C15 Engine

The C15 engine was basically a copy of the Triumph 200cc Tiger Cub engine, but as Triumph were owned by BSA at the time it was considered acceptable. The original engine going into production in 1958 was an increase version of the 150cc Terrier engine designed by Edward Turner, but unlike the Tiger Cub the cylinder was mounted vertically. The gearbox was also built as an integral part of the engine which was a radical change from the old method of having a separate unit and so this engine became identified as a 'Unit Single'. The basic design remained unchanged until 1965 when side points replaced the distributor. Other changes included roller bearing big end for the SS80 which had higher compression ratio and increased performance.

C15 / B40 Engine Parts

Engine Types

1959-64   Distributor engines with timing side main bush
1965-66   Side points 'F' engines with timing side main bush
1967-68   Side points 'G' engines with end fed crankshaft

Manufacturing at BSA

The manufacturing year was August through to July of the year given.
(ie. a bike manufactured November 1964 would be a 1965 model)

Distributor engines

Points housed in distributor unit fitted to the rear of the cylinder barrel towards the timing side. Rectangular sloping sump plate.

Side points 'F' engines

No distributor. Points in circular housing near the front of the timing cover. Distributor socket threaded to accept a chaincase cap. Rectangular sloping sump plate.

Side points 'G' engines

No distributor socket. Points as F engines. Square horizontal sump plate.

Power Output

C15 was 15 BHP
B40 was 21 BHP
SS80 was 20 BHP
C25/B25 was 24 BHP
B25SS was 22.5 BHP

Brake Horse Power

A horse power is the power of one horse, or a measure of rate at which a horse can work. A 15 hp engine for example do the work of 15 horses.

Now work can be done slowly or fast. The rate at which work can be done is called power.

The horsepower output of an engine is measured in terms of BHP or Brake Horse Power. The name is derived from the braking device which is used to hold the engine stationary while it is trying to move, while the horse power is measured in labs on a dynamometer.

The BHP of an engine indicates the amount of power an engine can produce at a certain engine speed at wide open throttle.

1958 Cut-Away view of the C15 engine.

Bigger Bore

A friend has a C15F with a round barrel and a 69mm bore. The appears to be a standard piston (stamped on the top), but the problem is that all the C15 to B25 models were 67mm, which you can go oversize to 3 sizes at +0.020", +0.040" & +0.060" with piston and rings for those sizes. A 69 mm is approximately +0.080" or 4 sizes oversize.

It would appear that the bore size of 69mm applies to a Triumph T100. Apparently it is or was a common practice to oversize the barrel from 67mm to 69mm and use a Triumph piston and rings, which increases the capacity and power output.