Apparently the reason so many frames were available with the big hole where the engine used to live is that they used to house Norton's big 500 single, and as they were the engine tuners choice when the Formula three club racers needed an engine for the rear of their Cooper framed car, they had to buy a whole bike as Norton refused to sell an engine independently. Consequently, many bought a whole machine simply for the engine.

The reputation they engendered with a twin duplex frame that rode like a 'featherbed' became popular when people shoehorned twin cylinder Triumphs in to replace the single. The best of both worlds in young mens eyes, and became the machine of legend it deserves to be.

I'd like to think that sticking a sidevalve engine from a previous generation was done simply because you can, as despite their rugged simplicity, they were very outclassed in the cafe stakes..! Oddly enough, it's probably made a nicely profitable end result too..!

Steve