Remove all traces of old gasket or sealant form the casing face.

Pop the casing in an oven at approx 150ºC. Put the main bearing and layshaft bush in the freezer.

Once at the correct temperature, the bearing and bush should both drop into the casing (or go in with a light tap).

Use Loctite 603 to fix the layshaft bush in place.

If you have the later type of casing with a locating hole for the layshaft bush, make sure the machined recess in the bush lines up with the hole as shown here.

Check that the face of the layshaft bush is proud of the casing. Some replacement layshaft bushes are too thin and will cause the layshaft gear to touch the casing.

Put some thread lock onto the thread of the selector pivot shaft and screw it in firmly.

Don't put the plunger bolt in yet; that comes later.