Check that all springs are of equal length; if it appears that any needs renewal, purchase a set of six, otherwise trouble with uneven lifting may be experienced later. Renew the springs it the clutch has been slipping enough to become overheated. This occurs when the shoe clutch has failed and the rider has attempted to drive too hard on the plate clutch alone. The latter, by itself, will propel the machine up to about 55 m.p.h. The usual cause of shoe clutch failure is oil saturation.

Remove the shoe-carrier retaining nut(C20/1/2); take care not to lose the hardened shim(s) fitted under it's head. Use a box or socket spanner to unscrew the nut. The shaft on which it is mounted can be locked by putting the machine in gear and applying the rear brake. A sharp blow with a hammer on the tommy-bar of the box spanner should free the nut. The thread is a fine one.

Do not lock the shoe carrier by jamming the nine hardened steel pins (C14) on which the primary clutch plates ride. The pins are held by a round-wire circlip passed through a hole drilled close to the base of each pin. The material is brittle at this point and breaks readily. There is, or should be, an oil seal below the carrier nut and it rests within the recess; it may be an O-ring or it may be a light-alloy ring to which is bonded a 'Neoprene' circlet. The carrier, with its shoes and nine pins, will now pull off the splined gearbox input shaft; an extractor is not required. If a previous owner has applied 'Bostik' to the splines, steady pressure may be needed.

The drum is now exposed. It is retained by six special screws (C2) with shake-proof washers under the heads. A Strong screwdriver is required to remove these screws for they are, or should be, very tight. They ensure the complete rigidity of the drum on the clutch chainwheel. The drum is an iron casting and liable to fracture if dropped. It should have an infinite life and can be machined if scoring is judged to be excessive, but it will put up with a considerable amount of marking. The surface is ground.

For normal clutch servicing it is unnecessary to remove the drum; little can go wrong with it. But it is obligatory to detach it to dismantle the primary drive because, until the drum is off, the chaincase cover cannot be removed.

A 1/16-in. dia. hole may be drilled in the corner of the drum between the face and the working surface to assist drainage of any lubricant which may have seeped past the chaincase seals. Do not enlarge the drainage holes and do not attempt to lighten the drum further. Being a cast component, it might consequently burst under centrifugal force and shoe pressure.

The shoe carrier assembly can be dismantled by removing the Seegar clips (865) and the split pins from the pivot arms and the link pins respectively. Do not break the pivot-pin fastening by removing the nuts, which are punch-locked. Some early clutches had ordinary nuts and bolts in this position and difficulty will probably be experienced in getting them undone. Again, a few subsequent models had the nut recessed so that a box spanner can be used. But removal of the clip, when it is used, is the best method.

Relining Clutch Shoes

Although clutch shoes can be relined by the owner, service-exchange items are available in pairs. Linings must not be replaced singly but in pairs only - two drive or two over-run. Drill two end holes first (1/8-in.) and countersink ¼-in. to within 1/16-in. of the base lining. Fit the shoe and drill the other holes, using normal brake-relining technique. Long and short rivets are supplied, according to the thickness of the metal at the points where they are inserted.

Moulded 'Duron' linings are recommended and these should have broad, angular saw cuts in the surface to provide a scraper action should the drum be fouled with oil. Two coarse blades mounted together in a hacksaw produce such a groove. Chamfer the leading edges of linings to prevent snatch. Ferodo MR41 linings have also been fitted. These cannot be slipped deliberately to burn off oil; nor may they be degreased with anything other than petrol.

There are several useful points about the shoes and their carrier to observe. The links and pivots should be perfectly free; binding can cause judder on take-up of drive. The centralizing plunger-spring adjusting screws (C12) are pre-set at the factory and are not screwed right home. If new ones are fitted count the number of turns involved in removal of the old parts. Punch-lock the screws. A pre-dismantling sketch of the location of the links on the carrier pins may be worth its weight in gold when assembling time comes. Check that the centralizing plungers (C11/1) are free in the tunnels. Finally, keep the unit free of oil if it is temporarily laid on one side.


