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Overhauling the post war Vincent power unit - Service Data on this famous Thoroughbred

The Vincent motorcycle engine is designed to give long periods of service without major attention, and whilst the handbook gives instructions for decarbonizing it does not cover more extensive overhauls. The following notes are intended for those who have the necessary mechanical aptitude to undertake the job themselves and familiarity with the general details of the engine thru study of the instruction book is assumed.

Fitting Crankshaft to Engine

Centering the crankshaft in the Vincent engine cases is an important part of the reassembly process. The stack distance of ET92, ET77, ET91 determine the position of the left flywheel and ultimately the position of the rods in their cylinders. The position of the inner most roller bearings (ET92) is critical on both sides. The clearance on ET92 is tight, but the rollers must fully engage their outer races on both sides. There appears to be several thicknesses of ET77 used over the years of Vincent engine assembly, most likely having to do with the roller bearing thicknesses of the time. Shims are available to insert between the bearings and the flywheels to achieve rod centrality, however with a too thick ET77, it would appear reducing the thickness will be required.

Cylinder Head and Barrel
The earlier type kit spanner for the push rod tube gland nuts was not always strong enough, but the prongs of the current pattern spanner will shift these nuts even if they are very tight. If the stainless steel tubes are difficult to lower in the synthetic rubber glands a smear of oil will help, and if the jaws of suitable pliers are wrapped in a rag the tubes will not be marked. The cause for frequent need for adjustment on one tappet should be investigated; it may be that undue loads are imposed on the valve operating mechanism by a coil bound valve spring or a valve stem collar hitting the lower valve guide. Rockers which have more than .010" sideplay in the bearing should be shimmed up as otherwise the inside of the fork will foul the valve stem (fig 2 ). Both fork ends should bear evenly on the stem collar, and if necessary the higher fork end can be carefully stoned down. Clearance of the bearing in the head is up to .002" when new, but as the bearing is surrounded by oil slightly greater play is not detrimental. Bearings are now available in .002" oversize, but fitting may require skimming the tunnel; usually a new standard size bearing restores the clearance to normal. A worn rocker bearing is unlikely to fail suddenly and renewal is mainly advisable on account of mechanical silence. Metering wires are recommended for all rocker feed bolts, but the bearing lubrication can be increased by fitting a thinner wire than standard.

Valve Clearance
Clearance of the valves in the lower guides is important; .002-.003" is recommended for inlet and .003-.004" for exhaust. Normally renewal is recommended when wear exceeds .003", depending upon operating conditions. For removal of the lockrings the tool shown in figure 3 is used and when dealing with the actual guide the head is heated up to 200 deg C. Valve springs are renewed when they have settled down to 2 1/16". Seat rings seldom require replacement, but they cannot be removed by heat; worn rings are carefully broken up by drilling or turned out in a lathe.Grinding stones dressed to 30 deg are used recutting. 
Piston Clearances
When new, piston clearances are as shown in Fig. 4, but while no hard and fast rules are laid down as to relinering and reboring, it is frequently accepted that wear in the top of the liner must not exceed .008in., but earlier attention is necessary if a liner is badly scored or worn oval.  Oversize pistons are available, but it is much better to fit Service Exchange barrels which are relinered to standard.  The same applies when high-compression pistons arre installed for using premier grade fuels.  Piston ring gap is .016in-.020in. compression and .010in-.020in scraper and, whilst a larger gap does not matter, rings eventually lose their strength, so that renewal is usually advisable. The gudgeon pin must be an easy push fit in the piston when cold and new pistons must be installed the right way around, i.e., with the larger cutaway facing the inlet valve.  When fitting genuine replacement high compression pistons it is not necessary to rebalance the engine.  Clearances are comparatively low and progressive running in for at least 2,000 miles is essential.

Timing Gear, Primary Drive
The plunger of the lubrication system presure relief valve must be an easy sliding fit in the timing cover, otherwise it may stick in the open position.  This reduces the flow of oil to the big end and premature failure will eventually follow.  If oil restrictor discs are used to reduce the lubrication of the cylinder wall these discs are fitted behind the rear rubber washer of each pair inside the timing cover. All timing gears are fully marked, but when dismantling note the position of various thrust washers (Fig. 5).  The idler gear is mounted on an adjustable boss for resetting to zero backlash against each cam pinion, and oversize half-time pinions in steps of .002in are available for use when idler gear adjustment gives too much clearance on the half-time pinion. Cam followers always show signs of wear in service, and if the grooves worn across the component are not more than 1/8in wide the follower can be refaced on an oilstone.  Badly worn followers can be built up and reground locally, but renewal is probrably easier.  A small ridge on a cam indicates that the follower is not tracking correctly.  The ridge can be stoned off and the follower is lined up by adjusting the thickness of the thrust washers.

