The Vincent Gearbox

Vincent Gearbox


The entire primary and dynamo drives are dismantled to get at the driveside cover plate, which is held by seven set-screws (279) and one similar part (G99), the hexagonal head of which is drilled to take the locking wire to the adjacent indexing plunger cap. The cover is also located circumferentially by a dowel at the bottom; this must not be left out during assembly. It is also necessary to detach the kickstarter parts from the other end of the box. A tap on the kickstarter end of the shaft frees the cover plate and allows the hollow input mainshaft to be withdrawn, complete with the 18 tooth constant mesh pinion (G6), which is assembled as a fixture on it. Symptoms similar to those of clutch slip occur if this pinion becomes partially free. It then rotates, seizes and then cools and frees again, repeating the procedure as a cycle.

It is now possible to inspect some of the contents of the box without removing the horizontally-disposed template, but for serious work, the plate and spindle (G34) will have to come out. Take off the dynamo. The spindle is machined with a screwdriver slot and with an internal 1/4-in. Whitworth thread to aid extraction. The surrounding casting will be peened down to secure the spindle, or there may be a tab-washer.

Inspecting The Gears

The seven gears can now be removed, so may the selector forks and the spindle on which they slide, the camplate, its associated bevel gear, and the layshaft. Remaining in the box are the final-drive shaft and 20-t. third-gear pinion (G12) held by the output sprocket nut. Normally, it is not necessary to extract this shaft for examination. Within it are two porous bronze bushes which can, with ingenuity, be extracted and renewed with the shaft in situ. Test for wear by rocking the right-hand end of the clutch shaft where it projects beyond the pivot plate.

Ball-journal bearings are used for all shaft support. Standard heating drill suffices to extract the mainshaft bearing at the output end (G14) but the layshaft one (G15) is in a blind hole. It is necessary to bend a tyre lever to a hook-shape. Grind down the end to fit into the bore of the bearing, and heat and quench the tool to toughen it. Heat the case. Some patience is required to prise this bearing out but the tool is a considerable help, and a new part goes straight in to the still hot case and must be tapped home squarely.

The two bearings in the clutch end of the box are normally only supplied as a complete assembly with the cover plate. The layshaft bearing (Gl5) is inserted from the inside, while the input shaft revolves in E91, offered up from the outside. This bearing is retained by a threaded ring (G45), outboard of which is a garter-type oil seat (G24). Failure of G24 will allow oil loss from the gearbox into the chaincase and it may also work along the clutch shaft splines into the clutch.

Wash out the box and set the gears up in a sub-assembly. Note that there is a 1/32 in. shim between the constant-mesh pinion on the clutch shaft and the bearing in the cover plate. This shim is not a thrust washer. It had been intended as a chip guard for the bearing but such a shield was found to be unnecessary and, rather than make the pinion 0.030 in. thicker to compensate for the omission, and so create service problems, the shim was retained with a reduced outside diameter.


Fit the selector forks into the grooves in the gears, the selector (G29AS) being outermost, slide the spindle (630) through and lay the camplate in position on top so that the bottom gear notch is in line with the spindle, i.e. bottom gear is selected. The camplate is, therefore, turned anti-clockwise as far as possible when viewed from above. Set the bevel on the operating shaft (640) for bottom gear by rotating it anti-clockwise when viewed from the right-hand end.

Now the sub-assembly may be entered into the shell. It is a help if a metal rod is slid down the inside of the layshaft and into the layshaft bearing in the gearbox wall to assist alignment. Finally, push the clutch shaft right home. The job is simple, though it requires some dexterity. Do not attempt to introduce components one at a time; use the sub-assembly technique. Insert the camplate spindle, fit the indicator lever and check for gear selection.

Smear jointing compound on the face of the cover plate and offer it up, seeing that the dowel enters fully. Use a new tab-washer on all the 1/4-in. screws except the one adjacent to the indexing plunger. The tension of the spring acting on the indexing plunger, which mates with the notches in the camplate, can be varied by adding or subtracting washers from under the bolt head.

