Engine Overhaul

Engine Overhaul

The 'Top Half'

Series 'B'and 'C' models should be put on the front and rear stands and blocks placed beneath the gearbox so that the rear springing is in the no-load position. A series 'D' machine should be placed on the centre stand and the crankcase shored up until the front wheel just remains on the ground but carries no weight. Enclosed 'D' models must have the cowlings detached. Start by disconnecting the wiring to the rear lights and unscrewing the oil-pipe unions. The rear cowl is taken off by slackening the nuts on the C-clamps at the lower front corners and pushing out the top pivot bolt. The skeleton rear frame of an uncowled 'D' model is removed similarly.

On 'B' and 'C' machines, remove the dual front seat bolt, disconnect fuel lines, remove the tank and note that there is a tie-bar between two lugs at the base of the tank, adjacent to the taps. Except on 'D' machines, it is necessary to take off the oil filler cap. Swing the tank forwards and upwards to avoid the head-clip. Do not allow the detached tank to rest on the petrol taps, the threads are ¼in. B.S.P. formed in brass and may be damaged.

Unscrew the rings to the carburreter mixing chambers, withdraw the slides, and tape them and the cables to the handlebars, being careful not to damage the needles. Disconnect the wiring and remove the battery. The Miller dynamo is connected to the loom by a simple plug, but the Lucas instrument used on 'D' models is fastened by a screw and plate over the twin leads. Detach the lower ends of the control cables - clutch and valve-lifter. On the 'B' and 'C' range, remove the feed and return pipes from the oil tank-cum-upper frame member and also the bleed pipe to the rear chain. Note that a stop-valve is used in the feed union and it is not necessary to take steps to catch oil. If the stop-valve drips, the nut from the base of a float chamber can be screwed on.

Three Major Units

Slip out the forward rear spring-box spindle and from the upper frame member on all except 'D' models, drift out the two bolts that hold the member to the head brackets. There is, of course, no head bracket at the rear on the 'single' but a tie-piece in the form of a light-alloy casting. The rear bolt has a large brazed-on flat head, the front one being really a spindle with a nut and lock-nut on both ends.

This spindle passes through a pair of 'top-hat' bushes that unite the frame-member lugs to the front head bracket. Tap out the steel bushes so that the upper frame member, complete with front wheel and forks, can be prised off and wheeled away. The rear fixing consists, in part, of an elongated hole; there is no need to mark the situation of the bolt head in relation to this; on replacement, the nut is fully tightened and then slackened back one sixth of a turn before the locknut is screwed tight.

Removing the rear frame is simple. Detach the rear chain and left footrest complete with brake cable. Unscrew the ½in. B.S.F. pivot bolt nut and drift the bolt out. The machine is now in three major units, all accessible for further attention.

Dirty deposits within the oil tank, trapped by the gauze filter above the stop valve (T32AS), can be difficult to remove after heat from the engine has 'cooked' it. Hot paraffin is a good 'starter' - beware of the fire risk; or a boiling solution of household detergent can be used. Finally rinse with clean paraffin and then with petrol. The 'D' tank also responds to this treatment.

The stop-valve can be dismantled, although this is seldom necessary. Washer No. 448 between the valve and the tank should always be renewed; leaks are difficult and messy to cure at this point when the machine is fully assembled.

Heads & Barrels

Remove the cylinder head brackets, noting where each one belongs and which way round it is offset. From engine No.8614 head bracketing was revised in 1951 to fit up with a modified head-lug on the oil tank. (Note that the oil tank is regarded as inseparable from the steering head casting for servicing purposes). On twins the offset mass of the front head bracket lies to the right. The series 'D' design of head bracket is extensively lightened.

