MKIII Interceptor prototype: 1969

Engine figures: number one complete prefix: prototype 1B. Engine machine numbers plus have two EXP1 extra suffix. engines. Production This is a bike that never was. Renowned classic motorcycle racer and Royal Enfield fan Steve Linsdell owns the only complete prototype MKIII Interceptor ever produced. The one you see pictured here is comprised of one of the two extra 800cc engines housed in a replica rolling chassis, and assembled by Hitchcocks Motorcycles. The prototype MKIII was timed at 128mph at MIRA, so the performance gains from the hike in capacity were clearly significant.

Engine: Bore and stroke is 73.5 x 93mm, according to factory drawings in Allan’s possession. Standard MKII crankcases are machined to accommodate wider, more squared-off barrels. Cam followers are set at a different angle to those on the MKII, meaning stock MKII cases cannot be retro-machined to 800 spec. The heads are enlarged to suit the wider barrel casting and fitted with larger valves with smaller diameter stems. Ports are bored to suit. Ignition: Allan has heard the prototype 800 engines were fitted with Boyer electronic ignition. Linsdell confirms his bike was fitted with a Boyer system when he bought it. Brakes: As MKII. Forks: As MKII Other details: The rolling chassis is similar to the MKII. The glass-fibre MKII export tank is used, along with glass-fibre side panels and a toolbox mounted above the rear mudguard.