Timing Gears

Timing Gears


Within a cast-alloy covered housing on the right of the engine are situated the timing gears, cams, breather and magneto drive with, on the singles, the addition of a dynamo drive. The cover carries a small detachable plate at the front, secured by six 3/16-in. Whitworth screws, giving access to the magneto drive so that the ignition timing can be reset, or the magneto detached, without taking off the main cover and the exhaust pipes. The timing unit will not pass through this hole.

At top-centre in the main cover a small hexagon-headed acorn nut shrouds the jet holder for the oil feed to the rear cylinder walls. Detach this nut but not the screwed plug beneath it; no washer is fitted. It is now possible to remove all the ¼in. Whitworth cover screws, noting their positions. It is also advisable to unscrew the big-end feed quill located at bottom centre. In the middle of this is a small plug with a washer, whose purpose is to provide a check on the supply of lubricant to the quill; removing it is not a means of verifying the onward supply of oil to the crankpin.

It may be that the ringed surface of the quill shows signs of rubbing; this is of no consequence provided that there is no sign of waisting that might cause breakage.

To dismantle, unscrew the feed bolt from the filter chamber (A22/1), and lift off the cover, checking that the four spindle seals (ET183) come with the cover. Behind the rubber seats are located oil restrictor discs (from engine No.9238 onwards). These limit the supply of oil to the bores but permit a full supply to the two camshafts. From 1954 onwards the size of the drilling was amended, making discs unnecessary.

The oil-pressure relief valve is contained in the cover. Check that it is working freely. The valve allows excessive pressure during cold running to blow off but, when the oil is warm, the valve remains closed. A valve jammed open will result in oil starvation at the big-ends.

On the twins there is now exposed an alloy steady plate (ET161) behind which are the steel camshaft, a large and small idler, the fibre magneto and the steel half-time pinions. Before engine No.4548, some of the large idler wheels were made of steel or bronze.

Cam-follower feet may show signs of wear. If the grooving is deep, say 0.010 in., the feet may be dressed up by grinding square to the bore and parallel to the original line, bearing in mind that the case-hardening is nominally 0.036 in. deep. Otherwise renew the followers. Slack spindles may be renovated by plating to restore a 0.001 in. interference in the case.

Thrust washers (ET98) are used between the followers and the case and one is fitted also between the front follower and the outside lug. The other followers require washers and distance pieces according to location. The spacers must be free to turn when the steady plate nuts are pulled up, yet the end-float on the followers must not exceed 0.010 in., the parts being arranged so that the feet track accurately on the cams.

Camshafts are mounted similarly to the followers except that the two spindles are hollow and have oil holes, the disposition of which is unimportant. One inlet and one exhaust cam is formed on a common camshaft which is a heavy-interference fit by taper in the camwheel. The entire assembly runs on two bronze bushes which need periodic replacement when wear exceeds (say) 0.003 in. The bushes are used as a mounting for grinding the cams off and their replacement is a skilled job because new ones require careful reaming. The camwheels are of hardened steel. Thrust washers are used only between the pinion and the steady plate. End-float must be present but not in excess of 0.015 in.

Cam Types

Ridged or badly flattened cams must be discarded. Difficulty may be experienced in obtaining new Mk. I cams; the D-type of Mk. 3 pattern are now substituted. Confusion need not exist about the designation of various cams. Mk. I was the original 'noisy' cam; units of this type at first bore no number but later were stamped '1'. From engine No.8343 the Mk. 3 cam was fitted. It has the same timing effect but incorporates quictening ramps. No alteration in performance should occur but some 'Ds' and a few late 'Cs' showed a 95 m.p.h. maximum due to bad valve timing. If that, and not carburation, is the trouble, it is overcome by advancing the timing a tooth - or only half a tooth if carburetter blow-back is then experienced when idling.

The designation Mk. 2 is not an official one but is widely used, even at the factory, to cover the 'hot' cam as used in the 'Black Lightning' and the 'Grey Flash'. Variations of this cam are in existence. All can be used in road models though there is some subduing of the output below 4,000 r.p.m.

It is impossible to alter the relationship between the inlet and the exhaust cams; note, too, that the geometrical position of the camshafts is fixed in the crankcase. But the meshing of the gear train can be adjusted at two points, i.e. In the idler wheel spindle and by the half-time pinion size.

The large idler is mounted on a triangular casting bolted to the crankcase wall. This casting takes two forms; some are an assembly of a light-alloy holder accepting a fixed steel spindle, others, the preferred variety, being a one-piece steel structure. With the latter there is no separate spindle to work loose. This holder can be moved about the crankcase wall to obtain satisfactory meshing with the camwheels, allowing zero to 0.003 in. backlash. If this condition cannot be readily obtained, oversize wheels are available. The retaining nuts are then punch-locked. The steady-plate hole is oversize to permit the adjustment.

Light alloy idler gears run more quietly than bronze or steel ones and all have 79 teeth to give a hunting effect. Because of this about 100 turns of the cngine are necessary to bring marks from alignment back into alignment again. Worn wheels should be renewed. Wear in the unbushed centre is very low.

Meshing Details

Moving the idler wheel upwards to deepen the mesh with the camshaft pinions takes the teeth Out of correct mesh with the half-time pinion (ET49). To compensate, the small pinion is supplied in grades. Backlash against the idler wheel is zero to 0.002 in. Both under - and oversizes are offered but there is naturally no service-exchange scheme.

The pinion is held by a nut with a right-hand thread and it is 2 BA tapped for extraction purposes. Behind it lies the oil-pump worm; some machines permit this keyed worm to be slid out through the outer track of the main bearing, but some do not. From engine No.2991 a steel worm was used instead of a bronze one and all worms are of the single-start variety. Some owners have fitted the two-start Picador target-aircraft-engine type which thus doubles the rate of oil circulation.

The 24-t half-time pinion has five keyways, so offering fine adjustment for the valve timing. The keyway in use is punch-marked on engine assembly.

Meshing with the front exhaust camshaft wheel is a small idler taking the drive to the magneto pinion. This wheel also doubles as a timed breather exhausting through a hollow, light interference fit spindle (ET159/1) fitted into the case and secured to the steady plate by a nut (142) and tab-washer. Four types of wheel and breather have been used of which the one-piece cast-iron type is the least desirable. The meshing cannot be altered. To minimize oil consumption, it is important that the breather port cuts off cleanly.

The magneto pinion, if of the automatic type, is of fibre material and should last 30,000 miles. Pinions are supplied loose for riveting to the timing unit. The magneto is positively located so tooth engagement cannot be varied.

On the singles the breather is a self-contained entity and the dynamo is also driven from the timing gear train. The back camshaft of the twin is used and this should be borne in mind when working out the timing marks. To simplify the spares position, parts are similarly marked for both types of engine. The space marked with a dot on the half-time pinion meshes with the line-mark on the idler tooth. The single dot on the rear camshaft meshes with a single dot on the idler; the front group has two dots and the tooth to mesh with the breather pinion is marked with 'B'. An additional 'B' on the idler is used for the single-cylinder engines.

Valve-timing details are contained in the tabulated matter. Engine builders having difficulty with Mk. 3 cams should resort to a degree plate and dial gauge and ignore the maker's marks. Beware of the quietening ramps with the Mk. 3 cams for there is less ramping on the opening side than the closing. Concentrate on the inlets only and let the exhausts take care of themselves.

A paper gasket is used between the case and the cover and also between the chest cover and magneto-inspection cover.