Slip the mixing chambers off the stubs, or flanges, marking them front and rear; the manifolds, which are bolted on, may be left in place. Detach the oil pipes to the rocker gear; unions are threaded ¼in. B.S.P. Undo the ¼in. B.S.F. set screws that retain the four rocker feed banjos. Detach the feed and return pipes by removing the hexagon-headed hollow feed bolts (A22). Remove the rocker inspection caps (ET24) and slip the push-rods up through the clearance adjuster holes. Remove the clearance adjusters only when the piston in the cylinder on which you are working is on compression, so avoiding strain on the valve gear. Thin-nosed pliers are useful for getting out the push-rods.

While the cylinder heads are still held firmly it pays to loosen the first thread of the spring-compartment caps (ET24) above each valve stem.

Remove the exhaust pipes by unscrewing the right-hand threaded finned rings. On the twins there is no need to break the joint of the two pipes; remove all piping and silencer as a complete assembly if required. Using the pegs on toolkit spanner (K1) completely unscrew the castellated nuts lying inverted at the top of each push-rod tube. Lower the tubes until the flanges clear the heads. Beware of distorting the tubes near the lower seals (ET104) so causing oil leaks later. All series 'B' and 'C' tubes are of stainless steel - preferable to the light-alloy 'D' type.

Slacken the four extended nuts that secure each head to the barrel by the through-bolts. Break the joint with the barrel and lift off.

Unsrew the bolts pinching up the mouths of the cylinder registers and slide the barrels out; both are deeply spigoted. Do not tap to loosen; the fins are soft and give way easily. Stuff rag into the mouth of each register as soon as the spigot clears and before the piston is out of the sleeve, thus trapping any pieces of broken ring. Wooden 'button sticks' support the pistons neatly and save the skirts from being damaged.

Gudgeon Pins & Pistons

Circlips are of the round-wire type and can be prised out to release the gudgeon pins; the pins should be a light push fit when cold. Tight pins are freed by heating the pistons, either by placing a domestic iron on the crown or by wrapping them with a rag soaked in boiling water. Another dodge is to place the lid of a boot polish tin on the crown and burn in it a little solid fuel of the picnic stove type. Mark the pistons. The front cylinder is regarded as No. 2: the larger valve cut-away is used on the inlet side. The respective barrels should also be marked No. 1 and No. 2 but the presence of an oil feed at the rear of each, plus the push-rod tube flats, show which way round they must be assembled.

Slip a new paper cylinder-base gasket over the studs. No jointing compound is used. If compression plates have been fitted it is not advisable to dispense with them unless the inevitable ridge formed at the limit of the top ring travel is removed. Failure to trim this away will cause ring breakage.

Scrape carbon from the piston crowns, but not from the sides above the top ring groove. The crowns should be polished. The usual checks for ring serviceability apply. Chromium-plated top rings may be fitted if required. Although pistons and barrels have been graded into 'A', 'U' and 'S' sizes, little heed has been paid to this by reconditioners since manufacture ceased. Engines subsequent to No. 3716 had graded pistons and barrels. Pistons are of the solid skirt type with 0.006/0.0065 in. clearance at the bottom of the skirt. Low-clearance split-skirt pistons have been fitted privately but not always with satisfactory results. The standard 'Rapide' compression ratio is 6.8:1, the 'Black Shadow', 7.3:1. Although both 8 and 9:1 pistons giving yet higher compression ratios are available, it is not advisable to go above 8:1 for normal road work even when 100 octane fuel is used.

New Liners

Liners may be changed when the ovality exceeds 0.010 in. Heat the complete barrel and the muff will fall free by its own weight. A new liner must be pressed right home until the flange abuts on the muff top. The oil hole must go at the rear and should mate with the gallery in the crankcase; the size is 3/32-in. and it is permissable to countersink the outside of the hole to get alignment.

Cylinder liners are supplied either bare and in muffs, the later as service exchange items. Oversizes up to +0.030 in. are available. Before refitting, the heads must be ground to the barrels. Smear coarse jointing compound on the outer flange and fine on the smaller and higher inner one. Work the faces together until a matt grey is produced on the complete inner flange and on 75 per cent. or more, of the outer one. Scrupulously wash away all traces of grinding paste.