Replacement  camshafts are normally only supplied complete with pinion; the assembly is not keyed and pressing out the old camshaft may affect the required interference fit.  In any case a jig is required to obtain the correct relation of the cam relative to the markings on the pinion.  Timing gear spindles are inserted after the case has been heated up to 200 deg. C., and if a spindle is found to be loose it can often be plated up to the required oversize and refitted after the case has been heated locally with a blow lamp.  A big end roller is peened into each exhaust lifter and this roller may come adrift if the exhaust lifter has been used at too high r.p.m.  It can be repeened into position if the lifter arm has not worn, otherwise a new arm is required.

Primary Chain
The primary chain requires renewal when stretched more than 1/4in. per foot, and usually this is indicated by all external adjustment having been taken up.  As fracture of the chain may cause irreparable damage to the drive-side crankcase it is a wise precaution to renew every 20,000 miles on account of the great power output of the 1,000 c.c. engine.  The component is endless and removed together with the sprockets (Fig 6).  Spare parts for the triplex chain are not supplied.  The tensioner blade always shows grooves, but replacement is called for when these grooves have worn half-way through the component.  A cracked blade must be renewed without delay. The chain features straight links for use with a tensioner and only genuine replacements may be fitted.

The shock absorber nut has a right-hand thread and must be dead tight.  Considerable leverage is neccessry if the tool kit socket spanner is used for removal or tightening, and it is permissable to lock the primary drive with a wooden or soft metal sprag.  A loose nut affects the location of the flywheel assembly and causes spring fracture; springs will also break when the shock absorber cam is badly worn.  It is advisable to renew the complete cluster of springs and reassembling the shock absorber is greatly facilitated by partly withdrawing the splined cam sleeve rather than trying to compress the spring cluster by hand!

Crankcase and Flywheel Assembly
On the twins the crankcase halves are retained by sixteen bolts and studs and there are three hollow dowels for alignment.  Jointing compound is used on the faces.  Splitting the halves to remove the flywheel assembly necessitates withdrawal of the half-time pinion with the extractor illustrated, but the remainder of the timing gear, magneto, oil pump and filter can be left in position if desired.  The matched cases are made from a low-silicon alloy which can be welded provided care is taken to avoid distortion.

Bearing races are fitted after the case has been warmed to 200 deg. C., preferably in an oven, and the crankcase metal is then staked as shown in Fig.7.  Races which are loose in their bores, but otherwise in good condition, can be plated up to the required oversize provided no plating solution comes in contact with the bearing track.  The required interference fit is .002in.; if more the race will contract to such an extent that the runnung clearance is reduced.  An interference fit of .001in for the spindles in the timimg side case is sufficient.  Spindle bores which have been worn out of round due to loose spindles as mentioned above can be bored oversize to take stepped spindles which will have to be made up.  Oil leaks past crankcase bolts are avoided by fitting with some jointing compound, and this also applies to the cylinder holding down bolts.  Sometimes the rear bolts break through into the cylinder feed gallery from which oil under pressure may work past the threads.  Make sure that these bolts do not pass oil by fitting with some jointing compound without blocking the cylinder feed.  Poorly fitted holding-down bolts cause oil to accumulate in the tunnels of the alloy cylinder jacket which eventually shows up as a leak from the front of the barrel below the head joint.

Big End Renewal
Big end renewal is not necessary before up-and-down play exceeds .003in, and crankpin wear by the seperators is not detrimental provided the roller tracks are in good condition.  Inspecting or renewing the big end is, however, a works or specialists's job, and the component parts are not supplied loose.  Oversize rollers are not available and the conrod sleeves are ground after pressing into the rod.  The works supply a completely assembled big end fitted to reconditioned service rods.  Clearance of the gudgeon pin in the small end bush is .001in., and the fitting of a new bush is not beyond the scope of the private owner provided he remembers to drill the oil holes and possesses a 7/8in. reamer.  Side clearance of a big end, felt at the small end bush, is not detrimental.

The flywheel assembly is located from the drive side without being affected by the shock absorber springs; no percepable end float is permissable, and .010in. shims are sometimes used on the drive side of a Black Shadow to obtain perfect centrality of the small end bushes in the cylinder registers.  Inner bearing races which are a slack fit on the mainshafts will turn and wear the flywheel boss away.  If this has happened the worn boss can be turned off and substituted by a hardened shim.

Owing to their generous dimensions main bearing wear is very low; renewal is necessary when up-and-own play exceeds .0015in. or when a bearing feels rough if spun by hand.  It is surprising how a little roughness in one main bearing may render an engine noisy.  When reassembling note that each roller race is refitted to its own outer ring.

Lubrication System and Clutch
The condition of the teeth on the oilpump plunger can be observed without dismantling the pump; excessive wear on these teeth indicates that the pump has beeb overloaded, possibly due to an obstruction oin one of the oilways.  If the plunger requires renewal the oilpump sleeve must be replaced as well, as these matched components are supplied only in pairs.  Use a ring spanner to unscrew the oilpump cap and a stout screwdriver to remove the locating screw, which is normally punchlocked.  The plunger is tapped 1/4in B.S.F. for withdrawal and the top end of the pump sleeve 5/16in. B.S.F.; to extract the latter part a crankcase bolt can be used, but it is necessary to dismantle the driveworm first.  Earlier oilpump worms were made from bronze, and if these are worn it is better to fit the current steel pattern.  Wear on the actual pump is usually very low, and provided the drive parts are in good condition it is rarely necessary to withdraw the sleeve.  When reassembling use jointing compound on the cap as well as on the locating screw, as air leaks here reduce the scavenge capacity.  With banjo washers in good condition there is no need to tighten the light alloy banjo bolts to the point of fracture.

