With the Thruxton, the company knew

it had a true world-beater on its hands

—which was proven at the inaugural 1967

Isle of Man Production TT when

the super-tuned Venom took

1st, 2nd and fastest lap,

thereby consolidating

Velocette as one of

the greatest

motorcycle

manufacturers

of them all.

Velocette Thruxton

     Buyers Guide

 

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Velocette Thruxton primary side

 

Minimal steering lock makes the Thruxton unsuitable for riding in heavy traffic, but on the track or open road there's little to touch one of these for sheer British riding class.
A sorted bike, note, starts on the first or second kick. Barrie James' beautiful 1970 model is a Geoff Dodkin special.

 

Velocette Thruxton brochure image

 

1967 Velocette Thruxton

 Specifications

Type: Air-cooled OHV pushrod twin

Capacity: 499cc (500cc)

Bore & Stroke: 86mm x 86mm

BHP: 41bhp (plus) @ 6200rpm

Compression ratio: 9:2:1

Transmission: 4-speed, multi-plate clutch

Brakes: Twin leading shoe (TLS) drum front,

single leading shoe (SLS) rear

Electrics: 6-volt, 60-watt Miller dynamo

Front suspension: Telescopic,

two way damped

Rear suspension: Swinging arm,
twin shock absorbers/dampers

Wheels/Tyres: 3.00 x 19-inch front,

3.25 x 19-inch rear

Weight: 390bs (dry)

Maximum speed: 110 mph

 

 

 

 

Velocette's "map of Africa" timing cover and high camshaft architecture. These bikes are very much "engineer's machines", but there's nothing in the design that a competent and reasonably dedicated home mechanic can't get to grips with.

 

 

 

CLASSIER THAN A DBD34 GOLD STAR? Some people think so. We’re talking about the Velocette Thruxton, of course; the ultimate expression of the redoubtable Venom/Viper architecture, the last truly great classic performance British single; a machine built, appropriately enough, by one of the most single-minded, most innovative, most well-respected manufacturers of
them all.

But the Venom didn’t get to Thruxton in one giant leap (and the Viper didn’t get there at all). Rather, there were a number of smaller hops along an increasingly rocky commercial road, each of which helped transform a machine that was merely hot into something that was little short of awesome.

Launched in 1956, the Velocette Venom was conceived and marketed as sporting alternative to the more sedate (read; flexible/touring) MSS; itself a more than merely-competent motorcycle which, its various updates notwithstanding, had served the company well since 1935.

Borrowing the ‘square’ 86mm x 86mm bore and stroke of the MSS, the 499cc, 36bhp Venom, with its (significantly higher) 8:1 compression ratio, monobloc carburettor, V-belt dynamo and full-width hubs front and rear, was an instant success. And for those ultra-traditional traditionalists (for whom the 350cc capacity still ruled), there was the ‘scaled-down’ 349cc, 27bhp Viper (8.5:1). Not a pint-sized Venom, mind, but a creditable (some would say ‘superior’) machine in its own right.

 

In 1959, fibre-glass engine/gearbox streamlining on both Venom and Viper was a factory option. But as Triumph, Norton and Vincent (among others) were to discover for themselves, the world—despite the insistence of 1950s-style focus groups—simply wasn’t ready for enclosed motorcycles and sales of these ‘Sports’ models were disappointing.

Nevertheless, the prestige of the firm’s OHV ‘staple crop’ singles was, despite declining company fortunes, riding high—and rose higher still with the introduction of its Venom/Viper Clubman models which, along with other mods, ran even higher compression ratios (8.75:1 and 9.3:1 respectively) and were fitted with a manually controlled BTH magneto (as opposed to Lucas units), a TT carb, rear-sets, low bars and an ultra-close ratio gearbox.

Fast, punchy, and ‘eminently steerable’, it was a Clubman Venom which, in 1961, snatched the 24-hour speed record at Montlhéry, France taking the 12-hour record at an average of 104mph, and the 24-hour at slightly over 100.

It’s a 500cc class record that stands to this day.

 

But good though the Clubman package was, it still wasn’t good enough for some; notably those anxious to break the sporting dominance of BSAs coup de grace, the all-conquering Gold Star. Which was why in 1965 the firm, aided by a bevy of Velocette privateers and aficionados, pushed the envelope a step further and issued a high-performance race-kit complete with radically revised cylinder head, an Amal GP carb, reworked cam-followers—plus all manner of other goodies that were guaranteed to separate the wannabees from the gonnabees.

