The problem... It is not easy to time the gears correctly when replacing the end cover..

Method:
1) Ensure all the gears are assembled into the gearbox shell correctly and the quadrant assembly is assembled correctly into the end cover.
2) Stick the end cover gasket (#C11905) to the gearbox shell with non hardening jointing compound such as Hylomar Blue or Gold.
3) Stick the cam barrel bearing thrust washer (#27598) onto the gearbox shell bearing face with high temp grease, to hold it firmly in place during the assembly.
4) Rather than use the method of aligning timing marks as described in the Workshop Manual, it is easier to assemble the end cover when top gear is selected.
To time the gears correctly for assembly, think of the cam barrel and the gear change quadrant spindle as two meshing shafts. They rotate in opposite directions. So, rotate the cam barrel fully anti-clockwise. Then with the end cover in your hand (gear lever fitted) apply the thumb of your holding hand to the quadrant to excerpt some pressure, repeatedly down shift the gear lever to position the quadrant in the top gear position.
5) Carefully position the end cover and gently work it into place.
6) Insert two opposing screws lightly and test for all four gears.
7) If all is OK, insert the remaining screws and tighten diagonally and evenly.
8) Replenish the oil. Silkolene Medium Weight Gear Oil 85W 90.
Quantity: 1 imp. Pint (570ml)