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Period Modified: Triumph Drag Bike

  • Engine: Stock TR6, 649cc air-cooled OHV parallel twin, 71mm x 82mm bore/stroke, 8.5:1 compression ratio, 42hp @ 6,500rpm stock (upgraded to 12:1 compression ratio, power unknown)
  • Carburetion: Dual 30mm Dell’Orto SS1 30 A
  • Transmission: 4-speed, chain drive primary and final
  • Electrics: Lucas Racing magneto
  • Frame/Wheelbase: ca. 1948 TR5 steel cradle type, 53in (1,346mm)
  • Suspension: Triumph telescopic fork front, rigid rear
  • Brakes: Front N/A, Rear 7in (178mm) drum
  • Tires: 3.00 x 21in ribbed Avon front, 4.00 x 18in rear M&H drag slick
  • Weight: unknown
  • Seat height: 24in at the lowest, 28in in at the  highest (609.5mm – 711mm)
  • Fuel capacity: .25gal (0.95ltr)

A competitor needs to focus solely on making it to the end of the run, everything else is just a distraction. But surely, back in the 1950s and 1960s when backyard builds were common at tracks across North America, one would be wondering if all the components of their machine would remain together at maximum velocity while hurtling 440 yards to the finish line.

Decades ago, many ambitious drag bike builders based their racing machines on commonly available motorcycles, such as Triumphs. With some mechanical ingenuity and parts from aftermarket retailers such as Webco and Harman & Collins, the Triumph 650cc parallel twin engines could be hopped up to offer tremendous power in a lightweight package.

Jeff Thompson of British Columbia, Canada, owner of the drag bike featured here, considers Triumphs built between 1955 and 1970 to be the most versatile motorcycles ever constructed.

Owner Jeff Thompson with his drag bike back in the truck. 

“During Triumph’s glory years, their 500 and 650 twins were the ultimate multipurpose bikes,” Jeff explains, and continues, “One of my favorite motorcycles in my garage is a 1970 Triumph T120R. I’ve owned that bike for 25-plus years, and it gets ridden at least weekly. It has been a picture of reliability.

“I love that bike and think 1967 to 1970 Triumph Bonnevilles represent the very best in motorcycle style. But it was my relationship with that 1970 Bonneville that set me on a quest to collect race bikes based upon the Triumph 650 twin platform. A flat tracker, road racer and drag bike.”

Two out of three

By the mid-2000s, Jeff had a period-modified Triumph flat tracker and a road racer in his stable but was without the drag bike. Not having much success in locating an original example, he was getting set to build his own machine based on a 1/8 scale 1964 Revell plastic model kit of a Triumph drag motorcycle.

Here’s where I play a small role in this story. I’ve known Jeff for years, and he’d mentioned to me his interest in locating a period-built Triumph drag bike. In 2007, while scanning eBay I came across a listing for a Triumph drag bike. I emailed it to Jeff, but by the time he’d found it the auction had ended, without reaching the seller’s reserve.

“I was able to reach the seller, though,” Jeff says. “I asked him what he wanted for it, agreed to the price and asked him to crate it up and drop it off at a Roadway terminal and had it shipped to Canada. It really was as simple as that.”

As delivered, the Triumph was complete, but it appeared somewhat cobbled together in some aspects of its construction. Cosmetically, it was rather challenged, too.

“It was cool, but I wasn’t going to leave it like that because it wasn’t good enough,” Jeff says. What Jeff had purchased was a drag bike he thinks was built circa 1968. It consists of several different Triumph models but is essentially based on a 650cc 1957 Triumph TR6 Trophy engine and separate Triumph 4-speed gearbox, in a format commonly known as “pre unit” construction. Although Triumph built its first unit-construction powerplant with the 350cc 3T/A in 1958 followed by the unit 500cc 5T/A in 1959, it didn’t convert its 650cc line to unit-construction until 1963.

 The rear tire is a 4 x 18-inch M&H drag slick. The seat is just a small cushioned pad on the rear fender. 

Looking back

Lindsay Brooke, noted Triumph historian and author, offers some insight into why Triumph machines were often used as the basis for constructing a competitive drag racing motorcycle. In an email to me, Lindsay writes, “Triumph’s 650 twin, particularly the pre-1963 models with separate engine and gearbox, was the popular choice for drag bikes because Triumphs were affordable, accessible, and responded well to speed tuning.”