The following year, the kit was incorporated into the factory produced Thruxton. And all the rest is a mixture of history and mystery—as evinced by the various Velocette idiosyncracies that, no doubt, have helped conspire to take the marque out of the hands of the often highly frustrated and leave it squarely in the preserve of none but the highly dedicated.

With the Thruxton, the company (now in serious decline) knew that it had a true world-beater on its hands—which was proven at the inaugural 1967 Isle of Man Production TT when the super-tuned Venom took 1st, 2nd and fastest lap, thereby consolidating Velocette as one of the greatest motorcycle manufacturers of them all.

Around 5750 Venoms were sold against the Viper’s 3589. The total number of Venom-based Thruxtons is said to be around 1108. The last factory-made examples were built in 1971.

The survival rate is fairly high, and getting higher, due to Venoms being upgraded (or even vandalised, if you prefer), and re-presented in Thruxton specifications.

 

Arguably the most distinctive feature of the Venom/Viper engine is the timing-side ‘map of Africa’ high-camshaft layout. Conceived by Velocette’s Eugene Goodman and designed by Charles Udall, the concept was intended to help keep reciprocating masses down whilst providing a cost-effective alternative to the more complex (and therefore expensive) overhead camshaft ideal.

It was a good, pragmatic design which—in a reworked form—was to serve rival Vincent well (in fact, Vincent’s Phil Irving had previously worked for Velocette and had doubtless cross-pollinated this and other Velocette ideas).

Driven through a series of helical-cut timing gears, the relatively soft M17/7 cam was fitted to the 349cc Viper, while the hotter M17/8 found its way into the 499cc Venom (although the M17/8 would soon be standardised). This gear train also spun the Lucas KIF magneto on the standard models. Whereas the Clubmans fitted either a Lucas KIF-TT or a BTH-TT mag (BKH1).

 

Housed within the timing chest, this arrangement would not only contribute to the Venom/Viper’s legendary reputation for quiet mechanical operation, but also to their famed longevity.

At the heart of the dry-sump engine resides a pressed-up steel crankshaft (featuring Velocette's shallow taper crankpin-to-flywheel fit) housed within a narrow, and therefore correspondingly stiff, pair of matched aluminium vertically-split crankcases. Spinning between a pair of taper-roller bearings, the Venom/Viper bottom ends are said to be good for upwards of 50,000 miles, and often much more—provided, of course, that assembly tolerances are rigidly adhered to, and that the oil is changed at never less than factory specific service intervals. The oil pump is gear-driven by the crank and is generally durable.

The deep-spigoted cylinder barrel was, until 1961, made from Alfin alloy on the Venom/Viper/Thruxton range. But from that year onward, a cast iron barrel was standard; a wise move that saw less distortion problems.

Pistons on the Venom and Viper are split-skirt (but not slipper type) with valve cutaways. While pistons fitted to the Clubman and Thruxton models are solid-skirted with larger rings and ring gaps.

Standard Viper carburation is via a 1-inch Amal monobloc (type 376/56), while the larger standard Venom breathes through a 1-3/16-inch (389/15). But in both instances, other chokes sizes are available. The Thruxton, with its heavily ported head (with re-angled and oversized valves), utilises a 1-3/8-inch T5GP2.

 

The clutch is either a mechanical masterpiece or mishap depending on whether or not you (a) come to grips with its novel operation and (b) learn to maintain it the way the factory specifies. With its six plain plates and six friction plates, the Venom/Viper clutch employs a unique tilting mechanism that peels plates off—as opposed to separating them in the conventional (i.e Triumph/BSA/
Norton) manner.

‘What this means,’ according to long time Velocette specialist, Colin East of East Restorations in Lincolnshire, ‘is that the clutch should never be held in at traffic lights. It’s a tip clutch introduced by Velocette in the mid-1920s. Pulling the handlebar lever causes the pressure plate to tilt—which means that the driven plates never entirely separate. This continuous contact can lead to overheating which in turn buckles the plates. And when that happens, you’re in a vicious circle that makes the problem progressively worse.’

For all that, most Velocette owners still broadly agree that it’s a good, smooth and progressive-in-action design—albeit one that requires a unique set-up involving the insertion of a small adjuster through the engine sprocket and into the clutch mechanism, and then rolling the bike forward or backward, depending on whether the clutch is slipping or dragging, with marginal slippage being the ideal.

‘But it’s not as complicated as it seems,’ according to John Hannis, Technical Secretary of the Velocette Owners Club. ‘And once you get used to it, it’s perfectly easy to maintain and live with.’

