A single-cylinder engine tuned for endurance racing
The noisy and oily centre of a Thruxton racer, pulled to bits and given a good coat of looking-at
The Venom Thruxton, sold from 1965 until Velocette’s Hall Green factory closed early in 1971, had a long ancestry. It evolved out of the factory’s first pushrod single, the 250cc MOV of 1934, which then spawned the pre-world War II 350cc MAC and 500cc MSS. In the mid-’50s, a new generation of singles with engines on similar lines included the sporty 500cc Venom.
In 1960 Velocette introduced the Venom Clubman, and in 1961 a factory machine set a 500cc world record at an average of 105.21mph over 24 hours, still unbeaten by any single-cylinder machine. Subsequently Velocette offered the 41bhp Venom Thruxton to homologate a 500 for production racing. With a bigvalve cylinder head and Amal GP carburettor, its design revisions were based on tuners’ experiments in several countries.
Velocette dealers entered UK endurance races, dominating the 500cc class in the mid-1960s. One was Geoff Dodkin, whose shop at East Sheen on London’s South Circular Road became a magnet for Velo fellows. Dodkin’s second attempt at the Barcelona 24 Hours race in 1965 saw his riders Ellis Boyce and Tom Phillips finish second in the over-250cc class. The engine seen here was built specifically for that ’65 Spanish marathon.
All Velocette ohv singles have a super-slim crankcase and narrow, very rigid crankshaft. Placing the primary-drive chain inboard of the clutch and final-drive sprocket enables the one-inch (25.4mm) diameter drive-side mainshaft to be short and stiff, with adequate support from a single main bearing. Main bearings on both sides are taper-roller units with an outside diameter of 2¼in (57mm).
Slow-tapered ends on the mainshafts press into the flywheels with an interference fit and are pinned by axial ¼in screws. The crankpin also has taper fixing, which is strong enough to dispense
with securing nuts, so recesses for them are not needed to clear the conrod; consequently a high proportion of the pin’s overall length is supported in wheel metal. There is an obvious Dodkin racing modification to these flywheels, which have bevelled outer edges to reduce crankcase oil drag and cut weight. Less obvious is that they are from an earlier MSS model, heavier in standard condition and made from a higher grade of steel than the Thruxton’s wheels.
The steel conrod, originally of H-section, is lightened and polished. It runs on the standard big-end’s single row of caged
9/16in x 3/16in rollers, while the small end has a plain bush with drillings in the rod and bush to admit oil. The long-skirted threering piston has a domed crown with front and rear flats for valve clearance. The compression ratio is set during build-up by measuring oil poured into the combustion chamber, with the cylinder tilted so the spark plug hole is at the highest point. Compression is varied by fitting shims of different thickness under the base flange of the cast-iron cylinder barrel.
The sand-cast alloy cylinder head fits on a shallow barrel-top spigot, with a plain copper gasket. Four long 3/8in studs passing through the head and barrel screw into steel sleeve inserts threaded into the top of the crankcase and nuts with washers hold the head down. The barrel’s lower fins are shaved back on the right side to clear the ‘map of Africa’ timing chest formed in the right-side crankcase half, which extends well above the barrel joint. Inside it, the four timing gears have silent-running fine helical teeth.
The camshaft, high up on all Velo ohv singles to minimise pushrod length and reduce flex, is driven from a pinion on the crankshaft via an intermediate reduction gear, which has a ‘hunting tooth’, to spread wear across all teeth. The ignition magneto’s gear is driven by the camshaft gear. The camshaft, a taper-fit in its gear, also turns on a fixed spindle and here Dodkin runs both gears on needle roller bearings. He also placed radialroller thrust bearings on either side of the intermediate gear to cope with lateral forces generated by helical teeth. The gears’
spindles are supported and located at their outer ends by a steel steady-plate bolted to the crankcase. Above and slightly to the rear of the camshaft axis, two triangular cam followers – called ‘bottom rockers’ in Velocette parlance – pivot side-by-side on a spindle pressed into the crankcase wall and held by the steady plate. They have hard radius pads to bear on the lobes and cups to carry the pushrods, while a dished Belleville washer behind the innermost inlet follower controls side-play. The Duralumin pushrods’ steel lower tips have ball-ends to fit the follower cups and smaller steel cups at the top that normally engage with threaded adjusters in the rocker arms. Here, Dodkin welded up the adjusters and set valve clearances for Barcelona by machining the pushrods to length. Telescopic plated tubes enclose the rods.
Not on spindles, the case-hardened rockers have external journals between their arms running in half-bearings formed by the rocker box alloy and alloy caps screwed to the box. Hard buttons on the left-side arms make linear contact with the valve stem tops. Two-piece hairpin valve springs are anchored at their open ends in blocks surrounding the tops of the valve guides. Shims are placed under the blocks to set spring preload. Holders restraining the springs’ top loops are held to the valve stems by collets, via recessed collars that allow the valves to rotate in the guides and reduce the likelihood of heat distortion.
A smoothly-flowed inlet tract leads mixture from the 13/8in (35mm) Amal GP carb to the Thruxton head’s huge two-inch (50.8mm) inlet valve. The combustion chamber is shallower than a complete hemisphere. Dodkin fitted a Viper exhaust valve – lighter than a Thruxton’s, it eases some load in the valve gear.
A spiral gear, secured at the timing-side end of the crankshaft by a nut on a left-hand thread, engages with the driving gear of the high-output oil pump, which is pressed into a housing at the rear of the timing chest. Oil is pumped from the tank into a gallery in the timing cover with four outlets. The first is a quill feed to the
end of the crankshaft to supply the big-end from a drilling that delivers oil to the rollers at an angle from the side, rather than through the crankpin. The next two outlets above lubricate the camshaft spindle and supply an oil jet in the steady plate aimed at the cams. The uppermost outlet is to a pipe leading to the rocker box, where oil is fed via a banjo joint to the inlet rocker and thence to the exhaust. To ensure adequate lubrication at Barcelona, Dodkin added external pipes on top of the rocker box to distribute oil evenly to both rockers. Two drain pipes run from the side of the head to a single union on the pushrod tube, so that oil falls onto the cams, followers and timing gears as it descends to the lower crankcase. From there, it is drawn through an internal pipe to the pump’s scavenging gears and returned to the tank.
For Barcelona, Dodkin devised an extended pump to prevent build-up of excess oil in the crankcase. A second set of scavenge gears is added externally to the standard pump’s mounting plate to draw oil through an external pipe from a union tapped into the drainage channel from the timing chest to the base of the crankcase.
An exhaust-valve lifter aids starting. A short shaft with a return spring, supported in the front of the timing chest, has a cam at its inner end to lift the exhaust follower when the lever at its other end is pulled by a cable. Ignition sparks are by a flange-mounted Lucas K1FC magneto. Its driving gear (not shown) incorporates a centrifugal auto-advance device.
Bevelled edges of flywheels are a Dodkin racing mod to reduce oil drag and cut weight Extended Dodkin oil pump (right) and standard item Oil supply is via timing cover Inlet tract feeds two-inch valve
Rockers are secured by alloy caps screwed to the rocker box
Extra rocker oil feeds were added externally to this engine for 24-hour racing