How would you identify this Royal Enfield motor?

Can you identify this Royal Enfield motor?

Identifying now rare Royal Enfield motorcycles from the past is a challenge. I was reminded of this recently when these pictures of a Royal Enfield single-cylinder motor arrived in my email. With them came this plea:

"I would appreciate your assistance in determining what year this motor is. It has been used as a door stop for about 40 years. Unfortunately some chrome was added in the 'swinging sixties.' I would like to sell it and am trying to find out its worth. The only numbers I can find are: 279 on rear S375 on front. Any help you can provide as to year and model would be greatly appreciated. Thank you, Scott"

Royal Enfield used inclined cylinders in the 1930s.

I'm no expert, but the unusual forward-slanting cylinder made me think I might quickly pin down the answer. Never seen one of those before!

Sure enough, the reference material I have showed that Royal Enfield used the inclined cylinder design only briefly, in the 1930s, sometimes with four valves in the head. I turned to blogger Jorge Pullin, who built his online Royal Enfield Early Years Virtual Museum to clear up such mysteries, up to 1929.

Jorge passed along pictures from three possible candidates:

1. The Model T of 1935-38. This was a 148cc motorcycle the catalog called "A real sports model with a remarkable performance and the lowest possible running costs." Bore and stroke: 56x60mm.

2. The Model S of 1935-36, a 248cc motorcycle, called "Probably the fastest machine of its type — certainly the most silent mechanically." Bore and stroke: 64x77mm.

3. The Model S2 of 1935-36, also a 248cc motorcycle, with high compression, called "A tuned sports model with the acceleration of many larger machines." Bore and stroke: 64x77mm

In an effort to identify our mystery motor, I cropped the catalog artwork for each model down to the motor, and set them side-by-side.

Which Royal Enfield model had the mystery motor: T, S or S2?

It is apparent that the catalog pictures were "stretched" a bit by the artists, no doubt to produce a lower, racier looking motorcycle. The actual photograph of the mystery motor shows it taller, and more narrow.

Allowing for artistic variations, the mystery motor could be any of these.

The "S" in the number found on the front of the motor suggests it is one of those, in any case. But how to be sure?

Jorge suggested that pulling off the head and measuring the bore (56mm vs. 64mm) would tell the owner whether it was a Model T or Model S motor.

And that is exactly what Scott did. I'll tell you what he found next time.