The stock 649cc TR6 engine ran at 8:1 compression and made 42 horsepower. Its current output is unknown.

He adds, “Because of the brand’s overall popularity, Triumph enjoyed the industry’s largest speed-equipment aftermarket led by Webco, Harman & Collins, and other suppliers aligned with Triumph’s eastern and western U.S. distributors. And, many of the tuning tricks that made Triumphs winners on U.S. dirt tracks transferred easily to the drag strip.”

Digging in

As Jeff took his drag bike apart, he was most keen to see what was inside the engine and treated the dismantling like a mechanical archaeological experience. He says opening up an engine of unknown background can be a nightmare, or it can be really interesting. “This engine was very interesting,” Jeff explains. “The original builder had his heart and soul in that engine. It wouldn’t have been cheap or easy to do.”

Starting at the top, the engine has a circa 1959-1960 eight-bolt Triumph Bonneville twin carburetor cylinder head that looked virtually brand new, with no wear or much evidence of the engine having been run. Inside the alloy rocker boxes, the rocker arms had been ground to lighten them up and polished. Instead of the 8.5:1 Triumph pistons that would have been in the cylinder bores, Jeff discovered 12:1 Robbins pistons. Manufactured by Charlie Robbins in Los Angeles, Robbins offered a line of performance pistons to fit many popular makes of the day, including Triumph, Harley-Davidson and Indian machines. To further increase compression on Jeff’s racing motor, there was no head gasket between the alloy Bonneville head and the cast iron barrels. To achieve a seal, however, the mating surfaces had been treated with some kind of aluminum paint.

“Although not street bike appropriate it was apparently not uncommon for a race bike like this,” Jeff says of the sealing treatment. Inside the TR6 alloy engine cases, one of the details that most impressed Jeff was the installation of the Harman & Collins No. 7041 Super Drag & Bonneville roller tappet camshafts.

“Installation of these cams was not a simple task,” Jeff explains. “It would have involved enlarging the cam tappet block holes in the cylinders to accept the larger ‘roller tappet’ blocks, and the stock push rod tubes had to be cut at the base and a new wider base welded on. Pretty cool stuff, and this was done very neatly.”

Harman & Collins

Harman & Collins was Kenny Harman and Cliff Collins of California. Harman was grinding camshafts for speed applications prior to World War II, while Collins was grinding cams for Kenner Engines, an aircraft powerplant company. The two joined forces in 1945 in Alhambra, California, working from a small shop behind a gas station. In 1950, Collins bought out Harman, and later merged H&C with Schiefer Clutches & Flywheels in 1961. That’s when Collins sold the motorcycle camshaft side of the business to Gary Robinson. According to the Crazy Horse Speed Shop website, a company located in Sturgis, South Dakota, they currently own the assets and intellectual property of H&C.

A period Harman & Collins advertisement says, “Most tuners are acquainted with the superior characteristics of Roller Tappet Cams, such as improved wearing qualities, reduced friction, and more sophisticated valve lift profiles than are available with standard cam followers. Harman & Collins is the only company that manufactures a successful Roller Tappet Cam for Triumph motorcycles.”

For comparison, Triumph’s most progressive camshaft in engines such as the Bonneville was the E (or 70) 3134 with a .310-inch cam lift, giving .350-inch lift at the valve. The H&C No. 7041 Super Drag & Bonneville had a .384-inch cam lift, providing .414-inch valve lift.

More goodies

On Jeff’s drag bike, under the right-hand side timing cover all of the timing gears had been drilled, lightened and polished. A K2FR Lucas Racing Magneto provides ignition. When Triumph designer Edward Turner first drew his 500cc parallel twin engine that appeared in 1937 in the Speed Twin model, the crankshaft was a bolt-together 3-piece affair. The left and right journals were connected to the central flywheel via six high-tensile steel bolts. Residing in these TR6 cases, Jeff discovered a later Triumph one-piece polished crank complete with polished Triumph alloy connecting rods.

This mixing and matching of Triumph parts was not unusual, Lindsay explains. He says, “Edward Turner’s iconic design offered ‘hemi’ combustion chambers and the flexibility of separate inlet and exhaust camshafts. The 650 Triumph was lighter than other British parallel twins, and engine builders could mix the best parts from Thunderbird, Trophy, Tiger-110 and Bonneville models. Twin-carb cylinder heads fed by proven Amal TT or Grand Prix carbs, brought more power potential.”