It may be necessary to commence assembly by renewing the large oil-seal held in the chaincase between the clutch drum and the chainwheel. The knife-edge of the seal (PD26/1) must be free of cuts, nicks or grit, and it must be sharp at the lip. A new one is inserted so that the spring exerting the garter action is innermost; that is, it is not visible from the outside of the machine. This is the usual source of oil leakage from the chaincase into the clutch housing. Note that the chainwheel will not centralize in the seal unit until the shoe carrier is offered up to the gear shaft splines.

Smear a little jointing compound on the face of the drum that contacts the chainwheel and run up the screws dead tight. New shakeproof washers are to be preferred. Wipe away surplus compound.

Put a little compound on to the splines of the shoe carrier and force the part home with a quick, hard thrust, checking that it aligns the drum and sprocket as it enters the two bushes within the chainwheel centre. Place the carrier oil seal in its recess within the carrier centre. Pre-1954 machines have a bonded light-alloy 'Neoprene' seal and the later O-ring calls for modified components to accept it. If the bonded carrier seal is in short supply, reversion may be made temporarily to the early all-copper item.

Run up the shoe carrier nut, making sure as this is done that the ground surface of the nut is not eccentric to the thread. When it has gone right home, insert the ear of the locking spring into whichever of the five slots mates with the shaft slot. When this nut is tight, the thrust washer(s) under its head should be free to turn but have a little end-float - maximum is 0.010 in. Graded washers control the end-float. End-float must be present or the carrier centre will not have essential freedom and may, therfore, bind, so hindering easy clutch operation.

Pressure from the locking spring (C45) makes it imperative that, when the outer clutch plate is offered up, it must be held in place lightly with one hand while the distance pieces, cups and springs are inserted and the pressure ring is attached with three of the nine screws. Then the other six screws can be started. Run down all nine tightly.

Clutch control Mechanism

Before replacing the clutch dome, the actuating mechanism must be set. The clutch-lifting mechanism is located in the gear selector housing on the right side of the machine. The hollow gearbox mainshaft carries a clutch pushrod and this is operated by a lever pivoting within the gearbox cover and controlled by a Bowden cable inserted through the Duralumin gearbox pivot plate. Watch for wear at the actuating lever. It mat be obviated by replacing an old lever by the later type with a 'Stellite' insert, or by building up the worn material with 'Stellite' and grinding freehand to shape. It is also advantageous to replace the one-piece push-rod by a two-piece component which is divided in the middle, the halves being separated by a ¼-in. dia. ball. It cannot fall into the gearbox!

The outer cable from the handlebar lever abuts into a conical sleeve which, in turn, is abutted into the pivot plate. It is important that the first stage of setting the clutch mechanism consists of varying the length of the adjuster in the cable outer so that the lever in the cover hits the conical abutment at the same instant that the handlebar lever touches the grip. It is not necessary to remove the inspection cap to determine this; the contact click is audible.

The second stage consists of producing about 3/16-in. to ¼-in. clearance at the handlebar lever when the clutch is fully home. Obtain this by means of the adjuster screwed into the exterior of the gearbox end cover. Further adjustment is provided in the pressure plate by an auxillary adjuster against which the clutch push-rod bears when the clutch is lifted.

Vincent clutches are more sensitive to oil than most. Often the difficulty is as great in detecting the cause of the leakage as in subsequent cure. A defective oil seal or engine breather can produce mysterious rises or falls in chaincase and gearbox oil levels. Detection of which oil level is being altered at the expense of another is practicable with the aid of colloidal graphite running-in compound. To the oil in either the chaincase or the gearbox add some graphite - it is black and betrays the fault. For instance, if there is black oil in the gearbox and green oil in the engine and chaincase, but only green oil reaches the clutch, then the chaincase-to-clutch sealing is obviously at fault. Oil can pass the clutch via excessive clearance in the mainshaft and the chainwheel bushings.

Do not tap the boss in the base of the chaincase and fit a drain plug (as on the 'Comet'). If the two lowest case cover retaining screws are drilled with a 3/32-in. hole, so that the housing for the screws is broken into from the inside, then removal of the screws will allow lubricant to be drained off. It is essential for this to be done when the chaincase cover is off so that swarf is extracted and the threads in the crankcase can be cleaned up with a Whitworth tap.

