Great engines in bits: 1948 350cc Velocette KTT Mk VIII

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This cammy Velocette engine is as idiosyncratic and beautiful as the bike it powered

hen Percy Goodman, son of Veloce Ltd’s German founder, set out to design a thoroughly modern engine in 1924, he placed the camshaft above the cylinder head, with rockers to open the valves. The camshaft was driven from the crankshaft by a vertical shaft and bevel gears, a method already used by JAP, Moto Guzzi and others. Advantages claimed for this configuration were simplicity, rigidity and durability, especially compared with pushrod valve operation.

The K-type Velocette (the model name for lightweights would replace Veloce as the marque name) proved unreliable in the 1925 TT, prompting Goodman to redesign the engine, with an improved lubrication system and camshaft drive. The next year Alec Bennett took the resulting bike to victory in the Junior TT by 10 minutes, setting a new lap record at 68.89mph – and demand for ‘cammy’ Velocettes forced the company to move to a bigger factory at Hall Green in Birmingham. In 1927 Goodman took on Harold Willis, a racer and brilliant engineer.

Velocette became a major force in TT and Continental GP racing, mainly in the 350cc class, although 500cc engines were also fielded. From 1930, the rival Norton factory adopted Velo-style ohc, launching their most glorious era in racing. In 1936, factory dohc Velocette engines preceded Norton’s by a year but, discouraged by camshaft drive snags, Willis did not pursue that development path. Sadly he died from meningitis in 1939.

Privateer racers and Manx GP competitors could buy the 350cc KTT racing model, the development of which closely followed works bikes, most notably ridden by superstar Stanley Woods. The first MkI issue for 1929 was followed in 1932 by the MkIV with hairpin valve springs and in 1935 by a MkV. The next evolution was the 1938 MkVII, with an alloy cylinder head, soon followed by the 115mph MkVIII for the following year. Post-war production was revived from 1947-52, although build quality deteriorated and the girder-forked Velo began to look obsolete beside Norton’s featherbed-framed Manx and the AJS 7R. Freddie Frith and Bob Foster won the 350cc FIM World Championship in 1949 and ’50 with dohc works engines, but after Peter Goodman, director in charge of KTT production, was seriously injured in a Continental event, the family turned away from racing. 

The KTT MkVIII engine seen here is number 1008, dating from September 1948. Its vertically-split aluminium crankcase is very narrow. Making the crankshaft as short as possible helped its rigidity, while being able to drop the lowest part of the engine down between the bottom frame rails was seen as desirable for handling. As on all Velo singles, the primary drive is inboard of the final drive, so the engine sprocket is close to the drive-side main bearing, with minimum overhang. The sprocket has a cam-type shock absorber, loaded by a hefty spring coiled from square wire. 

Built up from six pieces, the crankshaft has slim flywheels made from heat-treated carbon steel, rather than the commonly used cast iron that proved weak in the earliest K racers. The nickel steel mainshafts are pressed into tapered holes in the wheels and pinned with screws, half in the shafts and half in the wheels, then machined flush with the wheels’ inner faces. The shafts’ main bearing journals are very slightly tapered so that they lock tightly with the main bearings’ tapered inner races on assembly. Both mains are 10-roller units, normally an interference fit in the crankcase, but in added steel inserts here.

The crankpin, which has a shrunk-on hardened bearing journal sleeve, is also a taper fit in the wheels, secured by nuts on each end, fitting in recesses. The smooth-surfaced I-section steel connecting rod has a hardened track pressed into its big-end eye for rollers to bear on. A phosphor-bronze bush carries the stout 13/16in gudgeon pin at the small end where small holes in the rod and a scroll cut in the bush provide lubrication. Four studs threaded into the top face of the crankcase have female threads in their tops, to receive the bolts that hold down the cylinder head and barrel. The iron bore liner stands proud of the head joint to form a spigoted joint, sealed by a thin copper gasket.

Considered massive in its day, the head has an integral cam box and valve spring enclosures, 9in square finning (works engines had 10in heads) and some vertical fins in the air gap under the central part of the cambox. It is cast in Y-alloy, a heat resistant aluminium-based alloy invented in the UK during World War I.

The combustion chamber follows a pattern favoured by engineers seeking ultimate performance from the 1930s to the 1960s. It is hemispherical with two large valves set at an included angle of 70°, the 14mm long-reach spark plug slightly to one side and a domed piston crown to obtain a high compression ratio. Different ratios would have been used in the KTT’s competitive life, depending on whether the fuel was petrol-benzole or the low-octane pool petrol racers were forced to use in the post-war period. The standard figure was just below 11:1.

The valve seats, shrunk into the head at very high temperature, are of different materials. A hard iron is used for the inlet and an alloy with high heat conductivity for the exhaust. The valve guides are of phosphor bronze, used for its conductivity and bearing properties. The valve springs are two-piece hairpin types. Their lower ends slot into steel blocks surrounding the guides and their looped upper portions are bridged by caps with chamfered apertures in them. Wedge rings fitted in the apertures retain split collets in valve stem grooves made as shallow as possible so as not to weaken the stems. The valve spring chambers are closed by covers, each fixed on eight ¼in studs.