 The Triumph engine is fed by dual 30mm Dell’Orto carbs.

Instead of British-made Amal TT or Grand Prix carburetors, however, Jeff’s racing machine features two Italian-made 30mm Dell’Orto SS1 30 A “Green Label” units on the intake stubs of the Bonneville head.

While the engine was apart, Jeff simply cleaned and polished the alloy timing cover and rocker boxes and sealed everything up as he meticulously bolted it all back together.

Moving to the separate 4-speed gearbox, the outer and intermediate covers were removed to reveal the layshaft, mainshaft and sliding gears. “All the gears in the transmission had been polished, which I’m sure took a considerable amount of time to accomplish,” Jeff says.

Of note is the modified outer primary cover. Jeff thinks the builder, at some point, must have intended to fit a larger, more robust clutch than the Triumph six plain steel and five friction disc setup. In anticipation of that change, he says a large hole was cut towards the rear of the outer cover, where the clutch sits inside, and the bottom of an aluminum kitchen saucepan was welded over top. “Interestingly,” he says, and continues, “although this modification to the cover was made the clutch remains the original. Obviously plans change.”

Tying it together

Both the engine and the gearbox are mounted in an early Triumph TR5 Trophy rigid frame, a chassis that was first designed in 1948 for the TR5, or Trophy model. The first 500cc Trophy engine used the lower end of a T100, but the cases were topped with the unique alloy square barrel and head from a gasoline-powered portable electric generator set Triumph built for the Royal Air Force during World War II.

The frame, with its 53-inch wheelbase, was 1-inch shorter than the standard Speed Twin and T100 rigid chassis and the Trophy models proved successful mounts in International Six Days Trial competitions. But, by its very nature of being a shorter chassis, the Trophy frame would have made Jeff’s drag bike, with its well-built 650cc engine, likely something of a handful to launch down the drag strip. Mounted to either side of the frame top rail are two homemade alloy tanks, the right carries a quart of gasoline while the left holds a quart of oil.

The right foot shifter mounts to a footpeg on the rear axle.

Up front, the fork is a standard Triumph telescopic item that’s been modified so that it can be adjusted for height. The only instrumentation is a 10,000rpm Smiths tachometer that’s driven off the exhaust camshaft.

Wheels consist of a spool front hub laced to a 21-inch Borrani alloy rim with a ribbed Avon tire while the rear features a Triumph cotton-reel hub and 7-inch drum brake laced into an 18-inch Borrani rim. The rear drag slick is a 4 x 18-inch M&H.

During the rebuild, Jeff cleaned up a few areas including making a new seat, re-engineering the rearset controls and adding new exhaust pipes. The frame was powder coated black, as were the fork triple trees and lower sliders. He also modified the alloy tanks so they would accept proper filler caps, and petcocks on the gas side.

“To say the bike as it is today is a restored version of the bike as I bought it would not really be a correct representation,” Jeff explains. “Although I took great pains to rebuild and refresh the engine and its unique modifications, the chassis is more my own vision. I did make an effort to make sure the finished bike stayed reasonably true to the era the bike was originally constructed in, and I tried to retain some of that homebuilt/amateur character.”

The restoration took about a year, and Jeff says when he was done the bike, which runs on pump gasoline and not nitromethane, fired without too much drama. It does take a few people, though, to get it started.

Jeff says, “I utilize a gas-powered race bike starter setup that spins the rear wheel. With me on the bike operating the controls, a helper operates the starter unit. A third person, if you have one, squirts fuel into the carburetor bell mouths. Suitably prepped and primed it fires right up and I’ve run it through three heat cycles.”

Jeff uses a gas-powered race bike starter which spins the rear wheel.

As it exists today, the bike wouldn’t pass tech inspection at any racetrack, as it has no front brake, no chain guard, and no safety kill tether. Although curious to know how it would run under full power, Jeff won’t have to worry any time soon about whether it would stay together down the quarter mile.

“In reality,” Jeff concludes, “it has no real practical purpose other than to please me — which it does every day.” MC

Photos by Grant Robinson

Published on Aug 6, 2020
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