It’s well worth scribbling up a list of everything you can think of well before you start on the job. You will, of course, miss some bits, but it will nonetheless give you an opportunity to come up with a rough order of play, based on which items are likely to be long lead, need saving up for or, of course, need something else doing first.

I stuck it all on a spreadsheet, which makes it easy to add the bits I forgot and indulge in the unmitigated pleasure of ticking things off when they’re done.
For those who are sad enough to be interested, here’s the logic behind my particular approach. It may or may not work for you, but that’s your decision.

Stripping the top end of the engine may seem like a very odd place to start, but there is logic to it.
Firstly, it allows me to assess the state of the cylinder bore. This is important because knowing whether I need to have it re-bored (one of the few jobs I can’t sensibly do myself) and by how much, will determine which piston I need to buy.
I’ve already decided to fit the high compression SS90 piston, but do I want it standard, or oversize and by how much? Once I know that I can buy the piston and once I have that I can send it off to get the re-bore.
Incidentally, if someone says they don’t need the piston to do the re-bore, go somewhere else. A proper job involves machining to precisely the right piston clearance, not just an arbitrary plus 20, or whatever.

The other thing I can check is the big end wear. Alas, on the B40, a crankshaft rebuild is another one of the few jobs I can’t do. The crank is a press fit job and the combination of extreme force (typically seven tonnes or so) and precision needed to dismantle and re-assemble it requires a big hydraulic press and a lot of skill. Not to mention the fine art of truing and balancing it, so it’s a job for the professionals. That’s professional engineers, not smoothly dressed gentlemen with hot 1970s Fords and guns. I used to really enjoy that show.
However, dragging the ageing mind back to the matter in hand, it’ll make a big difference to the cost too. Quite apart from the cost of the crank rebuild, I may well then be tempted to fit a needle roller big end in place of the original plain bearing.
I’m not sure on that one. On the one hand, Jeff Smith got to second in the world championships on plain bearings and this will be a relatively low stress, low mileage road bike, on the other, I’ve no idea how often his engine was rebuilt and I want this bike to comfortably out last me. More thought required.

Having stripped the top end, reason dictates that I continue the engine strip. Again, there is logic. A very major factor here is clearing the bench space. I have only one small bench and as long as there’s half an engine sat on it I can’t use it for anything else. It’ll also give me a fair idea of what else I need to buy for it. B40 bits (especially tuning parts) are not always easy to come by, so having a shopping list to hand aids regular browsing of the web, autojumbles and any passing fellow enthusiasts. All of these will probably need to be enlisted at some point.
As an interesting (well, to me anyway) aside, it seems far easier to get BSA parts in the USA than it does here. I’m really not sure why, but beware import duty and the (sometimes higher) “administration fee” that the post office put on collecting it, when calculating prices.

Once the engine is safely packed away in bags and boxes (more on that later) I shall turn my attention to the front wheel, the reason for this is that I have managed to acquire a seven inch twin leading shoe brake which, while not correct for the Enduro or Trials spec’, is much a much better stopper than the SLS original.

According to the all-knowing RR (Rupert Ratio aka Dave Smith) it will fit in a B40, but may need some material removing to avoid interference with the fork leg. Since that’s all RR said on the matter, I have no idea how much or from where, so quickly fitting it, while I still have the forks assembled will enable me to quantify the job, or come up with plan B if it doesn’t work.

Sticking with the theme of long lead items, wheels come next.
The front will definitely require a new rim, since I’ve decided to fit a 21” in place of the original 18”. The enduro actually had a 20” front wheel, but good tyres in that size are few and far between, so most restorers opt for either a 19” or 21”. I’ll go into the reasons for my choice when I get to wheel building.

I’m not too sure of the condition of the rear rim, but the fact that it’s been painted over doesn’t lead me to suspect that it’s pristine. I’ll probably want to replace the spokes with stainless ones anyway and I’ll certainly have to dismantle it to refurb the hub, so that needs to come apart as well. Then I can order rim(s) and spokes.

