Two different types of Production Racer were produced from 1970 to 1972, for reasons that are best explained with a little Norton history. Associated Motor Cycles (AMC) acquired the bankrupt Norton Motorcycles in 1953, but maintained production of Norton models at Bracebridge Street in Birmingham until 1962. In that year, the aging factory was closed and production moved to AMC’s Plumstead, South London plant. By 1966, AMC itself was in financial trouble. Dennis Poore’s Manganese-Bronze Holdings bought AMC from the receiver, rebranding the organization as Norton-Villiers (M-B had absorbed Villiers a few years earlier) with production facilities in Plumstead and at the Villiers factory in Wolverhampton. The Norton range was rationalized around the 745cc Atlas engine, the same unit being fitted in AJS/Matchless cycle parts to create the G15, N15 and P11 models.
When the Commando was introduced in 1968, engine production and bike assembly was centered at the Plumstead factory, with frames being supplied by Reynolds. The first-generation (Plumstead) Commando engines carried the marking “20M3” in front of the engine number (for Model 20 Mk III) and had their contact breaker points in a housing behind the timing case where the magneto had been on the Atlas. These engines were used in the 1968-1970 Fastback and 1969 “R” models. The first (1970) Production Racers were based on the Plumstead bikes with their characteristic Atlas-type “cigar” mufflers and exposed, gray-painted oil tank. After the Plumstead factory closed, engine production was moved to Wolverhampton, with Commando assembly focused at the new factory in Andover. The engine was slightly modified, moving the contact breaker inside a small round cover on the front of the timing case, with a steel plate covering the hole where the points had been. Wolverhampton engines were marked “20M3S.” The new engine was used in the 1970 “S” model and the 1971 Roadster, with its upswept peashooter mufflers and oil tank hidden behind side covers. Thus, the Roadster formed the basis of the later 1971 Production Racers. MC
For more from Robert Smith see Yellow Peril: Norton Commando Production Racer.