The shafts were cleaned in paraffin, bearing seats heated with hot air gun to about 100C and the three main gear shafts reinserted and tapped home with a socket drift. I had a mate (who's gearbox this is) over who helped juggle as the three needed inserted together so the gears meshed correctly. Reassembly was just the reverse of disassembly - I was manky and had no free hands to take many pics, so I have tried to be as descriptive as possible.
This is the point I measured for shimming. Similar method to mikeyboy:
First the case (1). Using two 300mm lengths of rectangular 4" x 1" aluminium box section from ebay (
https://www.ebay.co.uk/itm/192509912855) and a depth micrometer:
I created a 'bridge' with the box section resting on the mating surface of the case. The 4" height (z1) brings the bridge above the height of all 3 bearings. Depth micrometer used to measure the distance (y1) from the bridge to the bearing surface. Height of bearing surface above the mating surface (x1) is calculated by [x1 = z1 - y1]. z1 was measured accurately with digital calipers, dont rely on it being exactly 4".
Then the cover (2), create bridge again with box section on mating surface, depth micrometer to measure distance (y2) from bridge to bearing seat. deduct known height of box section (z2) to get distance (x2) from mating surface to bearing seat [x2 = y2 - z2].
x2 should be larger than x1 by a tolerance of (bmw says) 0.05 - 0.15mm, though I read on advrider 0.06 - 0.08mm is better. Use shims on top of the shafts to bring gap within this range. The shims that came out of this box were pretty close, not quite within 'better' but well within 'bmw' and lacking any to hand to get it closer, I put them back in as they were.
Once the main shafts were in and shimming calculated, the shift forks were reinserted, swung out to the side to allow the shift drum and shift shaft to be put back in the box and tapped home. I had to 'fix' the 4 little rods in to the slots in the bottom of the shift drum with a dab of grease as they kept falling out. Located the tabs on the shift forks in to the corresponding runs on the shift drum and reinserted the shift fork pivot shafts, tapped home.
The metal baffle plates were then put back in, big one just sits in grooves in the case and the smaller one is fixed with 2 little bolts from the outside. Swing the arm on the end of the shift shaft in to position against the sprocket on top of the shift drum. All gears checked for alignment and seem smooth and correct when spinning the input shaft by hand.
Next the mating surfaces were cleaned with a green scotchbrite pad, wiped clean with alcohol, and a bead of anaerobic sealant Loctite 518 (
https://www.ebay.co.uk/itm/142651512831) was run around the mating surface of the case. The cover then heated with hot air gun (specifically around the bearing seats) and lowered on to the case. Once its down far enough, the spring of the shift shaft needs poked back inside with a screwdriver, it catches inside the lip of the case and stays put. Carefully tap the cover down with a rubber mallet, using the metal dowels to aid alignment. This took very little persuasion and tapped on no problem. This was a relief, as I was worried about all the shafts being aligned correctly, but turned out not to be an issue.
NOTE: This being my first attempt, I made a mistake and prior to putting the cover back on, stuck the little ball bearing to the depression on top of the shift drum with a dab of grease. Once the cover was on (not bolted), I rotated the shift shaft while looking through the hole (covered usually with an allen bolt from outside) to check that the ball bearing had stayed put, but it hadn't. I had to take the cover back off, and fish the ball bearing out the bottom of the case with a magnet. This was when I realised that the ball bearing actually drops through the hole in the cover AFTER the cover has been put back on the case.
Once the cover is back on, look through the hole above the shift drum, turn the shift shaft until you see the depression that the ball bearing sits in directly below the hole. drop in the ball bearing, then the spring, and replace the allen bolt.
Reinsert all cover to case bolts, tighten to spec.
Replace the circlip back on the keyed shaft end of the shift drum (behind where the neutral and gear switches go).
Turn the input shaft by hand (i used an old clutch plate) to check for smoothness / binding. All good here. Check that all gears and neutral can be selected, and that the output shaft is turning. Again, all good here.
Won't have time to put the bike back together until one free night next week, then the road test. Fingers crossed.