 

What makes the Venom/Viper clutch equally interesting, however, is not merely its operation but also its location. Contrary to general motorcycle practice, the Venom clutch sits between the gearbox and gearbox sprocket—whereas generally speaking, the clutch on British bikes sits outboard in the primary chaincase on the far left with the gearbox sprocket between it and the gearbox.

But if this novel arrangement makes the clutch less accessible, it certainly makes gearing changes very quick simply by removing the final drive sprocket cover and switching cogs. In the everyday world, however, this is of minimal advantage to the average street rider.

The gearbox is of Velocette’s own design; a constant-mesh mainshaft/layshaft close-ratio unit built in the conventional manner. Bearing removal and replacement notwithstanding, the unit is generally easy to work on in situ and gives little trouble in service. Venom/Viper gearbox shells, incidentally, have the numerals ‘12’ stamped on the top right hand side of the case. If a letter ‘R’ is visible, it denotes the Clubman ultra-close ratio gearbox.

 

The single (split) cradle frame is, broadly speaking, built according to traditional Velocette lug’n’braze method with most joints hand fitted and pinned. Post-1961, more welding appeared on the frame for selected lugs and brackets.

Less typical is the swinging arm design which utilises two separate arms instead of the single, two prong fork familiar to most riders. In practice it works well, but set-up alignment is another slightly fussy job that needs careful attention (and which is actually often in conflict with factory recommended alignment procedures, take note).

Sidecar lugs are the rule rather than the exception up until 1965 when they started to disappear. But finding a Thruxton with sidecar lugs is by no means unknown.

The forks are fairly conventional hydraulic single-damped units (of Velocette’s own design and manufacture). Clubman models, however, fitted two-way damping which is sometimes criticised for providing an ‘over-firm’ ride for road use, which is where most of these bikes—despite their sporting intentions—actually stayed.

 

Rear damper units are equally conventional sealed units; either Woodhead Monroe, Armstrong or—towards the end of the production run—Girling. Adjustment is, in all cases, provided for by loosening of the top bolt and sliding the unit through the arcuate slots. In practice, owners will find the best compromise between extremes and leave well alone.

Velocette electrics are, says Colin East, an ‘acquired taste’. Specifically, the Miller cartridge regulator is apt to give problems. ‘It’s has a solenoid type mechanism which often fails to provide the right voltage. The unit is best replaced with a solid state regulator—although the cartridge regulator can be left (and disconnected) for appearances.’

Prior to 1962, Miller electrics were fitted throughout the Venom/Viper range—the dynamo driven by V-belt to preclude slippage. Post 1962, Lucas components began to appear. Instruments—on the standard Venom/Viper—are housed within the pressed steel headlamp nacelle and include speedometer, ammeter and light switch. Rev-counters were an option and were fitted to a separate bracket on the right hand side.

 

On the Venom/Viper and Mk1 Clubman models, brakes are reasonably effective full-width single-leading shoe items; 7-1/2 inch at the front and 7-inch at the rear. For the Thruxton and Mk2 Clubman, a 7-1/2-inch twin-leading shoe brake was fitted—which has since been retro-fitted to other Venom/Viper owner/enthusiast machines.

Wheels front and rear are 19-inch with WM2 rims.

 

Engine weaks spots are few, according to John Hannis of the VOC. ‘But cylinder studs have been known to pull out of the crankcases, usually through poor maintenance.’

Roger Perfitt, an engineer from Dartford, Kent (and a Venom rider of 25 years standing) agrees. ‘Many of the threads of the Venom/Viper cases are small and fine. They strip very easily, so anyone buying a bike in parts should check these carefully where possible.’

‘Poorly set-up cranks generally reveal themselves in vibration damage,’ points out Colin East. ‘Check around the rear mudguard stay. If it’s cracked—or cracked and welded—it’s 90-percent likely that the flywheels are, or were at some time, misaligned. These bikes should be very smooth, and any abnormal vibration will become apparent very quickly in a test ride.’

‘Moreover,’ says John Hannis, ‘hard-ridden bikes that haven’t been set-up right have been known to push bearings right through the drive side crankcase. A four-thou bearing preload is essential.’

Timing side noise is sometimes a problem too.

‘That’s because the helical-cut timing gears need to be carefully lapped,’ explains Roger Perfitt. ‘Any mis-match will show up quickly enough to an experienced ear, but it’s not always obvious to a newcomer.’

‘Also,’ adds Colin East, ‘Helical gears, by their design, have a side-load that straight cut gears do not have. This means that if the camshaft wheels or intermediate wheels have excessive end-float, the result is an unwanted (and ultimately destructive) tapping between the crankcase and the outrigger plate.’

