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How to date and identify a Douglas BTH magneto

Started by cardan, 10 Oct 2022 at 07:24

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Looking back through posts on the forum over the years, there are quite a number of questions about the identity and date of BTH magnetos for Douglases. Many queries are still unanswered.

Starting from the EW (or maybe even some CWs?) Douglas mostly used the "early" style of BTH magneto, which featured a horseshoe magnet made from a tungsten alloy steel - for example the M2-W2 usually seen on the EW or the M2-AD1 and M2-AE1 used on the bigger twins.

From 1929 BTH introduced a new range of smaller, lighter magnetos (the MC and KC range) using 15% cobalt steel magnets, then from 1934 the MD and KD magnetos with ALNI (aluminium nickel) magnets, and finally the MH and KH magnetos with ALCOMAX III magnets. All these later magnetos have an id plate, with the model (eg MC1,KH2...), the "form" (eg B4, C6, AC-9), and a serial number (eg 7K551392). Between 1929 and 1969 (by which time the magnetos were branded AEI), all these numbers make sense and magnetos can be dated to the year and month pretty easily.

By contrast, the numbering on the early-style magnetos is a mess. I think M and K series magnetos with horseshoe magnets were available from the early 1920s (maybe even during WW1) until at least 1946, so there's plenty of scope to get mixed up with dating and correct usage.

Anyway, I think I'm in a position to answer questions about the various BTH magnetos, in particular about dating. I appreciate it's a pretty fringe topic so I won't be hurt if no one else cares!

I'd also be really interested to know if there was any special relationship between BTH and Douglas: certainly in the 1920s magnetos numbers in the 800000s seemed to have been set aside for Douglas of various dates, while the other BTH magnetos of the era carried on from 900000, so that the magneto M1-B2 7L927423 seems to have been built a couple of years before the Douglas BTH magneto M2-AE1 9E849795. Not to mention the 1926 M2-W2 magnetos for EWs with numbers in the range 10000-20000 at a time other BTH magnetos had serial numbers around 600000. Nothing like this happens with the numbers in later magnetos!



Tim OConnor

very interesting! let me know if I can help with any photos of my magneto.
1920 2 3/4 W-20


Great information Leon!,

I don't have anything much to add about BTH magnetos except that in the May 1915 Edition of "The Automobile Engineer" there is a chart of magnetos being produced by British Manufacturers to replace Bosch versions for the war effort. Here the BTH magneto types are listed as

U1 to replace ZA1
U2 and UV to replace ZA2 and ZAV respectively
W1 to replace ZE1
W2 and WV to replace ZE2 and ZEV respectively
U4 to replace ZA4
VW4 to replace ZF4
VWU4 to replace ZU4
Z4 to replace Z4

M and K series could have been introduced in WW1 but I have not found any information to support this yet.

If you don't have this chart I can PM a copy to you.




Hi Ian,
Yes BTH were part of the consortium of British manufacturers who swung into action at the outbreak of war to make magnetos and the like, mostly to replace Bosch instruments, but I don't know if they made motorcycle magnetos. If they did, they weren't amongst the prominent brands. Thomson Bennett, EIC, Fellows,...
From 1920, BTH advertised a combined magneto/lighting instrument, and from 1922 magnetos that looked like the well-known K and M series.
Not sure of the first Douglas use of BTH, but maybe on TTs in 1925 or so? Tim I'd imagine your W20 magneto is EIC?
I should have noted above that the serial numbers for the "horseshoe magnet" BTH magnetos don't fit in with the serial numbers for the later magnetos from 1929-on. Ditto numbers on other BTH product like their dynomag.


We wonder who the 10% who didn't join the BIAA were ??
Nice find ...


Maybe 10% of magnetos were American - Splitdorf, Dixi, etc.? Once Bosch magnetos dried up here in Australia (we had laws prohibiting "trading with the enemy"), American and British magnetos like the Thomson Bennett became the norm on locally-built bikes.


[Edit: Re-reading the advert I see what you mean. Not sure what British magnetos might be outside the consortium, but I don't see CAV listed...]


