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Differences M20 girder forks 1939 vs 1940

Hi all,

I know that it differences between early (short) and later war (long) M20 girder forks has been a subject on this forum before.

I have 2 early short girder forks.

One with partnumber 66-5023, making it a 1939 M20 Deluxe Fork (according to the partslist). I found this fork complete and under a layer of black paint I found original military green paint.

The other fork has partnumber 66-5014, making it a military fork from 1940 (according to this post: http://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&msgid=1183365&cmd=show). This fork was also complete and with original military green paint under a layer of paint.

I have noticed that there a several small differences between the 2 forks. Different markings on the fork links, difference in length were the mudguard fits to the fork and different steering stem yoke's. I have attached a few photo's below.

What is the reason for these differences?
Why is the 1940 fork shortened in the centre at the location were the mudguard is fitted? What was the purpose for this?

I am curious at your opinions.

Regards,
Bastiaan

Fork-1-0

Fork-2-0

Link-1-0

Link-2-0

Link-3-0

Stem-1-0

Stem-2-0

email (option): wdmotorcycles@gmail.com

Re: Differences M20 girder forks 1939 vs 1940

I don't know if this helps but the forks on my March 1941 WM20 is 66-5014 and I have rubber mounted arms for the handlebars.

email (option): cas.vanderwoude@gmail.com

Re: Differences M20 girder forks 1939 vs 1940

Hi bastiaan

Could one be from a sidecar out fit - different wheel size perhaps ?

Job

email (option): jonnyob1@googlemail.com

Re: Differences M20 girder forks 1939 vs 1940

Hi Bastiaan, I don't know the answer but I have other questions.

Image2

This part looks like the top grease nipple holes were added later, the two forward facing lumps look like unfinished/undrilled grease nipple mounts, is that the case?

It could be that as production was increased for the British Government earlier tooling was used to up production?

And the repositioning of the grease nipples to the upper surface could be because oil was used instead of grease after a certain period?

I'm also intrigued by the capital letter "D" & "K" over the plied arms logo, you see it a lot on tools but I have always wondered what it meant?

Rob

email (option): robmiller11(a)yahoo.co.uk

Re: Differences M20 girder forks 1939 vs 1940

Thank you for the reactions!

@John: I am not an expert on sidecar outfit's, but would in that case the stays for the front mudguard also have a different length compared to an m20?

@Casper: I have for both forks also the early rubber mounted top yoke's, these are original for the 1939 and 1940 forks, as far as I know.

@ Rob: The two forward facing lumps are correct for the 1939 forks, they are even drawn on the stem in the 1939 partslist (see photo below). The 1939 stem has no raised surface for the location of the grease nipples, the grease nipples are relatively deep positioned in the body of the stem which will be a bit of a challenge when the bronze bushes and fork links are fitted. I still have to see how this fits together.

I have no idea about the capital letter's "D" and "K", I know that later links don't have these capital's.

Regards,
Bastiaan

IMG-9778

email (option): wdmotorcycles@gmail.com

Re: Differences M20 girder forks 1939 vs 1940

Bastiaan, were your '39 DL forks intended for the valanced mudguard ? Did they alter something once the valanced guards were no longer being produced ?

Re: Differences M20 girder forks 1939 vs 1940

Bastiaan the sidecar outfits had 18" wheels which were fitted with 400 x 18 tyres which effectively gave the same diameter as the standard 325 x 19. So nothing had to alter. Ron

email (option): ronpier@talk21.com

Re: Differences M20 girder forks 1939 vs 1940

As far as I know the capital letters are date stamps used in the 20's and 30's, they stand for a year. Z stands for 1932, D is 1936 etc. On these prewar BSA's lot's of parts had such a stamp, from hubs to even very small parts...

And regarding steering stem, don't think you can fully rely on the drawing in the partslists, it has 3 numbers below and the drawing is not always exactly as the part. they could, for instance, have drawn the stem of the G14.

