Help the dating officer...

[simon.holyfield]

Currently I have one of the more challenging dating tasks of the year - a member with a period swinging arm conversion to a rigid frame, which doesn't have a straightforward frame number - though it looks like an AB frame from 1950. This means that the DVLA will need a catalogue based dossier of information to provide an age related plate, as we have successfully done recently with a 1926 bike.

A short perusal of the parts books reveals several changes to the rigid singles frame between 1937 and 1950.

Can anyone rise to the challenge of listing the changes to the singles frame between these years? I think there are three different frames in this period.

[Roger Gwynn]

There was a right side s/car fitting added to the rear fork at some date, I havn't recorded when, 1948 prop stand lug under clutch dome, 1953 side stand lug moved forward to front.

[paul.jameson]

Thank you for the question. The following may help but I cannot guarantee it is completely correct. All dates refer to model years (generally Sept previous calendar year - Aug current calendar year), not calendar years Jan - Dec.

1933-35 (and 1936 4F) the Y frame 4950-33.

1936 sees the change to the F frame 4950-36 (but not for the 4F). This was the major change when the steering head bearings changed and so the steering head changed to suit. The rear brake anchor arm was introduced in lieu of the peg in the nearside rear fork so the nearside rear fork changed and the cast / forged bracket for the brake anchor on the lower nearside frame tube appeared. Just to confuse the issue, early 1936 frames had the rear fork change but not the steering head, presumably until they had used up the old stock steering heads.

1937 sees the 4950-36 frame in the parts book but it is different from the 1936 version by the addition of the two brackets which hold the voltage regulator on. I suspect that these brackets were intended to go on from the start of 1936 but there is a 1936 note somewhere (I think in the Service Bulletins) saying that Lucas have yet to provide any voltage regulators. The frame prefix for 1937 is XF but the F and XF numbers form a continuous sequence which supports the idea that the X is an indication of the provision of an oil seal in the gearbox with the gearbox changing from 1BAPDH to X1BAPDH (or equivalent for the CP box). If this idea is correct, the change from F to XF should occur around the beginning of January 1937 because Burman changed their gearboxes in January each year. Incidentally, the F prefix was used as the next in sequence after E for the 250cc models in 1932!

1938 sees the appearance of the 4950-38 frame, prefix XG so the X is the oil seal and the G means a changed frame. As far as I can tell, and I can't (yet) prove it for the singles (but I can for the Square Fours) this change is to the tubes running to the rear fork(s?) to accommodate the wider brake drum introduced that year. The change is well nigh invisible and undetectable and I only happened upon it when researching the Square Four rear fork arrangements for a local member whose frame had been modified to swinging arm spec. He wanted to go back to rigid.

1939-45 sees continuation of the XG frame prefix, initially indicating that the 1939 frame was the same as the 1938 one, with a continuous numerical sequence but then I believe expediency took over during the war and they continued with the XG prefix for the higher ground clearance W/NG frames which also had the propstand lug.

1946 sees the AP frame (which was the W/NG type, but whether it had the propstand lug I don't know) and the BP frame which was the civilian type.

1946 - 1949 it would appear from the parts books that the frames were 1938 civilian type or 1940 high ground clearance type but there is a frame assembly drawing entitled 1946 machines which shows 3 changes. These are the prop stand lug under the clutch dome, the sidecar lug on the offside rear fork and the sidecar lug on the steering head.

1950 the parts book admits to a new frame 4950-50 which I suspect incorporates all 3 of the changes shown on the 1946 machines drawing. Whether there are also other changes I don't know.

If your member with the swinging arm conversion has effectively lost the rear part of his frame, the main dating item would be the sidecar lug on the steering head. The only way to find out when this appeared would be to ask people with 1946 - 1950 machines the date of their bike and whether or not is has the lug.

[Paul Slootheer]

Paul I can confirm that the 1946 AP prefix frame did not incorperate the side stand lug as on the W/NG frame (field stand). It shared the same dimensions as the W/NG frame which means it only accepted the 350 OHV engine.

My later rigid VB frame from 1953 does have welded on saddle spring lugs instead of the cast/ brazed ones. Also the headstock is different with some sort of reinforcement rib. Futher more the frame tube (which holds the petrol tank) is welded/brazed directly on the upper frame tube, instead it is fixed into a casting as with early frames. It also incorperates a ‘swing arm style’ prop stand lug.

[simon.holyfield]

Paul J, that's really interesting and just what I wanted. Thank you! I'd also be interested to know what you and Roger would have done in this situation when you were the dating officer.

Curiously, this frame has an AB number (but very much a restamp - font and spacing is all wrong) but no sidecar lug. I shall have a closer look at the pictures, but yours, Paul's, and Rogers information will give me an opportunity to show the DVLA it's a period frame despite having a suspect number.

Thanks again.

[will_curry]

If the frame were converted by one of the firms like McCandless there is a good chance that
they would have either stamped their own number on or restamped the original if it were
removed in the conversion process.

[simon.holyfield]

If the frame were converted by one of the firms like McCandless there is a good chance that
they would have either stamped their own number on or restamped the original if it were
removed in the conversion process.

Thanks Will. Not wishing to appear rude, but what makes you say that? If you have seen evidence of that happening, it would be very useful in my quest with the DVLA.

[Roger Gwynn]

Excellent research by Paul J as always. Interesting to hear from Paul S that the AP frame doesn't have the field stand lug, after all why would it? I have come across fabricated brackets on some 1953-54 rigid frames before, the assumption being that the factory ran out of forged lugs and the tooling was u/s or not worth making as the rigid frames days were numbered. The 1955 RF frame has several fabricated brackets as described by Paul S on his 1953 frame, which is how they can be separated from the 1949 RF frames (side stand lug is another identification point).
There are now 8 rigid singles frames 1936-55.

[Roger Gwynn]

Rabbit hole time. F or XF that is the question.
The frame prefixes are not listed in the despatch books but there are 149 F frames and 192 XF frames in the register and a quick perusal of them shows that, ignoring a few outliers the lowest XF frame number is 3823 and the highest F frame is over 5,000, quite an overlap. I then realised that all but 2 F frames were 1936 models and none of the XF frame were from 1936. So although the numbers continue from 1936 (F) to 1937 (XF) the prefixes do not, although, like Paul, because the numbers clearly very mixed up I assumed that the prefixes were as well.
This does raise the question of how the frames were numbered, after all it is pretty clear that at the change in model year the frame prefix was changed from F to XF but with an overlap in the numbers of 1,200 or so. In practice I don't see how the frames could have been numbered in a numerical sequence as one would expect by a lowly apprentice stamping each frame prior to being enamelled. The only possibility that springs to mind is that after the year change an X was added to the frames already made and a dab of paint added or were the frames stamped after painting anyway, which seems unlikely as there is no reason to. I have 3 frames that have certainly never been re-painted and I cannot tell if the stamping was done before or after painting.

[Paul Slootheer]

Photographs of the 1953 rigid frame.