Small-end bushes

Small-ends are unlikely to need renewing before the big-ends. Anything in excess of 0.001 in. gudgeon-pin play is regarded as excessive. A 7/8-in. nominal reamer should cut to size in a new bush; drill two 3/16-in. oil holes first.

Drop the barrels into place after lightly oiling the bores, piston rings and gudgeon pins. Because of variations in castings, barrel finning may trap the push-rod tube gland nuts (ET127) if these are not raised before the barrels are fitted.

It is not necessary to remove the rockers to change the valve springs or grind in the valves. But, assuming that the rockers are to be serviced, commence by unscrewing the locating bolts (ET231), using a spanner slipped over the hexagon. This hexagon comes very close to the head and provides insufficient purchase for the average ring spanner. Pull out the feed bolts, whereupon the rockers will be freed. To extract them it may be necessary to grasp the end of each rocker with a pair of pliers and tap the pliers away from the head. If the rockers are to be removed it is easier to take them out before the heads are off preferably with the engine hot.

Screwing in a 3/8-in. C.E.I. bolt part-way into the rocker eye so that a twisting action can be applied, helps to extract a stubborn rocker. The rocker bearings are not designed to have an interference fit in the tunnels and should not be difficult to extract.

The rocker is mounted on a hardened and ground captive steel pin which is a force fit in the rocker fulcrum and a running fit in the Duralumin housing. Any slackness of the pin in the housing indicates need for a new bearing. Vice pressure is sufficient to insert a new pin after the old one has been knocked out. It must be pressed in so that neither side of the pin protrudes past the bearing (ET26/1).

Putting back each assembly is simply a matter of lining up the bolt hole in the bearing with that in the head and tapping the bearing home until the bolt screws right in. No washer is fitted under the bolt head. Make certain that the rocker fork goes over the valve-stem collar by holding the rocker eye tilted down as it is driven home. See that the bearing and tunnel are clean; swarf must not enter the two rocker bearing oilways that carry lubricant to the pin.

Each valve assembly is contained in a well, closed at the top by a cap (ET24) and open at one side for the rocker fork to engage on the top of the collar (ET35). To dismantle, simply compress the springs by means of the light-alloy collar (ET37). Then prise out the two collets (ET36). Detach the spring collar and the inner and outer valve springs; triple valve springs were not used on 'Black Shadows' from engine No. 5337.

Wire circlips (ET38) can be eased off with a pair of pliers and discarded; new ones should always be fitted. Tap the tip of the valve gently and the valve will come out leaving behind the upper valve guide (ET40) and the hardened collar. The upper guide is the lightest of push fits and may even come free as the valve is extracted.

The rocker thrust collar should be a force fit on the stem. Much marking of the top of the rocker feet indicates need for a new component. Renew also if there is wear at the base of the collar where it touches the step of the valve stem. Wear at this point is the commonest cause of lost valve clearence adjustment, though, as a temporary expedient, an overlength push-rod (factory spare) may be installed. A loose collar must be discarded; so must a valve it the stem is below standard diameter.

Work On Valve Guides

Worn valve guides and stems are assessed by putting back the upper guide and slipping in the valve. Very little valve movement should be detectable. The upper guides alone may be renewed. Valves and guides should last for 50,000 miles or more. Valves are not interchangeable, the inlet being of larger diameter and of a different material.

Attention to the lower valve guides is really a specialist job. A service tool is needed to undo the locking ring and heat is used for a guide replacement. New guides are usually line-reamed after fitting, so renew the upper guides as well.

