Roy Henderson has brought this Thunderbird back from a previous life as a Speed Twin and made it his.
Words and photography by Stuart Urquhart
It’s been three years since I pestered SCMC member Roy Henderson into letting me write up his restored 1950 Triumph 6T Thunderbird for Classic Bike Guide. Covid, then a family house move from Fife to Ayrshire and back again, interrupted my promise to deliver this feature. But in October 2024 I arranged to meet Roy and his 650 Thunderbird once again to uphold my promise. We agreed a get-together in ‘The Fife Riviera,’ specifically in the quaint little fishing village of Pittenweem (Fife’s Penzance), the perfect location in which to photograph Roy’s lovely Thunderbird restoration. But as readers are about to discover, this is no ordinary classic. This gracefully ageing pre-unit Thunderbird previously enjoyed life as a Triumph Speed Twin.
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Before posing Roy’s motorcycle for the customary CBG photographs, we indulged in a brief catch-up chat. I was amazed to learn how this lovely example of one of Turner’s finest had been carefully and sympathetically restored by Roy. ‘Sympathetically’ might be mistaken as a loose term for ‘a less than perfect job.’ However, as Roy heartily explained: “I was on a mission to return this early Thunderbird to as near-original condition as possible, and as an example of a classic that has survived more than 70 years of continuous use.”
“I bought SO 9261, registered as a Triumph 6T Thunderbird, from an Aberdeen dealer in 2015,” he continued. “It was a non-runner and in barn-find condition. As expected, the bike was missing several parts and in quite a sorry state – however, it was complete. Unusually, it looked more Speed Twin in appearance than Thunderbird, largely due to its Amaranth red paintwork and a replacement 500cc Speed Twin engine.”
At this point, Roy decided to research his purchase before undertaking any project to get it sorted and running. With assistance from the Triumph Owners’ Club, factory records revealed that SO 9261 was dispatched to a Charles Farquharson, of Elgin, on May 13, 1950. The logbook next records the Thunderbird in Glasgow in 1956. Thereafter it changed hands five times before the sixth owner moved south to Andover, then sold it in 1962.
Roy speculates that owner number six was an exuberant rider and may have blown-up the 6T’s engine – possible, because records reveal that the 6T engine was replaced with a Speed Twin 500cc motor during this owner’s possession. Roy also believes that the Thunderbird was painted Amaranth red to match its replacement engine. Circumstantial evidence for this theory is that at some point a con rod had also exited through the crankcase of the replacement engine! According to tax and MOT records, the Thunderbird then moved to the vicinity of Hampshire and Middlesex until 1996.
Roy again: “On concluding my research, I decided to get the Thunderbird running first, and then I’d consider whether returning it back to its original specification would be worth the effort. I knew this would entail plenty of restoration work and sourcing a replacement 6T engine which could prove difficult to find. Meanwhile, I invested time in fettling… I added new fluids, a new battery, rebuilt the 276 carburettor, fitted new tyres, and then immersed myself in all the other tasks in returning a barn find back to the road.
“By the autumn of 2015, I’m pleased to say, it roared back to life! I was chuffed, and very much enjoyed runs on the Speed Twin – or ‘Replica,’ as I began to think of my rejuvenated Triumph. However, the more I used it in 500cc form, the more I wished for it to be returned to its original 650cc Thunderbird specification, so I committed to a long and protracted hunt for a replacement 6T engine.
“After several years of false trails and relentless searching, I secured a set of crankcases and a crankshaft from a 6T model. Eventually, all the other parts required to build an engine were found. The used crankshaft turned out to be a later model big-bearing crank, so its journal diameter had to be reduced to fit the small bearing cases already in my possession.”
Work begins
Roy entrusted skilled engine work to Anderson Engineering, of Kirkcaldy, which also carried out smaller jobs such as reaming the valve guides. The barrels had recently been bored to plus 0.060in, so there would be no possibility of future rebores. Grin Triumph, of Glenrothes, supplied bearings, gaskets, valves, guides and so on, as well as a new oil pump. The con rods had evidence of damage, so Roy sourced a new pair. Also required were new cylinder head bolts (more about which follows later). Next, the engine was carefully assembled and valve clearances set, along with the ignition timing. The gearbox had worked well during test rides, but it was stripped again and carefully examined before being passed as fit for service.
“I knew the rear rim was badly rusted, so I purchased a new chrome rim and stainless spokes from Central Wheels. After knocking up a DIY jig, I decided it was time I had a stab at wheel-building. I was surprised when this daunting job actually turned out to be quite straightforward; perhaps because a Triumph sprung hub is symmetrical and almost builds itself!
