Is this really as close as they get? The manual makes no mention of at what point in the engine's rotation this occurs. This is at TDC with the TS valves closed. Seems to me that if you're manufacturing the thing you could put the marks where they line up more exactly.
David, from your erstwhile previous posts about A65s, Mikunis on, etc., I'd have thought you were far past your disillusionment with BSA engineering.
But yes, in answer to your question, that's as close as the marks get. Crank position is not important, just as long as the OTHER marks line up when you rotate it.
If there's any logic to this, it's that the idler pinion can be inserted in either of two positions, 180 degrees apart, the two timing marks on the idler pinion being 180 degrees apart. Now since the idler pinion is not in a direct line between the crank and camshaft pinions, there's no way that the marks can all line up at the same time.
I suspect this is also the reason that there's no locating pin on the AAU, as there is on a Triumph AAU, which runs off the exhaust camshaft.
No disrespect intended, but it would be much clearer if they had cast the marks on the cam wheel and idler wheel two teeth clockwise and anticlockwise respectively. I idler wheel would only fit in one position, but there would be no doubt. But I forget that BSA folks like puzzles.
I think you are confusing your ease of setting the timing with their ease of manufacture. The keyways, the crank key is positioned for the right cylinder at TDC, easy to find when making the crank. The cam keyway is used to index the cam for grinding. I have not precisely measured it but it appears to also be at TDC right cylinder. The tappets are inclined 15 degrees forward and the right cylinder exhaust lobe is just coming around. There are only a couple teeth between the idler marks at 180 degrees and where the marks would align with the gear tooth mesh. If they put the marks, as you say, to align with each other, the fitter has a 50-50 chance to get the idler in the correct position or else has to rotate the idler around to the other mark. With 180 marks he only has to align the crank pinion and idler mark then rotate the cam so it is a couple teeth from mesh. Rotating the engine a little back and forth to check the alignment is not difficult. If you can count two teeth from mesh to mark you do not have to do that either. You can always add your own punch marks to the idler and cam gears that suits your convenience.
Rotating the engine a little back and forth to check the alignment is not difficult. If you can count two teeth from mesh to mark you do not have to do that either.
Back when the fitter were building hundreds of engines this worked best. Now that each "fitter" builds one engine each, it's obviously not the best setup. This becomes painfully obvious when you can't get the engine to fire and you start doubting whether you were off by a tooth or not. Unfortunately, it's the hands we're dealt.
Had BSA had the foresight to design machines for maintenance some 50 years later... they'd probably still be in business