Motorcycling Australia Museum

1953 Tilbrook 125cc Rotary Valve

Rex Tilbrook’s most radical

Adelaide-born Rex Tilbrook was a man of boundless energy and creativity who had a burning desire to go motor racing. At the age of 18, he booked a passage to Britain, arriving in early 1933 with high hopes of landing a job at one of the many factories and workshops located inside the famous Brooklands motor racing circuit in Surrey.  The place was a hive of activity and manna for a wide-eyed young lad with motor racing in his veins.  He soon befriended the owner of a Lea Francis racing car and struck a deal whereby maintained the car in return for an occasional outing on the fearsome banked track. His fame as a tuner and fabricator spread rapidly, to the point that he opened his own workshop, specialising in making the specialised exhaust systems and silencers required by the stringent local regulations.

With the outbreak of war imminent, Rex jumped a boat back home and spent the war in the munitions industry. In a shed in the back yard of his home in the Adelaide suburb of Kensington Park, he produced the first in a long line of motorcycle accessories – a universal pillion footrest. By 1947 business was ticking over sufficiently for him to expand to a proper workshop in the same suburb, where he produced a greater range of motorcycle components as well as the first of the stunning Tilbrook sidecars. In the same year he took a stand at the Adelaide Exhibition, set it up as an open workshop, and during the 54 days of the show’s duration, the first Tilbrook solo was constructed. The engine was based on a pre-war 250cc Zundapp, with castings made locally and machined on his stand. The engine was a complete running unit in 28 days and featured a 20 amp generator which enabled starting with a flat battery. His own design of pneumatic front forks was fitted to an all-welded frame in which the top tubes wrapped around the petrol tank in Coventry Eagle style.

To showcase his growing range of products, Rex decided to build himself a racing 125cc machine, and three were constructed prior to 1950. Evidence of Moto Guzzi design influence could be seen in the racers, such as the radial arm front forks with friction dampers. The 125cc engines were loosely based on the pre-war Villiers Y-port barrel, but with bore and stroke altered to 52 x 58mm. The methanol-fuelled power plant sported a two-staged induction system through Amal TT carburettors; the smaller one-inch unit opening fully at half throttle and the 1 3/16 unit taking over from there. Ignition came from a chain-driven BTH magneto running at half engine speed. The heavily revised barrel featured two inlet, two exhaust and four transfer ports, with useful power between 5,500 and 8,400 rpm – good enough for a top speed of 90 mph. The single-loop frame sported Rex’s own design of cantilever suspension, with the spring units located beneath the 4-speed Tilbrook gearbox. The handsome alloy fuel tank doubled as a fairing/front number plate. At race meetings, the immaculate bikes were accompanied by mechanics dressed in blue Italian-style overalls, decorated with Rex’s three-pointed star emblem.

Always searching for the competitive edge, Rex conceived his most radical design in the form of a 125cc rotary valve racer. Rex and Alan Wallis had taken their two strokes to Australia’s premier event, the annual TT races at Bathurst NSW in 1952, but were well beaten by the astonishingly fast Eric Walsh-tuned BSAs. This was just the sort of challenged that Rex relished, so he set his sights on having the rotary ready for the 1953 Bathurst races. Based on Villiers crankcases and using the standard 62mm stroke, the rotary engine had a hardened and ground crankshaft driving a train of gears in a handsome casting on the right hand side. Atop the barrel sat Tilbrook’s version of a twin rotary valve head, with the valves rotating at one-quarter engine speed. The rotary valve concept itself was not new – several local tuners having built their own renditions - but the twin problems of sealing the valves and the combustion chamber, and of lubricating the valves usually proved insurmountable. With clearances so critical, varying degrees of expansion among the components usually resulted in either seizures or loss of sealing, but Tilbrook’s answer was to attach the barrel to the head using a spring loaded technique. The idea certainly worked, with the engine spinning to 12,000 revs with useable power from 8,000.

As soon as the rotary engine was completed it was installed in the chassis of Rex’s two-stroke racer and he took off for a test run up the main street outside the factory! Within minutes he was back, declaring the project a failure since it offered no appreciable power advantage. With Bathurst only weeks away, the rotary engine was removed and the two stroke re-installed. His decision was vindicated somewhat with a fine fourth place in the 125 TT. Alan Wallis meanwhile felt the decision to dump the rotary was rather hasty, and undertook a quiet program of development over the next two years, machining new rotary valves and a special piston, and converting the engine to methanol. But despite Wallis’ enthusiasm, Rex had totally lost interest in the project and the rotary was never raced in anger. Fortunately, the unique machine survives in totally original form in the hands of Rex’s son-in-law Tom Johnston.

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