A final update, I hope, and I shall forego descriptions of stripped threads, gear selector lay shaft pawls incorrectly located, seating outer cover only to notice the kick start ratchet gear and main shaft nut had been forgotten and other such buffoonery.
So at my last posting I thought I had identified the reason for 4th gear not selecting was that the retaining circlip on the drive side had become unseated due to the outer retaining ‘land’ failing and allowing the gearbox main shaft bearing to walk out and consequently the final drive 4th gear following on so the dogs would not engage.
I had the circlip outer ‘land’ tigged up and the circlip groove re machined.
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Although the main shaft bearing didn’t run roughly there did seem a lot of play between inner and outer race and as I can’t think that the bearing location would require C3 clearance in this application which is what the play seemed to equate with I used the opportunity to replace it. This was done by taking advantage of the generous offer by A_HRD of a shielded bearing.
So all machining done the bearing freshly out of the freezer was dropped into the heated case and.....it would not rotate as it became apparent that the gear case wall that acts a stop/seat for the bearing had slightly mushroomed out and fouled the inner race. To make matters worse on closer inspection there were hairline cracks radiating from the centre main shaft porthole in the case; b*gger!.
So head scratch time. I cant see a reason for maintaining the small diameter of the casing bearing seat as the only purpose is to reduce the migration of the semi fluid grease through an open bearing and seeing as the new bearing was shielded all that was required was a sufficient seat for the bearing to sit/ register on. Back to the machinist and welding shop, that are luckily only a few units apart, for modifications to be made.
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I was slightly concerned that as there were no definite dimensions to work to and that the case had obviously distorted where the bearing seated that the machine work would establish it correctly from the sketchy dimensions taken pre work. On assembly of the bearing and main shaft output gear it was satisfying to see that the face of the output gear was 109.12mm from the face of the gearbox, this was at least in the ballpark of the dimension I had read elsewhere on the forum to look for.
So assembly finally complete bar primary drive cover, by rotating the rear wheel all gears appeared to be engaging. Due to rain all I have done is take it a couple of 100m up the lane and 4th now engages, however I achieved this result for a couple of hundred miles earlier in the summer so fingers X’d. Also appears to be some clutch slip in first gear, I thought initially the noise was the rear wheel spinning in the gravel so am hoping the clatter is just the clutch plate noise exacerbated by there being no cover; think Ducati.
So thanks again to A_HRD, other members for words of support, Malcolm the machinist and to add a bit of provenance to the Comet the welding work was carried out by the outfit that built John Surtees racing chassis.
If I post again on this topic it will be about how to fit a Norton/AMC box.