Oiled plugs - A cure

Persistent oiling-up of the plug indicates the need for special inlet guide renovation - assuming that the other more obvious causes, including worn piston rings or excessive feed rate, have been investigated. In this case have a new set of inlet guides fitted, specifying that they are not to be reamed. Chamfer to top so that the centre is the highest point; avoid any sort of cavity or concave shape likely to gather oil. Then lap in the valve, using metal polish, and continue the process until it is just about possible to move the valve freely without any trace of side play. This is zero clearance without the valve sticking.

The second essential is to provide good drainage. Aroung the top of each lower guide is a well; with a rounded file, break into this well and, according to the valve angle lead the lubricant so that it can flow by gravity to the rocker tunnel. Groove the rocker gear bearing at its lowest point in a line parallel with the axis of the tunnel and, therefore, between the two existing grooves. It should be possible, when the machine is upright, for oil to flow by gravity from the valve stems to the push-rods. Sufficient lubricant reaches the valves and guides by mist.


Valves may be cleaned by immersion in a caustic-soda bath or by rubbing with emery cloth. Assembly demands nothing more than tapping down to seat the rocker thrust collar before the upper valve guide is offered up. After complete assembly, hit the valve-stem top with a hide hammer, hard enough to jar the valve off its seat and so test the seating of the collets against the circlip. New collets are supplied in half-broken pairs; they should be parted and the rough edges trimmed up.

The cylinder heads can be readily cleansed of carbon with the aid of a scraper made from solder or Perspex and cut to the radius of the hemisphere. Detach the exhaust port sealing rings (ET160) and renew. Note that although some heads are black, they are not made of iron and must not be immersed in caustic-soda solution.

Valve-seat angle is 30 degrees and not the more common 45 degrees. This results in increased gas flow at low lifts. Usual grinding and lapping techniques apply. Deep pocketing effectively shrouds a partly-opened valve. Very badly worn seats can be turned out and renewed by a Vincent specialist or a service-exchange head obtained. Some new heads started out with a +0.010 in. seat dimension, shown by stamping this numeral within the hemisphere.


Before offering up the heads, fit the push-rod tubes - if they have been taken out - and use new upper and lower seals. The latter (ET104) can be particularly troublesome. Enter a tube, place the chamfer squarely into the seal and hit the flanged end smartly with a hide hammer.

The head nuts can be tightened to 30 lb.-ft. with a torque wrench but the average private owner can pull them up fairly hard with a short ring spanner. Note that a large washer goes under each. Always renew the push-rod tube upper seals. Spanner K1 is equipped with pegs for tightening the gland nuts. When the heads are in place the push-rods may be dropped through each rocker eye into the cam follower cups - which is easier said than done! Thin-nosed pliers help. Turn the engine over gently, without the valve clearance adjusters fitted when the push-rods, if correctly seated, should rise and fall easily in the eyes. If they are not seated they will not rise, and moreover, may jam between the cam followers and other contents of the timing chest.

If subsequently withdrawn, the ball-ended push-rod may be found to be 'chewed'. Dress the marks out and replace the rods. Self-hardening stainless steel is used and a fresh working surface is quickly formed.

Adjusting Valve Clearances

Set the piston on compression, so locating the followers on the base circles of the cams. Screw down the adjuster until no up-and-down movement is apparent at the rocker eye yet the push-rod is not nipped sufficiently to prevent it being spun by hand. This condition is known as zero-clearance. Do this work when the engine is stone cold; a hot motor will develop a little clearance. There is a tendency for the front exhaust setting to need attention more frequently than the rest - at intervals of something like 4,000 miles.

'Klingerit' washers are better for oil-sealing than the old-type copper-asbestos variety. Whenever the rocker oil feed banjos have been taken off, renew the 'Klingerit' washer around the feed bolt and the copper washer (ET188) under the retaining setscrew.

'Joggle' Wires

Many engines have a thin piece of wire slipped into the oil hole in each inlet side feed bolt. It has been described as a joggle wire and its purpose is to restrict the flow of oil to the rocker and the valve and so minimize risk of plug fouling.

