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Royal Enfield MkII Interceptor

  royal enfield MkII InterceptorWhat Jim Stothard really wanted was a Triumph. But a revised Royal Enfield Interceptor is what he got. It’s one of those stories, y’know.

Enfield’s Bigger Banger

Royal Enfield lagged Triumph and BSA in building parallel Twins, introducing their first in 1949. Like the Bullet, the dry sump lubricated 500 Twin’s oil supply was held in a tank cast into the engine behind the crankcase. Separate iron cylinders were spigoted into the crankcase, while two chain-driven camshafts mounted high in cases operated overhead valves via short, light alloy pushrods.
When BSA and Triumph both introduced 650cc twins in 1949, Enfield designer Tony Wilson-Jones saw an opportunity to combine the Bullet’s 70mm by 90mm dimensions with the 500 Twin’s crankcase, creating the 36-hp, 693cc Meteor of 1953. The Meteor begat the 40-hp Super Meteor and eventually the 51-hp Constellation of 1958. They were Britain’s biggest side-by-side twins until the AMC and Norton 750s of 1962.
With parallel twins, more capacity usually means more vibration. Uniquely in the British motorcycle industry, Enfield’s big twin crankshafts were dynamically balanced at the factory, making their engines easily the smoothest running of big parallel twins from that era. It helped that the one-piece nodular iron crankshaft weighed close to 40 lbs. and ran in two main bearings the size of hockey pucks!
royal enfield MkII InterceptorThe Meteor and Super Meteor twins were smooth and reliable performers, though a lack of crankcase rigidity caused problems in the fast but fragile Constellation. Fitted with Amal’s all-or-nothing 10TT9 racing carburetor and aggressive camshafts, the “Connie” was quick, but crankcase distortion and poor breathing led to oil leaks, and an unreliable oil feed caused many blowups.
Part of the problem lay in Wilson-Jones’s decision to use two separate cylinder barrels for his twin rather than having both cylinders cast into one block, as BSA, Triumph and Norton did, even though separate barrels would allow for more efficient cooling. The result was that, without the iron “block” to stabilize them, the crankcases twisted under load, allowing oil to leak. This was made worse by having the oil reservoir cast into the engine!
The Constellation engine also suffered from crankcase pressurization, the result of inadequate breathing, forcing more oil out. It was perhaps the Constellation that earned Enfield the nickname “Oilfield.”
When Norton introduced the 750cc Atlas in 1962, Wilson-Jones stretched dimensions again to 71mm by 93mm for 736cc. The Interceptor, as it was called, looked just like the previous year’s Constellation, only now the cylinder barrels were symmetrical and interchangeable from side-to-side. Internally there was a new clutch, and Cross rings replaced the always-suspect head gaskets. Bolted to the back of the transmission was an extra engine mount intended to stop the crankcase flexing. It helped.
A US-spec model introduced around 1965 featured separate tachometer and speedometer, a two-gallon gas tank, 12-volt electrics, longer swingarm, twin headers and a seven-inch front brake. With 52 hp and weighing only 420 lbs., the Interceptor recorded a fastest “out-of-the-box” standing quarter-mile time: below 13 seconds at over 100 mph.
Longtime Enfield Chairman Frank Walker-Smith died in 1962, leading to the sale of the company and closure of the Redditch factory.
And that should have been the end of the story, except that an independent subsidiary, Enfield Precision Engineers, was still in business, working out of underground caves near the hamlet of Bradford-on-Avon, Wiltshire. Established during WWII as a “skunk works,” EPE became engineering contractors after hostilities ceased. The new Royal Enfield brand owners, Manganese-Bronze Ltd, contracted Enfield Precision to continue building Interceptors.
Late in 1967, Enfield Precision launched a revised Mk 1A Interceptor, the type shown here on these pages. Styled as a street scrambler, the “TT” Interceptor had coil ignition, twin Amal Concentrics, upswept exhaust, and front brake “cooling discs.” Otherwise, though, it was mechanically identical, still with suspect engine oiling, single-acting front forks and the weedy seven-inch front brake.
The final Royal Enfield MkII Interceptor of 1968 married Norton forks and brakes to the Enfield frame (a result of the Manganese-Bronze takeover. The company also owned Norton.), while a change to wet sump lubrication cured the oiling problems.
Jim Stothard wasn’t looking for a Royal Enfield when he heard about the one his boss had in storage. He wasn’t even sure what an Enfield was. “I was really looking for a Bonneville,” he says. But the idea of buying it wouldn’t go away.royal enfield MkII Interceptor
The Interceptor had been stored in a garage for years, after some ham-fisted wrenching had cross-threaded a spark plug. So it just sat—oil, gas and all.
“I kept joking with my boss, asking when he was going to sell me the Interceptor. One day he said ‘right now’.”
So a deal was done. Stothard borrowed the company truck and a few buddies, bought a couple of cases of beer, and the Interceptor was shoehorned up the stairs and into his third floor apartment. He rebuilt much of the bike in his living room, though the engine and transmission seemed to be fine. It was time to see if the Interceptor would start…
Some more buddies, a few more six-packs, and the Interceptor was back on pavement. With fresh oil in the engine, fresh gas and a new battery, the bike coughed after a few kicks, and was soon running. But blistering chrome on the exhaust headers meant something wasn’t quite right. Stothard rebuilt the carbs, re-set the timing and the Interceptor was soon running like a top.
“It’s been flawless ever since,” he says, “apart from a couple of coil wires breaking. I’ve never had a single breakdown.”
The engine has 44,000 miles on the original bores, but Stothard suspects the pistons may have been replaced early in the bike’s life. He doesn’t ride in winter, but the bike’s ready to go again every spring. Stothard replaced exhaust valves a few years back, but the rest of the engine has never been apart, and in spite of some mechanical rattles, it runs reliably and cleanly. And the bellow from the scantily-lined Campbell mufflers when Stothard winds it up can curdle milk and stampede cattle!

– Robert Smith, July 2011 (issue #273)


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