Royal Enfield's Biggest Twin

Text: Robert Smith • Photography: Robert Smith

Jim Stothard wasn't looking for a motorcycle when he heard about the Royal Enfield his boss had in storage. He wasn't even sure what an Enfield was. But the idea of buying it wouldn't go away. It took two years of needling and cajoling before Jim was even allowed to see the bike.

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. But when the deal was done, Jim borrowed the company truck and a few buddies, bought a couple of cases of beer, and the Interceptor was soon installed in Jim's third-floor apartment.

It was the early 1980s, British bikes were out of favor, parts were scarce and advice even scarcer. One local motorcycle dealer even warned Jim not to bring the bike anywhere near his shop! The only information Jim could find on his new bike was in Bernie Nicholson's Modern Motorcycle Mechanics.

Fortunately, he stumbled across Vancouver's British Motorcycle Owners' Club, and a number of members helped out with parts and wrenching advice. It was time to see if it would start...

Some more buddies, another case of beer, 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 was wrong. Jim handed the bike to another BMOC member who rebuilt the carbs and set the timing, and the Interceptor was running like a top in no time.

"It's been flawless ever since," says Jim, "apart from a couple of coil wires breaking. I've never had a single breakdown."

Jim replaced exhaust valves a few years back, but the rest of the engine has never been apart, even though the odometer shows more than 40,000 miles, and in spite of some mechanical rattles, it runs reliably and cleanly. The bellow from the scantily-lined Campbell silencers when Jim winds it up can curdle milk and stampede cattle.

It's a tribute to Jim's spit 'n' polish that the bike looks as beautiful as it does. You can see your face in the carbs, a real achievement given Mr Amal's rough die-cast zinc alloy. The Mk1A drips chrome anyway - fork ears, headlight, gas tank - and Jim's polishing efforts have created a real eye-popper.

Jim Stothard's Royal Enfield is a 1968 Interceptor Mk1A, built close to the end of a development line that started long before, in 1949...

Enfieldology: The Twins
Royal Enfield lagged Triumph and BSA in building parallel twins, but once there, seemed determined to build the biggest. The prosaically named "500 Twin" of 1949 borrowed its basic layout from Edward Turner's Speed Twin, but Enfield designer Tony Wilson-Jones incorporated many ideas of his own.

Like the single-cylinder Bullet, the Twin's oil tank was cast into the engine behind the crankcase. The separate cast-iron cylinders were spigoted into the crankcase, clamped by the same through studs securing the separate heads to the cylinders. Two camshafts set high in the crankcase operated overhead valves via short, light-alloy pushrods running in tunnels cast into the cylinders.

Other advanced features included a full-flow oil filter, semi-unit construction (the transmission bolted to the back of the engine), and extensive use of aluminum alloy in cylinder heads, primary case, (and later) hubs, forks and the fashionable headlight "casquette."

When BSA trumped Triumph in 1951 with the 646cc Flash, the race for capacity was on, and Triumph was quickly back with the 649cc Thunderbird. At his drawing board, Wilson-Jones saw an opportunity to combine the Bullet's 70mm x 90mm dimensions with the 500 Twin's crankcase to create the 36hp 693cc Meteor of 1953. The Meteor developed into the 40hp Super Meteor and eventually the 51hp Constellation. They were Britain's biggest side-by-side twins until the AMC and Norton 750s of 1962.

With factory dynamic balancing, the earlier 700 twins were smooth, reliable performers, though a lack of crankcase rigidity caused problems when the Constellation arrived in 1958. Fitted with Amal's all-or-nothing 10TT9 racing carburetor and aggressive camshafts, the "Connie" was a fast, fragile bike with a tendency to leak oil - and explode. The torsional flexibility of the engine unit allowed the castings to move against each other, causing oil leaks. And the oil pump would sometimes lose pressure at high speeds, interrupting oil supply to the big ends with devastating results. Although Enfield experimented with different oil-pump specifications, it was always suspect.

When the 750cc Norton Atlas arrived in 1962, Enfield bounced back with one last stretch: a 93mm stroke and 71mm bore for 736cc. The new Interceptor 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 chamfered steel "Cross" rings replaced always-suspect head gaskets.

For 1965, A US-spec model appeared, with separate tachometer and speedometer, a 2-gallon fuel tank, 12-volt electrics, longer swingarm, twin headers and mufflers, and a seven-inch front brake. The home market model retained the Connie's headlight "casquette," six-volt sparks, "siamesed" pipes, 5-gallon tank, and twin six-inch front brakes. Neither of the front brake options - 2 x 6" or 1 x 7" - was very effective. But with 52hp and weighing only 420lbs, the Interceptor recorded the fastest "out-of-the-box" standing quarter-mile time - below 13 seconds at over 100mph.

Meanwhile, after longtime Chairman Frank Walker Smith died in 1962, The Enfield Cycle Company was sold to E&HP Smith Ltd., which had its eye on the Redditch, Birmingham factory site's development potential. By 1966, the Enfield range (which in 1960 featured 150, 250, 350, 500 single and twin, and 700cc bikes) was reduced to two models: the home market 250 Continental GT and the export-only Interceptor. In January 1967, the 250 was gone too, and in March, Dennis Poore's Manganese-Bronze, owners of the newly-formed Norton-Villiers, bought Enfield's remains.

And that should have been the end of the story, except for a set of caves in Bradford-on-Avon, Wiltshire. There, subsidiary, Enfield Precision Engineers Ltd, had been set up during WWII to work on secret weaponry, becoming engineering contractors after hostilities ceased. As part of the M-B deal, Enfield Precision was contracted to continue building Interceptors.

Late in 1967, Enfield Precision launched a revised interim Interceptor. Gone was the magneto, replaced by coil ignition. The MkIA as it became known, also had twin Concentrics, upswept exhaust, and front-brake cooling discs. Export models got "scrambles" type bars and a two-gallon chrome tank. A fiberglass battery box replaced the traditional Enfield tin toolbox, while a large-diameter oil tank breather addressed crankcase pressurization problems.

Mechanically, though, the MkIA was identical to its predecessor and suffered the same propensity to self-destruct if not maintained properly. Enfield finally got it right with the MkII, which arrived late in 1968.

The new bike had a revised engine with wet-sump lubrication and contact-breaker points in the timing chest. The finned crankcase helped cool the oil, while the filter was moved to the top of the sump. But the biggest change was to the bicycle, with Norton's Roadholder forks and eight-inch brake fitted at the front. The result was a significantly more reliable motor in a package that could now stop as well as go. Enfield Precision struggled on for two more years, finally ceasing motorcycle production in 1970.

TECHNICAL SPECS:
1968 Royal Enfield Interceptor TT Mk1A

Owner 60 x 54mm
Engine 736cc high camshaftOHV four-stroke parallel twin
Bore x Stroke 71mm x 93mm
Compression 8:1
Output 52.5 bhp
Lubrication Dry sump withreciprocating pump, in-line filter
Carburetors Amal Concentrictype 930 30mm (2)
Ignition Battery/coil
Transmission Chain primarywet multiplate clutch withAlbion 4-speed gearboxautomatic neutral finder
Final drive Chain with snail-camadjuster and cush drive in rear hub
Front suspension Enfield single-actingfork with alloy sliders
Rear suspension Pivoted fork withcoil springs and dampers
Lighting Alternator, 12 volt
Frame Steel tubeengine as stressed member
Wheels WM2 x 19" front, WM3 x 18" rear
Front brake 7" SLS with"cooling discs"
Rear brake 7" SLS