Excessive chaincase oil nearly always causes clutch slip. Habitual use of the left-hand prop stand may cause oil in the main-bearing housing to drain out. A seal can be fitted to obviate transfer at this point but only in exceptional case is it necessary. A breather can be fitted into the chaincase level plug. This not only allows the case to breathe to atmosphere but maintains the oil level automatically, albeit with some mess unless a proper breather pipe is fitted to a ¼-in. banjo union held on by the level plug. Oil passing down the clutch carrier shaft can be arrested by an O-ring of 1-in. bore slipped along the shaft until it rests against the gearbox collar. If a recess is turned in the end of the carrier sleeve so that about 80% of the ring sits in it, then the action of running up the carrier nut will force the ring to accomodate itself to the space available as the sleeve is pressed against the collar. The shaft to carrier joint will then be proof against oil.

Primary Drive Overhaul

Stripping the primary drive after dismantling the entire clutch is straight-forward, although there are some noteworthy points about the assembly. First, slacken the tensioner blade. Four screws retain the dynamo-drive cover; two are long and do double duty in securing the chaincase cover as well. Remove the four. Turn back the tab-washer under the head of the dynamo bolt and remove all the drive parts; an adapter is used here with the Lucas instrument. The dynamo sprocket is drilled and taped ¼-in. Whitworth to take a chaincase screw to provide a 'Handle'. Behind the sprocket is an oil-thrower disc; remove this. The dynamo is now only retained by a clamp and may be left alone.

Remove the remaining chaincase screws, noting the lengths; tap the case downwards gently with a hide hammer and pull it off. The triplex chain wil now be slack but it cannot be removed at this stage. Slide out the tensioner, which consists of a single curved blade for the 'B' and 'C' or a double blade, with a double-slotted holder, for the 'D'.

A stout box or ring spanner is needed to get the engine-shaft shock-absorber nut undone. A sharp blow with a hammer will release the nut by acting against the flywheel inertia. All pressure on the springs is relieved before the nut comes free. Dismantle entirely and slip off the chain and two sprockets in one motion.

Fitting An Engine-shaft Seal

An engine-shaft seal can now be fitted. The outer main bearing is held by interference and by a large circlip on each side, so positively locating it endwise. Detach the outer clip and replace it with the seal which is held to the crankcase by several 2-BA countersunk screws, holes for which will have to be drilled and tapped. There is not much metal around the bearing and beware of swarf entering the race. The seal is a rubber garter with a knife edge. It was intended, had manufacture continued, that these seals should be used in conjunction with a modified sprocket. The other main bearing oil seal is an internal one, not accessible at this stage.

Primary Drive Assembly

Before replacing the engine sprocket, inspect for signs of case-hardening wear on the faces of the shock-absorber cam and also for chipping of the face that abuts up to the track of the outer main bearing. The hardened collar has oil grooves but may be replaced either way round.

Check that the clutch chainwheel teeth are in good order and that the bushes in the centre are not sloppy on the carrier. Between then is an oil seal which should be renewed if the bushes are replaced and may need renewal anyway. Bushes are a simple press fit and do not require reaming after insertion. The primary chain will have to be discarded only if no adjustment is left on the tensioner or if the rollers are missing or cracked. It is strongly recommended that the shock-absorber is replaced by the 'D' type on all 'B' and 'C' machines. It goes straight into position and so does the 'D' tensioner. Replace the tensioner and offer up the chaincase cover, entering the tensioner carrier carefully in the process. Tighten the screws fully and assemble the clutch to bring the the chainwheel into its correct attitude. Temporarily tension the chain to about ¾-in. up-and-down play.

Now insert the dynamo sprocket so that the teeth mesh in the middle row of the chain, the thrower being behind the sprocket with its flange facing inwards. Align the pegs on the driving dog; these pegs must be firmly clenched in the dog face. Tighten the dynamo bolt and peen back the tabwasher, using a new washer at each assembly.

When the unit is thoroughly hot - and that means after at least 12 miles motoring - re-adjust the primary chain tension so that is has about ½-in. up-and-down movement at the tightest point. Change the oil in the chaincase at least every 5,000 miles to keep the contents in good condition. If there is a sign of sludge formation, this can be overcome by the regular use of the 'Filtrate' compound marketed for its prevention. Tensioner blades last some 30,000 miles, becoming due for replacement when the grooves worn in them by the straight-edged chain links cut in for more than half the thickness of the blade in the 'B' and 'C' types.