Stamped out of hard steel and shaped to be light yet strong, the valve-operating rockers have floating phosphor-bronze bushes between them and their spindles, which are threaded into the cam box on the drive-side and made eccentric, so they can be rotated and fixed by lock-nuts to set valve clearances. Velocette fitted hairpin return springs around the rockers to keep them in contact with the cams, but most owners omit them; they are not seen here.

Two integral cam lobes fit onto the camshaft, with a Woodruff key to locate them. The shaft, which runs in a ballrace in the drive side of the cambox and caged rollers at its driven end, has a flange carrying four studs onto which the crown bevel gear is secured by nuts. Housed in a casing fixed to the cambox by studs, the gear is driven by a smaller bevel with a short integral shaft in a housing and bears in a bush. Another small bevel is in a similar housing mounted on a timing chest cast integrally with the timing-side crankcase half. The two gears are joined by a solid steel vertical drive shaft, via Oldham couplings that cope with minor misalignments or dimensional changes due to heat expansion.
A steel tube enclosing the shaft holds plain bearings for it to run in and is held in place by ring-nuts threaded onto the bevel housings, where asbestos string was used for sealing.

In the timing chest, a bevel keyed to the crankshaft drives the shaft’s lower bevel. Odd numbers of teeth on the gears ensure that the same teeth are not always in mesh, causing wear.

A spur gear outboard of the driving bevel, locked to the crankshaft by a taper joint and secured by a nut on a left-handed thread, drives a gear of twice its size that has an integral shaft, supported by a plain bush in the inner magneto drive chaincase.

A recess in the gear’s inner face carries a toggle coupling to turn the oil pump’s drive shaft. In its cylindrical steel body, the pump has pairs of gears for feed and scavenge, the latter having wider teeth. Ports on the pump body mate up with oilways in the crankcase. Oil drawn from the remote tank is pumped through a gauze filter in the magneto drive inner chaincase. On the way, it passes a ball-and-spring check valve to stop oil seeping into the engine when idle. From the filter it goes to the bottom end and up to the valve gear. A metering jet delivers oil to the big end through drillings, with surplus dumped into the crankcase by a pressure relief valve. A flexible line runs up to the upper bevels and cambox where a rigid length of pipe has two banjo unions surrounding oil jets. One jet aims oil at the meshing point of the bevels and the other squirts it at the rockers where they contact the cam lobes. A three-way drain attached to the timing side of the cambox returns oil from the valve gear to a sump in the lower crankcase, where it is scavenged by the pump and returned to the tank.

A platform on the timing-side crankcase half carries the BTH ignition magneto. The front cover completing the chaincase is used to carry a rev-counter take-off from the top sprocket. 

Engineers' notes...

Alloy muff

On the MkVIII, Velocette replaced the iron cylinder barrel used on earlier KTTs with an iron-linered alloy barrel. The iron inner not only forms the bore, but incorporates the bottom flange that sits on the crankcase, with a spigot projecting downwards more than an inch into the case. It has a corrugated surface onto which the finned aluminium muff was cast in a sand mould. This particular method was chosen to reduce the weight of the engine, while maintaining strong construction and optimising the transfer of heat from the cylinder to the fins.


Big big-end

The large 13/8in (35mm) diameter of the crankpin helps keep the hard-worked crankshaft rigid. A case-hardened sleeve forms the inner track for the 16 big-end rollers (9/16in x 3/16in). They are held in a Duralumin cage to prevent them being crowded together as the crankpin follows its path at high rpm. The bars of the cage are relieved on their inner surface so only its outer circles contact the crankpin. A relatively soft aluminium alloy can wear away hard steel through rubbing.    


Running one today

Surely one of the most beautiful motorcycles in history, the KTT MkVIII cammy Velo is now fairly rare (49 were sold in 1939 and fewer than 190 built post-war) and worth more than £35,000 in good running condition. Ivan Rhodes, author of Velocette: Technical Excellence Exemplifed, is the ultimate authority on all KTTs as well as the works racers, which are surprisingly different in detail. “If you are working on a KTT, the usual common sense applies,” he says. “But as with any racing engine, you do need specialist knowledge. It’s vital to get all the clearances right in the camshaft drive.”


1948 350cc Velocette KTT MkVIII

Engine/transmission Type air-cooled ohc single Capacity 348cc Bore x stroke 74 x 81mm Compression ratio 10.94:1 (petrol-benzole) Valve sizes inlet 45mm, exhaust 40mm Valve timing (0.020in tappet clearance) Inlet opens 55° before TDC Inlet closes 65° after BDC Exhaust opens 75° before BDC Exhaust closes 45° after TDC Valve clearances inlet 0.012in, exhaust 0.025in Carburation 13/32in Amal 10TT Ignition BTH magneto Spark timing 30° before TDC Lubrication system dry sump Oil capacity 8 pints (4.5 litres) Primary drive chain Power output 29bhp @ 7,000rpm Weight 60lb (27kg) without magneto

Words: Mick Duckworth. Photos: Simon Hipperson

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