Timing with auto advance/retard unit.

[Groily]

Not really I don't think. If your bike is a '55 then it was the first year of the SR1 mag with the atd according to the Lucas parts lists. The other magneto that goes on these beasts is the N1 as you'll have seen, with manual advance and retard, but swapping yours / buying one in a decent state might leave you no further ahead by the time you've got a cable with its handlebar lever etc.
There be many who'd rather have the SR1 as they're easier to maintain, and of course there's the 'originality' thing if it bothers you.
I think if you can sort out the fastener on that atd you'll probably be OK as long as there isn't a massive amount of wobble and it opens and closes OK against the springs. Your hassle lies in getting the stuck bolt out, but that's 'time not money' so you could be good to go for the cost of the bolt, even if things aren't perfect. (Yours is probably no worse than many of the 2nd hand ones that come up on ebay etc, nor, I bet, than many that are in service.)
If the 3/8th BSF threads clean up, and the atd can be re-attached without even changing the bolt, and you have an extractor that can get the thing off by other means, then you might sort it just for the cost of the tap and die. There are many atds in service which have lost their auto-extract, in the hands of those who have other ways of getting them off - but it should be noted that tyre levers aren't an ideal method!

[clive]

If I have borked my auto advance unit, is there a cheaper manual option that can be fitted?

You could fit an N1 mag with manual advance but there is no option for advancing the mag if you put a fixed sprocket on thd SR1. Buying a working N1, sprocket, cable and handlebar lever is likely to be more expensive than tracking down another auto advance unit. If it comes to it I suggest a wanted ad in the mag.

Edit
I see Groily beat me to it. I agree tyre levers are a no no as you risk damaging the inner timing case. While you are about it i would check the mag is properly positioned on the mag platform for the run of the chain from the sprcket on the exhaust cam. Looks like yours may be too far across causing the witness mark on the case. This was not caused by a loose unit as it seems to have no wobble in the mark. You can check the run by measuirng from the mating face on the timing cover at both sprockets.

[The Vandal]

I managed to extract the bolt from the ATD today and screwing it onto the magneto without the ATD shows that the threads are not going together as smoothly as they should. I wasn't sure if it was that or the tapers binding.

Tap and die didn't arrive tomorrow so will be a few days before I get back to working on it.

@clive your assessment that the sprocket is too far into the case is interesting, I eyeballed it when putting the mag back on the platform but have new bolts coming for that so it will be moving anyway so I'll double check with pucker measurements. Thanks.

[Groily]

Well done - that's a bit of seriously good news!

You can see if the tapers are OK now the bolt is out just by offering the two together to see if they mate firmly - no rock, fully 'on'. If it's loose, then there is a serious problem of extreme wear - that's a 'worst case' bridge to cross later if it must be. It is far from unknown however, and much more common with ATDs than with plain sprockets or pinions.

It's important, meanwhile, that the relative lengths of threaded bits allow full engagement when the ATD is refitted - ie that the thread doesn't bottom before the taper has gripped properly. A moderately worn taper can cause this.

Ways round this failure to engage do not include wrapping aluminium foil round the male part, nor loctite or similar! No bodge is exactly desirable, but moderate wear can often be sorted by the use of an extra spacer under the head of the bolt, or maybe by shortening the bolt by half a turn's worth of thread so it doesn't run out of thread on the mag spindle before the ATD has gripped.

If running a tap & die over things sorts it out though, so much the better.

It is not impossible, if the tapers are very tired, that that is also responsible for the misalignment of the atd / lack of clearance to timing cover. WIth all tapers, a 'little wear goes a long way'. With any luck, per Clive's thought, there's enough adjustment of the mag on its platform to compensate for any problem, or the slots could be very slightly elongated.

[limeyrob]

I've recently converted a BSA from auto A/R to manual and I don't think there was change out of £100 once it was all done so its worth getting those threads cleaned up. I'm assuming it has the fibre gear? They are easy to damage when you are working on the unit.