'The Devil's in the details' as the old saying goes..
There are various oil pump drive spindle retaining pegs for different BSA models which share the same dimensions apart from the length...
However, of particular note is the fact there are two different ones used for the M20/21 depending when the bike was built...
Wartime models and those produced until the beginning of the 1948 model year used Part No.66-2579...This was used in conjunction with Part No. 1-4621, which was a washer (oil pump drive spindle retaining pin washer)...
Machines built after this used Part No. 65-2580 and the washer was deleted...
The difference between the two pins is a length difference of approx. .050"...
From the outer end of the 'short' pin to the shoulder where it is reduced to engage the pump drive shaft is approx. .800" and this must be fitted with the washer to ensure correct engagement...
The longer, later, pin measures approx. .850" between the same two points....
Another point worth noting is an error in the BSA workshop manuals...The manual advises using a timing cover screw (1/4" whitworth) to screw into the end of the pin to assist in its removal from the crankcase...
The pins in fact have a 1/4" cycle thread internal thread...
The error dates back to the pre war period when 1/4" cycle thread was used for cover screws...
And finally...If you have the pin with the washer fitted (at its outer end) the screw won't pass through the hole in the washer. The washer must be removed first...
The washer is not a standard 1/4" washer...As well as having a smaller hole in the middle the outside diameter is smaller...
It is a snug fit into the crankcase and will need to be prised out using a small screwdriver or similar...
If making a washer from a standard 1/4" washer carefully file the outside diameter until it is a good fit in the crankcase....and don't forget to check its thickness..There are some standard washers close the right dimension....Ian
email (option): ian@wright52.plus.com
Ian, you must spend hours researching this stuff, both in the workshop and in the armchair with a note-pad................mind you, a good way to spend semi-retirement and I for one can relate to this..............
But I wouldn't mind betting that there are a few Forum members who would be thankful for your hours of work within a few minutes of encountering an "issue" in the garage...........
Well done mate and total respect for your efforts and contributions for the WD bike community.............
Ian, you must spend hours researching this stuff, both in the workshop and in the armchair with a note-pad................mid you, a good way to spend semi-retirement and I for one can relate to this..............
But I wouldn't mind betting that there are a few Forum members who would be thankful for your hours of work within a few minutes of encountering an "issue" in the garage...........
Well done mate and total respect for your efforts and contributions for the WD bike community.............
And how's the leg doing more to the point !!!!?
I admit I am a bit obsessed with the mechanical details...
No more though than others who are interested in different aspects of these bikes...
I admire the efforts of you chaps who plough through the archives for example...I couldn't do it myself....
The key thing is though that we all share the knowledge we have gleaned...
It's great to think that we are all getting 'fast tracked' up the learning curve in one way or another due to the exchange of knowledge on the forum...
As for my ankle..Thanks for asking...I've been banned from riding until the spring, working on the theory that if I keep kick starting bikes it will never get better...
It's been a long 18 months and a slow process, much more so than I expected to be honest...Ian
email (option): ian@wright52.plus.com
Is it self-discipline with the ankle til spring Ian or Mrs "W" taking charge !? - Lol 
A bit of both Steve...Once she convinced me I might do better if I didn't ride the bikes for a while I agreed to go along with it...but only till the spring...
Then I'm going to ride anyway...I'm not going to miss another Summer...Ian
email (option): ian@wright52.plus.com
Good plan Ian ! I regularly get "advice" from Mrs M that somehow "convinces" me..........normally with consequences if I don't follow said advice.......
Ron knows my wife and at only 4'10" you wouldn't want to cross her, despite me being 6'4"...........
I'm glad the leg is improving mate......
Hours in the workshop measuring and checking parts is no less time-consuming and diligent than hours spent pouring through paperwork..........sadly, with the latter, many years in the police have numbed me to do this pretty effectively.......
As I've said before, we simply couldn't do without you on this Forum and God knows how we managed before you came along...............I've known you for years Ian, you're a damn good engineer and a perfectionist...........nothing wrong with that either...