“The electrics, however, were a different matter. Bench tests revealed my dynamo would only deliver 9v at high revs, so earlier thoughts of a possible 12v conversion evaporated. The battery hadn’t been charging, and following detective work I discovered the old mechanical voltage control box was the culprit. I purchased a solid-state regulator and LED lights from Paul Goff.
“Next, I hid the new regulator inside the old regulator’s metal box, before a rewire restored normal service. Spinning the magneto produced plenty bright sparks, so it was set aside for re-use. My 6T model would originally have had an Amal 276 carb with separate float chamber, but prudence dictated fitting a new 1 1/16in 376 Monobloc as the more reliable solution. Suddenly, I’d arrived at a finish and my transformed 650cc Thunderbird was now ready for a test run.”
First test run
Roy added fresh oils and fuel to his restored Thunderbird, and it started on the second kick, before settling to a nice tickover. A new pump guaranteed good oil pressure and Roy observed a nice and steady return to the oil tank. After warming up the engine, it neither rattled, smoked or made worrying noises – even the exhaust note sounded sublime and so typically Triumph. Roy could hardly contain his excitement in taking the 6T for a run to test the new 650 engine.
“I set off on my usual route. All was going well when the engine’s power suddenly dropped, to be followed by clouds of blue smoke. Moments later the engine abruptly stopped. More alarming was discovering there was no compression when I attempted to kick the engine over. I was truly baffled. The only solution was to collect my van, load up the stricken Thunderbird, and return to my garage to investigate the problem.
“After opening up the top end, I discovered the replacement head bolts I had fitted were a few millimetres longer than the original set. On barrels that have been bored out to plus-60, the bolt holes are getting closer to the cylinder walls. My assumption was that the slightly longer head bolts had reached into virgin thread, and when I investigated these lowermost threads I found them to be heavily rusted. I deduced that tightening of the head bolts had expanded these rusted threads and introduced excessive pressure that damaged the bore. Unfortunately, the resulting high spots caused the piston rings to overheat and seize to their respective pistons, catastrophically reducing engine compression.”
Obviously devastated, Roy thought he would learn from the experience and just move on. The purchase of new barrels and pistons saw the Thunderbird (and Roy) make a rapid recovery and the engine was carefully run-in. The summer of 2021 saw Roy using his rebuilt Thunderbird to tick off several SCMC Scotia Challenge checkpoints and enjoy memorable trips around Scotland’s stunning scenery.
Much-needed restoration
As Roy racked up the miles on his Thunderbird, he confided that its tatty and rusted Amaranth red paintwork was becoming a concern. Being the wrong colour for a 6T, he also felt it was a poor reflection of his meticulous rebuild. So, in the winter of 2021, he once again embarked on a full strip-down of his 6T with the sole intention of changing its colour. Roy informed me that during the first year of Thunderbird production, the factory supplied bikes in Thunder Blue – however, this rather dull colour did not win favour in Triumph’s most important market, the US. So, in 1951/52 the colour was tweaked to a lighter metallic blue. It is Roy’s belief that this could be the most common colour seen on classic Thunderbird’s today.
Back to Roy: “My approach has always been to refurbish any classic bike to the standards of its heyday. For example, in the 1950s and 1960s, motorcycle owners would simply fix mechanical problems and tidy rusting paintwork. ‘Complete restoration’ was unlikely to be considered appropriate for a working, everyday bike.
“So, after blasting the tinware, fuel tank and frame, the tank was treated with a chemical ‘de-rust’ solution, followed by an ethanol-proof liner from Flowliner – as supplied by Ace Classics. After rubbing down and priming all previously painted parts, I carefully brushed on several coats of Thunder Blue cellulose supplied by RS Bikepaints. Using fine emery paper, the paint was then ‘keyed’ before being finished in the same colour aerosol cellulose. I then rebuilt the bike using original fasteners, after which a new saddle completed the refurb/restoration.
“In the winter of 2023, the front end was treated to an overhaul, with new fork stanchions, bushes and seals. The poorly performing front brake drum was also skimmed by Anderson’s Engineering and the front wheel rebuilt with new stainless spokes and rim, supplied again by Central Wheels. The ‘bars I changed to US-style in the expectation of a more relaxed and comfier ride, and new control cables throughout completed the fettling.
“Generally, I find the Thunderbird is now a first kick-starter and only requires flooding the carb when cold. A larger 24T engine sprocket was fitted to give relaxed 60mph cruising, with 80mph-plus available for overtaking. The rigid handling is predictable but does require a degree of caution when negotiating damp, twisting or severely potholed roads. The usual reservations about using classic brakes and planning well ahead also apply. The brakes are of their time, and new shoes on the front hub have yet to bed in, but I’m hopeful their performance will improve with use.”