Hi Leon and EW-Ron,

After the British Magneto Chart in The Automobile Engineer May 1915 there is an article on British Manufacture of Magnetos for the war effort.  A list of current magneto manufacturers and those intending to manufacture them in Britain at the time is given. I have no idea if some of the lesser known manufacturers in this list did actually produce magnetos during the war. It appears this article possibly predates the formation of the BIAA as there is no mention of it?

The article does give some interesting information of the state of magneto production in 1915.




Here's an identification plate from a typical BTH Douglas magneto. Before 1929, much the same information was stamped directly onto the magneto body.

There are four pieces of info on the plate:

The magneto type: in this case "M2" means a evenly-firing twin-cylinder magneto with a tungsten alloy horseshoe magnet. All BTH magnetos fitted to Douglas - from the mid-1920s to the Aeros of the late 1930s - were Type M2. [Edit: Should have said M magnetos have 35mm armature height, K magnetos are 45mm.]

The magneto form: in this case "AE1" means an anti-clockwise magneto with enclosed advanced/retard, as fitted to ohv twins at the end of the 1920s. I think of the "form" (some late BTH magneto id plate actually label this box as "form") as a description written on a card in a card file somewhere at BTH, so that the customer (Douglas in this case) can just say "M2 AE1" and everyone agrees on how to configure the magneto.

The date code: in this case 9E means June 1929. Unfortunately the "9" can mean 1929, 1939, 1949, 1959 or even 1969, so dating the magneto needs to be informed by the style of the magneto and the serial number. Beware! There is mystery in the serial numbers, particularly when Douglas is involved. A BTH magneto for a different customer in June 1929 would have a serial number 1xxxxx, rather than the 8xxxxxx number on the Douglas magneto. Magnetos made before early 1926 don't have a date code.

The serial number: in this case 849793. As mentioned, Douglas BTH serial numbers are weird, particularly for a batch of EWs in 1926 and all Douglases from 1927. So far as I can see, from 1927 until the end of the Aeros all BTH magnetos fitted by Douglas had "8" as the first digit of the serial number.



And here are the Douglas magnetos I know about. No doubt there are more - for example what type of magneto was fitted to a 1928 ohv (presumably anti-clockwise with external a/r mechanism).

M2 W2  clockwise with external a/r used on EW until about 1927
M2 AD1 clockwise with external a/r used on sv twins until about 1929
M2 AD2 clockwise with enclosed a/r used on sv twins from about 1929
M2 AD4 clockwise with enclosed a/r used on Aero models in the mid 1930s

M2 AE1 anti-clockwise with enclosed a/r used on ohv twins from 1929

There are academic questions like "What is the difference between W2 and AD1? Or AD2 and AD4?" but I haven't worried about the details. There might be significant differences - say in the detail of the armature taper or the base plate - or it might something not readily seen - maybe different windings of the armature. There are probably 100 or more different "forms" used by BTH over the years.

My knowledge is not complete, but I'm happy to have a go at any BTH questions, particularly "When was my magneto made?" or "What type of magneto am I looking for?"


Bob M

Hi Leon,
I have in front of me a copy of the Second Edition of "Magnetos, The theory and practice of magneto ignition for all forms of internal combustion engines" by A P Young and published by Iliffe & Sons.
The preface of this Second Edition is dated May 1st, 1920 so essentially gives information on the state of play at the end of WWI.
At that time the British Standard Nomenclature and German equivalent as printed in the book was as follows,
M1 = ZA1
M2 = ZA2

K1 = ZE1
K2 = ZE2

G4 = ZF4

E3 = ZU3
E4 = ZU4

E5 = ZU5
E6 = ZU6

In a later chapter  a brief summary of available BTH motorcycle magnetos advises "A complete line of types 'M' and 'K' magnetos for single-cylinder, 2-cylinder and 'V' engines has been standardised by this company and is now being manufactured in large quantities".

This is followed by an illustration of a "smaller type M2 magneto, suitable for 2-cylinder engines(cylinders 180 degrees apart)". Ring a bell?