The fitting of a new oil filter element is advisable and all external pipes must be clear with the rubber hose in good condition.  If the earlier type inspection-cap spanner does not fit the filter chamber cap the outer periphery of the tool can be ground down without detriment to allow for slight variations in crankcase castings.  Damage to the taper of the chain oiler calls for replacement of the screw, which is a standard Amal carburetter component.  Swilling the oiltank with degreasing fluid will release sludge from the trap at the rear, and as the stop valve gauze is surrounded by a shroud this job cannot be done with the tank in position.  The big-end quill must be scrupulously cleaned; signs of slight rubbing of the quill in the timing side mainshaft do not matter provided the quill is not weakened.

Clutch Lifting Mechanism
Normally, only slight wear takes place in the clutch lifting mechanism, and the standard length of the pushrod is 12 5/16in. If worn prematurely the rod can be divided, hardened where cut, and refitted with a 1/4in. dia steel ball in the middle.  When cutting the rod make due allowance for any wear which has already taken place, as well as for the diameter of the ball.  A worn lever can be built up with the Stellite or similiar process if no replacement is available.  Clutch linings must be free from oil and the seals in good condition. Duron moulded linings which are groved (see Fig.8) may powder to some extent.  They may be washed in clean petrol, roughed up and the grooves scraped out.  A new bonded carrier seal is always necessary and if not readily available a hard copper washer of the same thickness and o.d. makes a good substitute.  The leading clutch shoe-lining edges must be well chamfered to avoid fierceness in operation and steel plates which show signs of distortion should be refaced on a surface plate. Clutch shoes which are badly worn on the pivot holes cause judder rather reminiscent of a chain jumping a sprocket in bottom gear.  If rebushing is not possible the shoe must be renewed.
Gearbox and 500c.c Models: Final Instructions
It is rarely necessary to dismantle the gearbox to cure jumping out of gear; trouble of the nature can nearly always be rectified by careful adjustment of the selector mechanism, the fitting of the later type pawl carrier centralizer, one-piece footchange lever or new selector pawl spring.  The box must be dismantled, however, to split the crankcase, although new oilseals can be fitted without disturbing the internals.  Both seals are identical, of the spring-loaded synthetic rubber pattern and fitted with the spring and the lip facing inwards.  Access to the gearbox is gained by removal of the clutch, primary drive, dynamo and kickstart cover.  This exposes on the left-hand side the gearbox cover plate (Fig. 9) and on the right-hand side the kickstarter ratchet parts are pulled off the shaft after removal of the Seeger circlip.  After unscrewing the eight retaining setscreews (locked by tab washers) the cover plate is released by a tap with a copper mallet on the right-hand end of the clutch shaft.  The plate is then withdrawn complete with shaft and constant mesh pinion and this exposed the other gears which are still retained by teh camplate spindle.  A large screwdriver is neededto unscrew this part (usually punchlocked) from the top of the driveside crankcase and it is tapped 1/4in. B.S.W. to facilitate final withdrawal. The gearbox internals can now be taken out with the exception of the final drive shaft; the third gear pinion can be examined in situ.

Rounding of the dog clutch teeth does not matter so long as the gear changing is not affected, but gears with badly pitted or chipped teeth must be replaced.  Side clearance of the selector forks should not exceed.010in. and selector pins should be renewed or turned in the forks when they have worn flats more than .005in. deep.  A camplate must be renewed when clearance between the slots and selector pins exceeds .020in., backlash on the bevels can be taken up by shimming.

For reassembly the gear cluster is put together on the bench with the double gear to the right and the large pinion away from the operator.  On the layshaft the order of assembly by the number of teeth on each of the four gears is 29, 23, 18, and 27, the two middle gears being embraced by the grooved selector fork. The camplate is placed on the selector forks with the bottom gear notch in line with the selector bar and nearest to the assembler and the cluster entered into the gearbox shell.  Check the indicator lever is back in bottom gear position before the camplate is fully pushed inwards, refit the camplate spindle and complete the reassembly of the box in the reversed order of dismantling.

With the exception of the gearbox, practically all the foregoing is equally applicable to the 500c.c. machine.  The idler gear drives the dynamo on which the pinion is tapped for withdrawal with the extractor shown in Fig. 10.  Note that on 500c.c. models the slot in the breather spindle does not face forwards, but downwards.  The single-row primary chain has a spring link, but  the shock absorber is identical to the one used on 1,000c.c. models; there is, however, a spacer behind the sprocket.

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