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 English Model A's and Early US models

These have the same dimensions:
Bore 2.5". Stroke 2.75". Capacity 13.5 cu.in. = 221cc.
Compression ratio (simple) 3.8:1 (actual) 2.88:1

 English Model B's
Bore 70mm. Stroke 74mm. Capacity 285cc.
Compression ratio (simple) 5:1 (actual) 4.05:1
These are identified by the removable cover on the transfer port on the right hand side of the cylinder, with an entwined "S S" on it.

US models from late 1924
Bore 2.625". Stroke 2.875". Capacity 15.5 cu.in. = 255cc.
These can be easily identified as they have vertical fins on the top of the cylinder. Earlier ones have horizontal fins.

 US models have the inlet port at the FRONT, with the exhaust port on the right hand side.
English models have the inlet port at the REAR, with the exhaust port on the left hand side.

Ignition and Electric systems
 English models have a brass flywheel with flywheel ignition up to chassis number 3928.  From that number on, they have a steel flywheel, with a chain-driven magneto situated on a platform which replaced the older flywheel backplate. I know of 7 bikes with this, and 4 of them have Bosch FB1b magnetos, suggesting that this was the usual magneto fitted.

Model A's have both 4-volt AC electric and gas lighting, with the electric lamp mounted on the right mudguard stay and the gas on the left. The rear lamp is electric and gas in the same housing.  

Early Model B's have electric lighting with a dry battery for parking lights, and later ones gas lighting.

 US models have Eisemann flywheel magnetos and electric lighting. The later models have 10-pole generators. Early models seem to have had twin lights, later ones a single light mounted on the front mudguard/fender.

Wheelbase - English models 59",  US models 55.5".
Seat height 27".
Weight approx 190lbs.
Tyres - English 26" x 2.5", US 26" x 3.00"
 English Model C's
 These have the Blackburne 350cc 4-stroke engines fitted, with a Sturmey Archer 3-speed gearbox, and conventional clutch. Dimensions are the same as Model A and B, except for the weight, and the tyres, which are 26" x 3.00".