Further on still the author advises, "The BTH Co has specialized on the manufacture of 2-cylinder magnetos suitable for 'V' engines, having standardized type KV for the following 'V' angles:- 42, 45, 50, 55, 60 and 90 degrees. The staggered form of pole piece and armature core construction referred to in detail in Chapert III is adopted for all angles with the result that the sparking characteristics at low speeds are distinctly good and the intensities of the sparks delivered by No. 1 and No. 2 terminals reasonably uniform at all speeds.

Whether or not BTH followed through in all their efforts I don't know but it does show their intentions in 1920 and helps in identifying a start date.

All in all this is a very helpful magneto book if you can find yourself a copy.

Cheers, Bob


Hi Bob,

My copy of Young is newer than yours, so it's coverage of BTH is more comprehensive on the later models, and less detailed about the origins. But clearly 1920 is an early date for a BTH M2 (flat twin) magneto. Not sure what this looked like, but by 1920 it may have been quite similar to the late magnetos.

Re quantity: after the war the major manufacturers went their different ways, Douglas with EIC until about 1925, Triumph and Sunbeam with ML, Rudge, Norton and BSA with CAV, and so on, so I don't think BTH had a quantity market for its M2 magnetos until Douglas adopted them in the mid 1920s. The Eighteenth Edition of Motor Cycles and How to Manage Them (Sept 1917) has a description of the BTH (single cylinder) magneto, but at that time it was a near-copy of the Bosch ZE1... At some time (maybe 1920-ish) this was reborn in the familiar M1/K1 design and appeared on bikes from AJS, Raleigh and others. Luckily for the Douglas part of the story we don't need the exact details of the early models!



Bob M

Hi again Leon,
It would appear this is another book where I need to collect multiple editions to get a broad coverage of the subject over the years.
From remarks in the preface of my second edition I think the first edition was more aviation oriented but once the war was over the emphasis became more concerned with volume production and the civilian market thus more motor car, motor cycle oriented.
What edition is yours? How many editions am I looking for?
I have found that AP Young was still writing books on ignition for the armed services during WWII but by then the emphasis had moved away from purely magnetos so I haven't bothered with that one.
As for the 1920 M2 flat twin BTH it doesn't look any different to the later ones to me but I am no expert on BTH mags so take that with a pinch of salt.
Cheers, Bob


Mine is by AP Young and L Griffiths, "Automobile Electrical Equipment", also published by Iliffe - the first edition was in 1933, no doubt incorporating relevant info from your earlier AP Young magneto book. I have the fifth edition from 1955, which is good because it covers the entire development of the BTH magneto in some detail, all the way to the last BKH style where the magnets were actually cast into the alloy.

Interesting to know that the 1920 BTH M2 looks just like a later one. Sadly the early BTH magnetos don't have a date code, so if one turned up, loose, I'd be able to say "Wow that's early," based on the serial number but not much more. Need to find them attached to an original machine.



Hi Leon and Bob,

Interesting to read about the works on magneto's by A.P Young (and L. Griffiths). I don't have any of these books but when I was looking at the info. on magnetos in The Automobile Engineer (also an Illffe production BTW) I came across a technical paper on Magneto's by Young in March 1915. I wonder if this was the basis of all his later works?

He does mention BTH magnetos in the text (e.g. type W single cylinder). The paper is 9 pages long and technical - so not sure that it worth posting here? Let me know what you think or I can PM it to you.

I finally got to put the scan of the chart that goes with the article (Edit:- From reply #6 above) from The Automobile Engineer together - the format is larger than A4 (so not so easy to scan on my home scanner) and the combined picture file size was then too large to upload onto the forum.... but here it is now.




Thanks Hutch. Personally I have enough info on magnetos in genreal. But as the topic of the thread says, happy to discuss identifying and dating Douglas Bosch magnetos.



The 1932 TT Douglases used an interesting BTH M2 magneto, fitted with a "TT" points enclosure. In a post I wondered whether any of these magnetos had survived.

Bonhams sold a bike some years back that was fitted with a magneto that looked similar (see below), but it turns out that the serial number on this magneto body dates it to the mid 1920s, while the enclosed TT points housing (with its 3-screw cover and waterproof air vent) was first seen at the 1931 TT. So this particular magneto seems to be a "recent" build of mixed parts.