Sometimes, perfection is needed to identify the faults and problems further down the scale.............
I think it was Kalle, though pre war machines aren't really my thing...
I'm basing my statement on a 1936 G14 I owned..That had the fine threads, so I'm guessing other machines in the range shared the thread form...
I haven't had a single that early though, apart from my 1918 Model K and I haven't got inside that yet...
Perhaps some forum contributor who is a pre war enthusiast can tie down the specs. more accurately
I should have written...'I believe the error dates back... etc. etc.' to indicate I wasn't 100% sure on that particular point....Ian
email (option): ian@wright52.plus.com
That's interesting...I recently sold a set of AA crankcases but can't say I noted the thread form for the cover screws....
Looking in the appropriate parts list with a supplemental section for AA components neither crankcases or screws are listed under different numbers, which they would be...
To me this seems an unlikely variation but never say never!...I'd appreciate it if you would check when you can..
AA alternator models were only made to special order for the AA, between 1961 and 1963, when production of the side valves ended...Ian
email (option): ian@wright52.plus.com
Thanks Leon....Yes, my '36 G14 had the fine threads...I'd forgotten about that spring retaining screw though...
WD Models and post war models retained the fine thread for that application..(worth noting as well that the screw was hardened)...
There's also the cable adjuster thread on the gearbox...
No post war singles had the fine thread for covers that I know of and Kalle says his '37 and '38 engines don't...
So maybe 37 was the year the threads were changed to Whitworth?...Ian
email (option): ian@wright52.plus.com
It could be too short...or too wide for the slot in the headlamp.. ...Anythings possible...Ian
email (option): ian@wright52.plus.com
More details for rivet counters. Did they use 0BA (M6) for kneepad screws? I always thought that they are 1/4 cycle? At least on my M20, these threaded holes were rusted from 0BA to 1/4 cycle
Hi Kalle...It can be difficult to positively identify some of these threads as they are very close to each other...
For example, it was some time before I concluded the primary cover screws were 3/16" Cycle and not 2BA...
Other candidates for confusion are the clutch dome screws, rear hub felt cover screws, speedo mounting studs and the the tank rubber screws you mention...
BA was also called 'instrument thread' and was used for switches, carburettors, speedos and surprisingly, handlebar levers...
However, for other cycle parts I would consider Cycle Thread to be the more likely choice and I use 1/4" Cycle screws for the tank rubbers...
As mentioned earlier, the speedo mounting studs are O BA and the clutch dome and hub plate 3/16" Cycle....Ian
email (option): ian@wright52.plus.com
I found the same problem with my 1970 Bonneville...It is in the 'changeover period' when Whitworth, Cycle Thread etc. were being replaced by UNC, UNF etc...
The problem is they did it gradually and bikes from this period are a real mixture...
In comparison the 40's and 50's bikes are simple.. ....Ian
email (option): ian@wright52.plus.com
Although BA thread sizes are expressed in Imperial dimensions it is actually a metric based thread system...
Coincidentally, 0BA was the 'base' size for this system and has a 1mm pitch and a major diameter of 6mm (.236") so 6mm nuts fit perfectly at this size, although the size across the flats is not the same....
At other diameters pitches etc. do not match...Ian
email (option): ian@wright52.plus.com
Looking in the appropriate parts list with a supplemental section for AA components neither crankcases or screws are listed under different numbers, which they would be...
To me this seems an unlikely variation but never say never!...I'd appreciate it if you would check when you can..
AA alternator models were only made to special order for the AA, between 1961 and 1963, when production of the side valves ended...Ian
Bit of an 'old' thread now but today was the first chance I've had to check these Ian. Certainly not 1/4 Whit, although I am guessing a few previous owners thought they should be. Due them having been beggared about with it is difficult to say with certainty if they are Cycle or BA. I've tried both in and both had varying success in different holes. So whereas I wont say definitely is 'xyz', I will say definitely isn't '1/4 Whit' - on my cases. No claims for any others.