“Fortunately, the rear brake is a belter! Let me just add that any classic rider who habitually tailgates, especially in modern traffic, should reconsider their choice of mount!” Roy laughed. “I’m overjoyed that my Triumph 6T Thunderbird is now a very pleasant and thrilling motorcycle on which to explore the back roads and quiet highways of northern Scotland. I can’t get enough!”
Brief history – the Speed Twin
The Triumph Speed Twin was launched at the 1937 Earls Court Motorcycle Show. With girder forks, rigid frame, a 500cc OHV pre-unit engine and striking Amaranth red paint finish, it was an instant hit. The Speed Twin engine is credited as being one of Edward Turner’s finest and the forerunner to Triumph’s successful twins. However, before leaving Triumph, talented engineer Val Page designed sidevalve parallel twins as early as 1913, and his designs and ideas were known to have had an influence on the young Turner.
Turner’s Speed Twin engine was a pre-unit (separate gearbox and engine), four-stroke, OHV parallel twin with a capacity of 499cc. Fitted with a cast-iron barrel and cylinder head, it relied on two lower camshafts fore and aft to operate the overhead valves via short pushrods. A single Amal side-float carburettor took care of fuelling.
In 1946, telescopic forks replaced the girder forks, and a rear sprung hub became available as an optional extra to add limited suspension to the rigid rear frame. Strengthened rods and new profile camshafts were added in 1951. Coil ignition and an alternator were introduced in 1953 (with the 6T model following just one year later).
By 1955 Triumph had introduced rear swinging arm suspension, with Girling rear shock absorbers across all models. Other upgrades that year were iconic grill-styled tank badges and a full-width front hub. The Speed Twin and the similarly styled 650 Thunderbird became the definitive Triumphs to own at the time. The esteemed Speed Twin enjoyed a production run spanning almost 30 years before it made way for the new unit-construction 5TA model, produced from 1958 to 1966.
Brief history – the 6T Thunderbird
Acclaimed as one of ‘the best-looking motorcycles ever made,’ the Triumph 6T Thunderbird 650 was launched in 1949 as a direct result of the Speed Twin’s universal success and growing customer demand for more power both at home and in America. The new Thunderbird model’s name was influenced by a North American tribal mythical bird of immense power and beauty and its adoption is attributed to the charismatic Edward Turner and the Meriden publicity department.
It became a legend motorcycle in its own time.
To publicise the Thunderbird’s launch, senior management devised an ‘endurance test’ that would capture both press and public attention. As proof of the Thunderbird’s reliability and endurance, three Thunderbirds would be ‘tested’ for 500 miles at a sustained speed of 90mph at the famous Montlhery Racing Circuit in France, thereafter completing a final lap of the circuit at a flat-out 100mph (secret practice laps were undertaken before the public event). After the test, the three Triumph’s would be driven back to Meriden, stripped, tested, and declared fit under scrutiny of the press, such was Meriden’s confidence in its product.
The publicity stunt was a resounding success and the Thunderbird became another sales sensation for Triumph, both at home and in the US. Another trump card for Triumph was Hollywood’s use of a 1950s Thunderbird by the actor Marlon Brando in the classic biker film The Wild One.
As with other models, handling was improved in 1955 with the introduction of swinging arm suspension and Girling shock absorbers, and the engine was upgraded with a bigger main bearing and shell-type big ends. An updated fuel tank with famed ‘mouth organ’ chrome badges was added in 1957. In 1960, an unpopular bathtub was fitted (then dropped), along with deeper valance mudguards, and by 1961, the 6T was given an alloy cylinder head and 8in front brake. 12v electrics arrived in 1964, along with revised tank badges and ‘fashionable’ 18in wheels.
Meriden, however, had another famous model waiting to steal the 6T’s thunder – the pre-unit Triumph Bonneville. Announced in 1957, the Bonneville was launched the following year to a receptive motorcycle press at Earls Court. Arguably, the Bonneville became Triumph’s most celebrated and iconic model throughout the 1960s and beyond. The popular pre-unit 6T Thunderbird, however, remained in production until 1963, before it was replaced by the unit construction 6T Thunderbird model until 1966.
Demand for classic 6T Thunderbird remains very strong, and early models are becoming scarce to find.
6T specifications
Bore and stroke: 71 x 82mm
Capacity: 649cc
Compression ratio: 7.0:1
Power: 34bhp @ 6300rpm
Top speed: 96mph
Transmission: Four-speed
Brakes: Drum 7in (F); 8in (R)
Fuel tank: Four gallons
Engine oil tank: Six pints (SAE 20/50)
Suspension: Telescopic (F); sprung hub (R)
Ignition: Magneto
Electrics: 6v
Carburettor: Amal 276
Dry weight: 385lb