Once those few bits are out of the way the strip of the cycle parts should be pretty straight forward.

I’ll need to check the state of the swinging arm bushes early, as this is the other job that I can’t do. It requires putting the whole frame in a big press to either remove or fit the swinging arm pin. My impression riding the bike home when I bought it (about 50 miles) was that it was actually in pretty fine fettle, the handling felt surprisingly tight and stable, but it needs to be checked properly. Casting about on the web I was surprised to find that leaving it in doesn’t seem to provide any barrier to cleaning or painting the frame. Indeed quite a few people have had the frame powder coated with the swinging arm in place. I’d have expected the heat/grease to cause some problems.

Storage
It has probably not escaped the notice of the more astute reader that I will rapidly end up with a large pile of bits, which need to be stored carefully and somewhere near sensibly, for a period of at least twice the estimated duration of the build!

In my case this is exacerbated by the fact that I don’t have a nice, dry, heated garage. Indeed, I’m still building in a tent. It’s a bigger tent than it was in the days of Little Suzi, but still a tent, I did invest in a tiny shed (3x4ft) for storage.

My suggestion of the basic requirements, when dismantling are as follows:-

Large and small Freezer Bags – the ziplock type work best and can accommodate parts up to cylinder head size.

Sharpie Marker – For writing on the freezer bags. You will definitely forget what some bits are (or which versions , if you’ve bought used replacements) and it’s really helpful to write on what sort of order they assemble in i.e. where spacers go and such.

Takeaway boxes – For small parts, or groups of small parts, cylinder head fastenings for example. One can of course buy lots of takeaways, but if you don’t want to risk your health, fifty new ones is only a few quid on the internet.

Big plastic storage boxes – the stacking type are good. Cheap from places like Willco and QD. You can put the freezer bags and takeaway boxes in them.

Mobile phone – PHOTOGRAPH EVERYTHING before you take it off. Digital photography is wonderfully free, so don’t skimp. Back up your pictures in the cloud (also free) and you have a secure record of what went where.

Vacuum cleaner – With a bit of imagination and some gaffer tape, you can suck the air out of the freezer bags and effectively vacuum pack your bits to stave off oxidation.

Silica gel packs – also available cheaply on the web. Stuff them in your freezer bags to keep the moisture at bay.

Tie on labels – old fashioned, but still the best way to label some parts.

All that may sound a bit anal, but it’ll add about twenty quid to your rebuild cost (unless you buy a vacuum cleaner specifically for the purpose) and could save you hours of work, and possibly some expensive bits, later on. In the words of the great Eric Burden (obscure 1967 song reference, just humour me) “it will be worth it”

Clean or grubby?
Personally, I prefer to clean off the very worst of the muck, just to make them easier to handle, but leave the parts pretty much as they came off the bike until I need them.
At build time it’s nice to pull out lots of clean and shiny bits, but clean parts are a lot less stable than mucky parts, by which I mean that newly cleaned surfaces are open to oxidation and scuffing, far more than those protected by a layer of grime.

To take this to it’s logical conclusion, try cleaning a cast iron part. I boiled my barrel (is it just me or does that sound like a Glaswegian insult.. “g’away and boil yer barrel, pal”) in caustic soda and it, literally, rusted faster than I could paint it.

The counter argument is that it’s not until a part is cleaned that one can properly assess its condition and consequently, whether a replacement needs to be sourced.

I suggest use of that wonderful oxymoron “common sense”. The barrel, for example, had to be measured accurately to resolve the rebore question, so that had to be cleaned straight away. The cylinder head isn’t going to be attacked straight away, so that can stay grubby for now.

Just remember that if you do clean something, figure out how you’re going to protect it first. In the case of the barrel, Granvilles cylinder black on the outside, WD40 on the bore and gasket faces and vacuum packed with silica gel.