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).

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.