An extra washer makes a softer gearchange but predisposes the box towards poor gear holding. Too much spring pressure makes the gearchange needlessly stiff, and can make gears 'overshoot'. Wire the bolt head to the screw with the head drilled for the purpose (699).

Selector Mechanism

A weak plunger spring penmits the camplate to pass a desired position under momentum; a few of the early camplates responded to deeper notches for the indexing plunger. The neutral notch can advantageously be deepened if the machine is used much in traffic. But maintain a smooth profile or it may be difficult to get out of a notch once selected. Difficulty in getting into first gear can be caused by a slack camplate spindle.

To gain access to the ratchet, take off the kickstaner crank and gear lever. Remove the inspection cap (ET24/3) and take out the clutch cable by pressing in the actuating lever with a hammer shaft. Remove the 5/16-in. stiffnut and the cheese-headed screws. No washers are used except for a plain one under the stiffnut. Unscrew the 1/4-in. set-screw securing the indicator lever and pull off the cover. No oil should be present unless a seal or bush has failed. The clutch mechanism comes away in the cover except for the push-rod which may be pulled out of the hollow clutch shaft.

If the kickstarter return spring (687/1) is prised off, the quadrant complete with captive shaft, can be readily slipped out.

The final drive sprocket retaining nut (623), has a normal right-hand thread but is punch-locked into the splining so that it cannot unscrew of its own accord. By prising out the pivot plate, the saucer-shaped pressing (G51), the felt seal (652) and cup (622) are exposed. Removal of the nut allows the sprocket (621/21 or 22) to be pulled off by hand.

The selector arm, operated by the pedal to push the ratchet through the desired degree of rotation, has been available in three forms. Originally produced as an assembly under the number G66AS, it was prone to spline stripping; a one-piece item overcame this trouble and is numbered G66/2AS. G66/1AS was a lightened version.

Return movement is effected by strip-steel springs 670 and G71. A new version of the mechanism was brought out in 1953, with stops to limit the travel; this part (661/1) is interchangeable with the predecessor. Pawl return spring 664 is similar to 670 but not identical.

The Indexing Plunger

A check on the indexing plunger and camplate notches can be made from this end of the gearbox. Rotate the rear wheel until a position is found in which the indicator lever can be moved from bottom to top gear and back again in one smooth motion without the gears having to be rotated at all. Select bottom gear. With the heel of the hand jab at the indicator lever which should then 'sail' through to second, its progress being arrested by the plunger dropping into the second gear-notch in the camplate. Repeat for all changes. If this condition is obtainable and yet, when the pedal is suddenly and sharply moved to make a change, the gear is not correctly meshed, then the box is either under-changing or over-changing, depending on how far the indicator lever has moved.

Before replacing the cover, grease the components, especially on the working and bearing suifaces; do not oil. The cover should slide on smoothly. If it does not then the indicator shaft may be out of line with the hole in the cover casting. This points to the pivot plate being bowed, but rectification should not be attempted until the condition is verified on the road. It will usually show up by the gearchange being free when cold and stiff when hot.

If this is so, the distance piece, formed by the battery carrier tube shrouding and footrest hanger bolt, must be lengthened. A nut and lock-nut are provided for the purpose immediately behind the pivot plate. Turning the nut anticlockwise will lengthen the tube and so push the pivot plate outwards.

All series 'B' machines and those of the 'C'-type fitted with linkage gear pedals, can have an improved gearchange by fitting the one-piece magnesium-alloy pedal, which is more accurate in changing and also results in less resistance to operation.

Kickstart Mechanism

Worn quadrant teeth can be reshaped by grinding and if the splines accepting the k.s. crank are unserviceable, a new shaft can be pressed in. On the outboard side is a loose steel distance piece (G86); do not omit it during re-assembly.

Two bushes carry the starter spindle, and both are a simple press fit and subject to considerable wear.

Kickstarter springs are prone to stretching, evidence of this being given by a gap in the central coils. It is possible to replace this spring through the inspection cap with the aid of suitable wire hooks.