A properly lapped-in and set-up timing gear train should be exceptionally quiet.

Speaking of gears, the transmissions are usually reliable. But jumping out of first is not uncommon with high mileage bikes. ‘It’s generally the dogs, of course,’ explains John Hannis. ‘But teeth strippage occurs too. So buyers should check gear selection carefully and listen for suspicious noises.’

Handling, adds John, should be ‘smooth and predictable’ and advises that ‘all day long 70-80mph cruising’ is possible. Modern laser-cut clutch plates and bonded friction plates help preclude the likelihood of an occasional overheating clutch.

 

Overall, Venom/Vipers are capable of high mileages requiring little more than routine maintenance and overhauls. Provided, that is, you can start them. More than most British bikes, Venom/Vipers, due mainly to low kickstarter gearing, are notorious fussy starters, and its part of the mythology that ‘real Velocette men’ can fire ’em up in about five seconds. The rank-and-file rider could well be there all day—which is another reason to ensure that the clutch has been set-up accurately.

Wet-sumping is another Velocette bug-bear. Oil inevitably creeps past the ball and spring on the pump feed side; the solution being regular use (which is always the best way to maintain a classic) and possibly an oil line shut-off linked to the ignition switch to prevent accidental—and terminal—oil starvation.

Vibration fracturing is another problem—albeit one that’s certainly no more common with Velocette than other marques. Check the front mudguard bracket and oil tank lugs. Colin East, meanwhile, reminds buyers to check the top gearbox lug for cracks. ‘It’s a common problem,’ he says, ‘caused both by vibration and by poor (i.e. ham-fisted) primary chain adjustment.’

Also, check fuel tanks for corrosion, notably behind the knee grips. Petrol tanks can also crack around the headstock, usually due to a loose or missing tank brace.

 

Barrie James - an owner’s story

 

"I got into Velocettes through my father who was friends with Geoff Dodkin, the well known and respected Velocette specialist. My dad met Geoff when they were about twenty years old they and spent a lot of time riding round together and hanging out in the usual way. A few years later they went off to do their National Service, both in the RAF. My dad became an instrument fitter on Lincolns and Lancasters. Geoff, as far as I know, was an engine fitter.

When they left the air force, Geoff began his career working for the police as a mechanic. He was looking after both cars and bikes, and later worked for Stevens, a Velocette dealer in the Goldhawk Road, West London. Geoff soon came into contact with some very influential people at Velocette such as Tommy Mutton, a technician who was instrumental in building/creating the famous Velocette Roarer.

 Geoff eventually opened his own business in the bike world, and my dad ended up as a keen club competitor riding for Geoff and racing Gold Starsoften being paired with Vic Willoughby. 

Geoff remained a friend of the family throughout. When he retired, he had amassed a lot of parts from the factory, enough to build a replica to the racer that Geoff built which came 2nd in the 1967 Production TT ridden by Keith Heckles. When that bike went through the speed trap at the Highlander on the Isle of Man TT circuit, it was timed at 124mph. It was sold recently by Bonhams and fetched £36,000

A few years ago, having pestered Geoff for many years for the parts he had amassed with a view to building a replica, he finally agreed I bought the bike off him as a dry build.

We went to great pains to get the fairing correct and fitting properly. The Smiths Grand Prix rev counter in the front of the fairing was never a standard fitting, The silencer has an upswept exhaust that was never from the factory; this was Geoff Dodkin’s rework to get extra ground clearance. The oil tank is extended to carry approximately one extra pint of oil. The ally heat shield is a one-off period addition. 

The engine is a standard Thruxton unit, good for about 105-108mph. The gearbox sprocket is a standard 19 tooth item. 

The seat is a one-off. Velocette did make single racing seats, but this is my interpretation. It’s been cut and shut and upholstered in industrial vinyl with a suede panel in it for grip.

The rear brake stay is Mk8 KTT brake stay in alloy. The battery box is a one off to copy the endurance racers.

The Thruxton is insured for about £15,000, but I’m sure it would easily fetch more than that.

 It's a great bike. I love it.

 


 

 

Top Velocette Links

 

Velocette Owners Club

www.velocetteowners.com

 

www.velocettedorkingcentre.btck.co.uk

 

KTT Services

Kevin Thurston produces and sells a range of products/goodies for performance Velocettes including belt drives, hubs. billet timing covers, barrels and yokes. Check out his website.

www.kttservices.co.uk

 

 

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All this and the open road