1961 Matchless G12CS. Owner: Steve Eorio, Paso Robles, California.

The 1950s and ?60s were the era of the UBM — Universal British Motorcycle — a parallel OHV twin sitting upright in the frame, in the 500cc to 750cc range. The original UBM was the Triumph 5T Speed Twin of 1938, soon to be copied by half a dozen of the major British motorcycle companies. Matchless, which built its first motorcycle at the Plumstead works in southeast London around 1901, came up with its own version in 1948, the 498cc G9, with a 66 x 72.8mm bore and stroke. And a fully sprung frame, with a swingarm rear suspension.

It should be noted that in the 1930s Matchless bought the AJS marque and the company became Associated Motor Cycles, Ltd., or AMC, the major difference between the two brands being the lettering on the gas tank.
The G9 engine differed from other UBMs in that it had a third bearing on the crankshaft, between the two connecting rods, to give added strength. The engine?s dry sump lubrication system used the camshaft to run two oil pumps, one on each side of the crank, aiding in efficient lubrication; apparently these engines could go 75,000 miles before any major work was needed. Quite remarkable for a UBM of the era, when top-end jobs were often done at 20,000 miles, bottom-end at 40,000.

The two cylinders were separate, as were the heads, and while this seemed to work well with the 500, as the engine grew larger the lack of rigidity appeared to enhance vibr...