The diameter of the feed bolt oil hole is 0.031 in. and the standard joggle wire 0.020 in. Thicker wires may be fitted. None is necessary on the exhaust side, where excess oil cannot reach the combustion chamber.

Difficulty in screwing down the inspection caps is generally traced to the adjuster in question fouling the inside of the cap: this happens if you do not keep the cylinder on compression while working.

Valve Gear Modifications

There are two methods of dealing with loose rocker bearings. Knurling is rarely satisfactory; it is the tunnel that wears rather than the bush. Hold the bearing in a vice and drill through close to the edge to take a taper tap. Replace the bearing and, with a grub screw in the taper-threaded hole, force metal out against the tunnel wall.

A better method is the positive locking of the bearing to the tunnel in line with the direction of the push-rod/valve-spring thrust - that is upwards. Turn up a top-hat bush from phosphor-bronze or a good steel, to fit into the feed-bolt hole at the top of the bearing. File a flat on each side so that this bush can be inserted from the inside of the bearing. Thread the bush vertically ¼in. B.S.F.

Discard the standard feed bolt and replace it, and the banjo retaining bolt, by a long, ¼in. B.S.F. stud to pass right through and thread it into the top-hat. Do not over-thread this or it will foul the rocker at maximum angularity. Obtain a B.S.A. collared nut, threaded ¼in. B.S.F., and drop it into the feed bolt hole so that its flange rests on the top of the head. This nut, as it is run down the stud thread, does not damage the 5/16 in. thread. A reversion to standard parts later is possible. As it tightens it pulls the rocker upwards against the roof of the tunnel. File a groove to allow oil to pass. Replace the fibre washer, the banjo and the copper washer and tighten down with an acorn nut.

It is possible to quieten the valve-gear appreciably by riveting a fibre disc, 1/16 in. thick, to the large idler wheel.

Light-alloy, high-expansion push-rods have been tried by private owners. These are made from suitably strong tubes into which are pressed hardened ball-ends from Velocette push-rods. Because they cannot be threaded through the rocker eyes, it is necessary to remove these to fit them and if too thick they will foul the cylinder head metal.

The Crankshaft Assembly

Two fully-circular flywheels are used on the twin with pressed-in main-shafts and a common crankpin. Location is effected by the assembly being pulled across to the drive-side by the mainshaft nut at which point the axial thrust is taken by a ball-journal bearing. Three other bearings are parallel roller components.

The cylinder angle for post-war twins is 50 degrees and the two rods are mounted side-by-side, the drive-side being for the front cylinder, and, when the cylinders are viewed from the front or rear, they appear staggered.

The average owner can do most jobs but splitting the flywheels is not one of them. The tools needed to get the flywheel nuts undone and tightened again are beyond the resources of the ordinary workshop. Moreover, the 3 mm. by 5 mm. big-end rollers are not supplied in oversizes. Extensive damage can be done to the complete engine-gearbox unit by the careless or unknowledgeable owner.

Dismantling Routine

To split the cases and reach the crankshaft it is necessary to remove the clutch, primary drive, gearbox cavity contents and the timing-side main-shaft nut. Though it is possible to do the job with the timing gears and magneto in position, these parts, too, are best dismantled.

The cases are held together by sixteen bolts or studs, the location of which can be usefully marked on a sketch as they are withdrawn. Behind the front cylinder on the singles, and on each side of it on the twins, is a stud which does not pass right through but is screwed into the timing-side case from the drive-side. It can be left in place.

Careful work with a hide mallet will seperate the cases, the flywheel assembly remaining in the drive-side half because there is a light interference fit between the drive-side mainshaft and outer main bearing (ET91), ball-journal. Three dowels align the cases. They should be captive and not removed.

