Just a thought on the Idler Gear shaft:
It has an oil groove cut into the bearing surface that should be positioned uppermost when the shaft is bolted into the crankcase.
As you cannot see it (the gear is on the shaft by then) Lister cut a groove across the face of the threaded end of the shaft with a machined flat to show which way the top is.
If you don't fit the shaft the right way up, it will still get oil into the gear/shaft region, but it won't carry inside the joint as well as it should.
Peter