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Looking at the various lists available, it seems that there are gaps
in the numbers known versus the numbers expected. One way to try to
fill these gaps is by making an inventory of all existing numbers. This wil
inevitably be an incomplete inventory (many bikes lost, not everybody
responding etc) , but it will hopefully give some insight in the
numbers.
The S numbers are the original War Department numbers from late 1939
onwards. Prewar military bikes had numbers without the prefix in line
with the civil bikes.
Like with WD16H engines, also engines with an AS prefixed spare
engines are found.
Norton did not make a distinction here to separate 16H and Big 4's.
Dating
the bikes based on the contract numbers is a tricky issue. Bikes made
on a contract date in one year could very well have been delivered the
next year. I added the most likely manufacturing year without
pretending to know it exactly!
The frame number is found on the left hand front petrol tank frame lug
casting. The engine number is found on the left top corner of the left
crankcase half.
Sidecar chassis number is found on top of the front lower sidecar to
motorcycle fixing.
(See pictures below).

On
(WD)16H´s
I have found different casting numbers on head stock and saddle
fixture.
I'm wondering if this has happened with the BIG4's as well.
If anyone has these numbers, please add them.
Frame casting numbers are found at the left, aft side on the steering
head, saddle casting number is found on the left-hand side below the
saddle.
Petrol tanks have individual numbers at the bottom, front forks have
individual numbers at front lower cross brace (left hand side when
viewed from the front).
If
you want to help unravel the past, please send me your frame, engine and sidecar
chassis numbers (and preferably the others as well).
If you want to have your bike on the website, send a picture.
Rob
(WD)BIG4
Motorcycles
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