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Frame
The frame on the C15 consists of two main members. The front engine cradle
and the rear seat  support
are bolted together, the top
joint having a steel bush fitted through the members, and the lower at
the swinging arm pivot where it is retained by bolts. The rear seat section also provides attachment of the rear suspension and mudguard.
The C15 and B40 frames are similar have different diameter top tubes, the
B40 is bigger. 
The easy way to tell them apart is by the front mounting lug for the
petrol tank The tank front sits on 2 rubbers, which are mounted on a
pressed steel bracket. On C15 this bracket loops up and over the frame
tube, saddle fashion, and is welded to it. On B40 this bracket is a
straight channel shaped pressing which is bolted to two small lugs welded
to either side of the top tube.
Click on the pictures view detail
Swinging Arm
The swinging arm suspension is mounted onto the rear of the frame by a large
steel king-pin.
 This has grease nipples
in the bolts at each end to lubricate the brass
bushes within the swinging arm unit. The brass bushes fitted to the rear suspension swinging arm are unlikely to
need replacement for some considerable time. When it is found necessary to renew
them, first remove the suspension units by detaching the top pivot bolts and the
bottom retaining nuts. Remove the rear wheel and chain guard. Then undo the swinging arm spindle nuts and carefully tap out the spindle using a suitable
drift. Lift the rear fork until it is clear of the side plates, when it can then be
turned and pulled away from the rear. The bushes should be removed with a suitable drift and the new bushes will need to be reamed out to size after fitting into the swinging
arm.
Front Suspension Units
  The front forks consist of a steel stanchion, this slides within the lower fork tube which contains the
damper oil. The bearing surfaces consist of two brass bushes, one fixed to the bottom end of the stanchion, and the
other at the top end of the lower fork tube, through which the stanchion slides. Fixed to the bottom of the lower fork tube
is the damper restrictor rod which passes through the stop nut that is screwed
into the bottom of the stanchion. The restrictor rod is tapered at the bottom
end to give progressively harder damping as the suspension begins to bottom out.
At the top of the lower fork tube there is a screwed collar, which is chrome plated
and contains an oil seal. The cover tube pushes into the top of this collar and
on later models also contains an oil seal. On reassembly it is recommended that grease
is smeared around the bear metal of the fork stanchion to prevent corrosion
while in use.
Click on the pictures view detail
Rear Suspension
 The rear suspension units offer little opportunity for maintenance. You can
remove the springs, but the damper units are sealed. The C15 models were fitted with a damper spring of 124 lb/inch,
which is retained by
chromed collets fitted at its base. A service tool was
made by BSA,
part number 61—5064 to facilitate removal. This is assembled as shown and when the nut is screwed down sufficiently
the spring is compressed releasing the collets.  The collets can
then be
extracted through the apertures in the tool and the spring comes away when the
tool is removed. Reassemble is in the reverse order.
As the above tool is unlikely to be available, I have designed a similar tool
which will do the job. Approximately half the tube on one side has been
removed (B) to
facilitate the extraction of the chrome collets.
Click on the pictures view
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