Main Bearings

To change the main bearings, peen back the indentations around each roller outer track, heat the case to 200 degrees centrigrade in an oven or by blowlamp and jar the tracks out. Remove the outer clip (ET125) locating the ball-journal bearing; drift the bearing outwards from the inside and an new one inwards. The outer track of the inner drive-side main bearing (ET92), roller is pressed in with a light-alloy distance-piece (ET19) firmly sandwiched between it and the inner circlip. The outer race of the inner timing-side main bearing is obstructed by the oil-pump body. Withdraw the body by once more judiciously applying heat and using one of the 5/16-in. B.S.F. bolts as an extractor. The pivot plate will have to be removed to get the sleeve out. Replacement is effected under heat, pressing it in up to the flange and taking care that the holes line up.

The oil-pump worm is driven by two means - the Woodruff key fitted and by its being trapped between the flywheel and the timing-side shaft nut, the two main bearing 'inners' act as distance pieces. The inner timing-side main bearing is also ET92, as for the drive-side, but the 'outer' is ET94, a smaller component.

Essential Rigidity

When inspecting flywheel parts for serviceability make sure that the bearing bores are a snug fit on the shafts, that the shafts are rigid in the wheels and that the big light-alloy distance-piece (ET19) on the drive-side is not a sloppy fit with the oil thrower (ET77). If the last-mentioned has too much clearance, the reverse-scroll action to throw oil back into the crankcase will not occur. It is vitally important that the distance-piece is firmly trapped between the circlip and the outer track of the inner bearing.

Factory-serviced flywheels are returned with the shafts trued to a total run-out of less than 0.0015 in. Shafts are secured by a Mills pin arrangement and are supplied as spares. A caged big-end assembly is also available for owners intending to race or use revolutions in excess of 5,800 r.p.m. For road work, the standard uncaged big-end assembly is perfectly satisfactory. The choice between the two is quite clear cut. Some troubles with proprietry big-end assemblies have been traced to the oil holes being incorrectly disposed; this cautionary note must also be extended to those fitting a new timing-side mainshaft; shafts require a pressure of more than 2.5 tons for fitting.

Big-end Renovation

Big-ends are tested for wear by the traditional up-and-down pressing method. A total movement of 0.001 in. is to be expected but anything more than 0.003 in. shows need for a new assembly. Check with an oil-free bearing. There are many instances of owners dismantling, finding some up-and-down play within these limits, putting the engine together again and then satisfactorily covering 30,000 miles or more before taking another look. Rock at the small-ends should not be confused with big-end wear.

Assembly Work

It is advisable to centralize the connecting rods in the bores to within 0.010 in. and an engineer's steel rule is good enough to achieve this. If the rod is found to be too close to the drive-side, the crankshaft can be shimmed over towards the timing-side by inserting a hardened steel disc of the right thickness between the flywheel boss and the inner track of the inner drive-side main bearings - and vice versa. Various thicknesses of shims are supplied for this purpose. Because it is very hard indeed to get the inner track of this bearing off the shaft once it has been pressed on, it pays to make a dummy distance piece. The shaft diameter is nominally 1-in. Do not place the shim(s) outboard of the inner main bearing or the rollers will not run correctly on the track. The track is 5/8-in. wide, instead of the more common ¾in. using a wrong bearing results in the crankpin nuts touching and eventually coming undone.

When assembling the 'bottom half', slip the flywheel assembly into the drive-side bearing first, do not use a paper washer between the two cases but smear non-setting compound along all the joints, including those of the gearbox. It is advisable to coat the crankcase bolts with jointing compound also, to prevent the seepage of oil. If the cylinder studs have been out (their removal is unnecessary) insert the right-hand rear one of each cylinder with jointing compound on the threads formed above the lowermost waisting. Oil is supplied under pressure via this waisting where the bolt passes through the gallery feeding the rear of the bores. Seepage of oil up this stud can produce a mysterious, and apparently incurable, leak at the head joint.

It is not necessary to rebalance the flywheels when changing the pistons of a different compression ratio, even though the h.c. ones are up to three ounces heavier.