Bee-Za... Julian's picture also shows the oil hole drilled in the gear. Locate the pinion gear on the splines so this hole coincides with the corresponding drilling in the layshaft. Do the same with the remaining sliding gears.
The fixed pinion gears are a tight press fit on the shafts. When correctly located (circlip acts as a stop) the spline on the shaft end should be level, or more usually very slightly below the face of the gear pinion. This allows the pinion face to act as a bearing surface against the blind bush top hat, in the case of the layshaft, and as a location against the inner race of the smaller ball bearing in the case of the mainshaft. Swing arm and plunger box layshafts have the circlip in different places, S/A circlip location stop groove is at the tip of the drive dogs, see bergs previous note, and as shown in Julian's picture. Plunger circlip is at the base of the dogs, adjacent to the gear teeth. The layshafts are machined with the locating groove holding the circlip positioned to match. S/A layshaft also has a scroll on the blind bush bearing surface, shown above, the plunger type is plain so you can tell at a glance which is which.
The layshaft you have shows a typical wear pattern. The remedy here is a new layshaft (rare/expensive), or re machining the bearing surface and a custom underside bush. In view of the problem with the loose fit of the pinion gear, a better used layshaft complete with pinion is another option.
The selector forks fit back to back, and it is easier to assemble the gearbox (Camplate set to neutral) by loading the complete layshaft and its selector fork but less the big bottom gear into the case first, positioning the selector fork against the camplate, followed by the mainshaft, gears and its selector fork. With both forks located in their respective tracks in the camplate, the fork shaft can then be added, but make sure the grub screw is removed completely. The grubscrew location cracks off when folks hammer the shaft into place, and the grubscrew is obstructing the hole. The selector fork shaft should slide in with little force when the selectors and case line up. Leave the grubscrew as the final finishing touch.
Swarfy.
