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Civil versus military details guide  (Big 4)

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The differences between civilian and military Big 4 combinations were much more pronounced than with the 16H/WD16H.
Because the military Big 4 combinations were conceived at a later stage than the WD16H, they were slightly more modern, for instance by utilizing the enclosed valves as also seen on the civilian motorcycles (16H and Big 4) from 1938 onwards.
As they were provided with a driven sidecar wheel, it required some very obvious modifications of the frame. There is no controversy whatsoever to determine the origine of a military Big 4 frame.
The pre war civilian Big 4 frames were identical to civilian 16H frames.

The frame front end including the saddle and tank tube was similar to the the 16H. Behind that the changes can be found at the rear axle mounting and the lefthand side upper chain stay.  The lefthand upper chain stay was provided with a bracket to mount the sidecar wheel  dog clutch engagement lever.
The forward bolt on rear upper sub frame, to which the saddles and pillion passenger hand grip was mounted, required an additonal fixing onto  the top saddle tube casting. Saddle studs were not required anymore.  
The castings holding the rear wheel axle were modified to carry the wheel bearings and has an additional hole to mount the vertical rear subframe stay.

Here a list giving a number of details differences between civil and military bikes between 1936 and 1947. This is not definitive and probably not complete but based on the best information available to me at the moment.
The military details variations page shows the variations found on military motorcycles only. At the bottom some details of military origine.
 

PART

1936

1937

1938

1939

1940 -1942

1946

Rubber cushioned handlebar clamp introduced in 1935

C

C

M C

M C

M

-
1 inch handlebars C - M M M *11 ?
7/8th inch handlebar - C C C - -

Open valve cover

C

C

M

M

M

-

Enclosed valves

-

-

M C

M C

M

C

Oil tell tale

C

C

-

-

M

-

No oil tell tale

-

-

M C

M C

-

C

Front brake plate left hand side *1

C

C

M C

M

M

-

Front brake plate right hand side *1

-

-

-

C

-

C *2

19" wheels *5

C

C

C

C

-

C

18" wheels *5

-

-

M

M

M

-
Front fork standard width *3 C C C C - -
Front fork extra wide *3 - - M M M -

Front Mudguard standard width *3a

C

C

C

C

-

C *4

Front mudguard extra wide *3a

-

-

M

M

M

-

Standard front fork spring *6

C

C

C

C

-

C

Stronger front fork spring *6

-

-

M

M

M

-

Saddle spring studs vertical

C

C

-

-

-

-

Saddle spring studs horizontal

-

-

C

C

-

C

No saddle spring studs *7

-

- 

- 

M

M

-

Gear position indicator small    *13

C

C

M C

M C

M

-

Gear position indicator large

-

-

-

-

M

-
Gear change lever straight   *14 M C MC - - - -
Gear change lever hooked down *14 - - MC MC MC C
Dolls head gear change mechanism MC MC MC MC MC -
Enclosed gear change mechanism - - - - - C

Nut front fork shock absorber

-

-

M

M

M

-

Handwheel front fork shock absorber

C

C

C

C

-

C

Footrest tubes welded to engine plates

C

C

C

C

-

-

Footrest tubes interchangeable

-

-

M

M

M

-

Serrated foot hanger long

C

C

C

C

-

C

Serrated foot hanger short

-

-

M

M

M

-

Rubber cushioned footrests

C

C

M C

M C

-

C

Rear brake pedal short

C

C

C

C

-

C

Rear brake pedal long

-

-

M

M

M

-

Semi-loop (Diamond) frame

C

C

M C

M C

M

-

Cradle frame

-

-

-

-

-

C
Oil tank standard pre war *8 C C C C - C*9
Oil tank increased capacity - - M M M -

Petrol tank overhead valve engine cut-out

C

C

C

C

-

C

Petrol tank with cut out at right hand side over sparkplug only

-

-

M

M

M

-

Silencer 1935 model

C

C

M

M C

M

C

Silencer 1938 (Cows udder)

-

-

C

-

-

-

Toolbox wide

-

-

M

M

M

-

Toolbox narrow

C

C

C

C

-

C*10
Single rear numberplate top fixing bolt C - - - - -
Two rear numberplate top fixing bolts - C M C M C - *12 C

Tyre inflator under petrol tank

M C

M C

M C?

M

M

-

Tyre inflator on rear chain guard

-

-

C?

C

-

C

 

Notes:

       *1   Different front fork to accommodate righthand brake plate reaction arm,
*2   Telescopic Roadholder front forks, brake plate on right hand side.
*3   Civil Big 4 utilised same front fork as the civil and military 16H. Military fork widened to accommodate wider tyres.
*3a Civil Big 4 utilised the same front mudguard as the 16H. Military Big 4 front mudguards were wider to accomodate the 18” x 4" knobbly tires,
*4   Mudguard changed to fit Telescopic Roadholder front forks.
*5   Military Big 4 hubs are not interchangeable with military WD16H or civil Big 4/16H because they only have provisions to hold one wheel bearing, at the right hand side,
*6   It used a heavier/stronger fork spring to cater for the added sidecar weight. Prewar civil 16H and Big 4s, and the WD16H all used the same fork spring,  
*7   The saddle springs were mounted to horizontal studs on the rear sub frame.
*8   Standard oil tank used on civil 16H and Big 4.
*9   Not clear, might have been prewar civilian design.
*10 In 1947 a completely new design toolbox was introduced.
*11 Spare part number change change after contract C8631, not known what changed.
*12 No rear numberplates after 1940.
*13 Gear change lever (hooked) clinch bolt fitting from bottom in combination with small indicator,
        top fixing on hooked gear change levers in combination with the long indicator.

 Based on:  1937, 1939, 1940 and 1946 spareparts lists,
1938 and 1939 Norton Catalogues,
1938, prototype pictures
1938 to 1943 military motorcycle pictures

  
The military details variations page shows the variations found on military motorcycle combinations. At the bottom some details of military origine.



 
Frame number with S prefix Engine number with S prefix
Large valance fixed on near side of front mudguard Cut out through front mudguard to accommodate front fork
Toolbox provided with caution plate to disengage Side Wheel Drive on hard ground (Plate not always present on contemporary pictures) Large frame/axle cut out to accommodate wheel bearings. No provisions for rear stand.
Sub frame bolted onto rear of  frame onto which saddle and  handle for pillion passenger are fixed. Rear axle to sidecar wheel drive (minus dog clutch) and upper chainstay with mounting for dog clutch engagement lever.
Engine crankcase shield    

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