My guess is that the 39 stem is actually a little earlier [1936??] and maybe not even from an M type, probably fitted in combination with a forged steel backbone and had protruding parts faced backwards to limit the steering angle.

Hope this helps, Leon Hop will know for sure. :slightly_smiling_face:

Re: Differences M20 girder forks 1939 vs 1940

Bastiaan,

The fork link marked with a K is in fact 1939. The fork link marked with a D is 1936 (and it looks like an Blue Star/Empire Star fork link). The bottom yoke with the bulges is for a pre war bike. Like Michiel said pre war parts were stamped with the year pre fix. You can reliably date parts as of 1930:
X 1930
Y 1931
Z 1932
A 1933
B 1934
E 1935
D 1936
H 1937
J 1938
K 1939
As of 1940 this practice was dropped (also for civilian bikes).
The bulges hit spots on the frame to create a steering lock to ensure the handlebars don't hit the tank.

I can't post pictures right now.

Regards,
Leon

email (option): leonhop3_at_planet_dot_nl

Re: Differences M20 girder forks 1939 vs 1940

Leon KM20.1 I believe is correct in that the bottom yoke with the bulges was to restrict the rotation of the of the forks via the handlebars to protect the fuel tank. I don't think it was to protect the fuel tank from the handlebars themselves though but instead to protect the fuel tank from the forks.

Re: Differences M20 girder forks 1939 vs 1940

That's interesting, despite having owned a G14 and a Y13 (many years ago before I was that interested in those sorts of details) I didn't realise pre war parts carried a date stamp...As my experience is largely with 1940 onwards BSAs I missed that altogether...!!....Ian

email (option): ian@wright52.plus.com

Re: Differences M20 girder forks 1939 vs 1940

How many parts on an M20 actually have a piled arms stamp, I can't think of very many?

Rob

email (option): robmiller11(a)yahoo.co.uk

Re: Differences M20 girder forks 1939 vs 1940

I have early clutch & brake levers and I think I have seen it on a kick start?

Rob

email (option): robmiller11(a)yahoo.co.uk

Re: Differences M20 girder forks 1939 vs 1940

Found lots of 'piled arm' stamps with a Z above it all over my 1932 Sloper.

They are, off course, hard to spot as very shallow stamped, and lots will not be visible after a modern multi layer of paint, let alone under powder coating. That is one of the reasons I spray very lightly, these details are great to be seen as well as machine markings, casting texture, etc.

Re: Differences M20 girder forks 1939 vs 1940

The letter code was also used in the twenties.
However it do not follow the frame code
I have many examples of P R. S T all on parts from 27-29.
Parts books are also bad at displaying correct parts, for instance the first drum brakes in 27 show rivets.
In fact the first hubs don’t have rivets.
Parts books were compiled in January of the following year.

I have NOS parts for 20’s bikes that have a date stamp of 1936, considering the part was stopped being used 10 years prior.
Engine cases were stamped the year of use, I believe as I have 35 internals with 36 engine numbers.

Re: Differences M20 girder forks 1939 vs 1940

Thank you all for the information. Very interesting to know about the stampings.

I have just checked the other fork links from the 1939 girder fork, they all have the capital "D" stamped in them. The strange thing is that there are also some remains of bronze green WD paint on the fork links. This paint matches the paint which I have found on the early top yoke and the girder frok itself. Below a few photo's of the other fork links with the bronze green paint.

Regards,
Bastiaan

20200423-092605

20200423-092551

email (option): wdmotorcycles@gmail.com

Re: Differences M20 girder forks 1939 vs 1940

It's probably not impossible that with the rush to fulfill the 1939 orders, they actually got down to the bottoms of the parts bins, or the back of the stores. If a part was functionally interchangeable then there would have been no reason not to use it...or even in the case of the Dutch orders which included quite a special specification that parts may have been set aside prior to the commencement of the contract.

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