Thruxton is a motorsports circuit built in an ex Royal Air Force airfield that was used by the RAF and the USAF during World War II. In 1950 it was decommissioned and it was then that motorsports, in particular motorcycle races started taking place. At that same time (November 1948) Royal Enfield had introduced its first 500cc parallel twin. This was Enfield's response to the enormous interest generated by Turner's Triumph Twin. But the Enfield machine was very different from any other British twin: it had two separate cylinder heads made of alloy with separate valve gear. The skirts of the barrels were sunk very deeply in the crankcase. As in the Bullet oil was kept in the crankcase and the oil pump was in the timing cover. The oil filled telescopic front suspension and swinging arm rear suspension were as in the Bullet. A 692 version of the twin was launched in Earl's court in 1952, it was called The Meteor. In september 1955 the Super Meteor was introduced. It put out 40bhp at 5,500 rpm, which gave the Enfield a 100mph potential for the first time. To handle the additional power several elements in the bike had been beefed up: crankshaft, cylinder heads, frame. In spring 1958 an even hotter bike, the Constellation was introduced, which put out 51bhp, had two carburettors and lighter camshafts. The Enfield twin, initially perceived as a tourer, had now grown into a serious contender for competition.
An endurance race of 9 hours of duration was created at the Thruxton raceway. Several riders entered Constellations. They achieved impressive feats: posting the fastest laps, clearly outclassing the competition, but never managed to win event. The Enfield seemed to have too much power and always ended up self-destructing in one way or another. The best performance was in the 1958 race when Syd Lawton's team of Bob McIntyre and Derek Powell placed second behind the Triumph team of Mike Hailwood and Dan Shorey. Both teams made four pit stops. The Triumph team had a better coordinated pit response. An analysis showed that the extra time the Enfield team spent at the pits in the stops was more than the final difference in the race time for both teams! Pictures: Top: Geoffrey Brown in 1962. Center: Bob McIntyre overtaking the Velocette Venom of D. Hamilton in 1959. Bottom: Bob McIntyre about to leave the pit in 1960. He later crashed due to mechanical problems.
(Source: "Bob McIntyre: the flying scot" by Mick Walker)