A different spin on things
TILBROOK ROTARY VALVE FOUR STROKE ENGINE When Rex Tilbrook was at Brooklands from 1933 to 1938 he was associated with many riders and engine builders who either had workshops there or came to test and compete on the track as well as preparing machines for
During this era most tuning work was by the time-honoured system of experience and trial and error. Works-prepared machines generally were superior to most privately tuned models but successful ideas were jealously guarded. The problems for individuals included access to special metals and how to treat them for specific purposes as they were expensive to produce and generally limited to volume buyers. One of the problems experienced by most engine builders was poppet valve failure with heads snapping off and generally causing considerable damage when they did. This was exacerbated as engine speeds increased and valve overlap caused them to sometimes clash or hit the piston. Another problem was valve bounce that was solved in some cases by stronger valve springs that put further stress on other parts of the valve train. Rex thought there must be a better way to control inlet and exhaust control and looked at alternatives with one being rotary valves that had already been tried with different versions but were prone to seizure and not seen as being a viable alternative. He drew up a system using separate rotors for inlet and exhaust with a straight slot in each to control port openings. This meant that they had to revolve at one quarter of crankshaft revolution thereby reducing revolution speed. His idea was to make a cylinder head for an engine that was an already designed for high speed and chose the chain driven overhead camshaft 350cc R10 A.J.S. to prepare working drawings, but his return to Australia in 1938 and the outbreak of World War 2 in 1939 put the plan on hold. When the Ultra Lightweight 125cc road race class became popular in Australia in 1949 a team of Tilbrook racers was formed to compete with fields of predominantly B.S.A. Bantam machines but overseas the dominant 125 machines were overhead camshaft four strokes. The dream of a rotary valve was reignited and Rex sponsored his friend from Brooklands days, Dennis Minett to migrate and work on the building and development of a rotary valve 125 engine. Dennis had been employed in the Vincent race shop under Phil Irving and was familiar with this type of development work. The plan was to use the gearbox and crank case of the Villiers 10D engine as the Tilbrook machines were fitted with Villiers 6E and 10D units and both had identical frame mounting lugs. A full circle builtup flywheel assembly replaced the Villiers bob weights and this carried an external flywheel within the primary drive and clutch housing on one side and the drive assembly to the valves as well as a large capacity oil pump on the other. A one-off barrel was fabricated with welded-on fins similar to the prewar German Auto Union air cooled works race car engines while the cylinder head assembly consisted of a fabricated steel casing to contain the bronze cylinder head, rotary valves and valve caps. It must be borne in mind that this was an experimental project with the object to test the theory before embarking on the costly process of producing patterns for aluminium castings. Patterns were made for the bronze cylinder skull and caps that were cast at a nearby foundry and machined in the Tilbrook tool room with the skull spigotted over the extended cylinder liner that had a piston ring seal. It was secured by a ring of compression springs as were the caps, these being important features to prevent seizures. This and the fact that the chamber containing the components was flooded with circulated oil from a large oil tank situated in the tail piece of the machine proved successful and no problems arose during extensive bench testing.
While the engine was being built a complete new machine was constructed with a fuel tank extending around the head stem and recessed for the riders chin and rev counter. This as well as a new rear end holding the seat and containing the large oil tank were all hand made from fusion welded sheet aluminium. It was entered for the 1953 Bathurst meeting but road tests did not come up to expectations so the rotary valve was replaced with one of the two stroke race units for Rex to gain a creditable fourth place. As with such projects there often comes a time when the inventor having proved a theory either loses interest or comes to the realization that the expense of continuing is not worthwhile and such was the case with the Rex Tilbrook dream of an alternative to the poppet valve. After twelve months of machining, fabrication, assembly and development the project was shelved and put under a bench in the experimental shop and virtually forgotten. Dennis had moved on and eventually settled in Victoria. About a year later another attempt was made to see if it was worth proceeding with the concept. New rotary valves were tried with different timing and alterations made to the head contours to vary the gas flow. The compression was raised and methanol fuel used. These changes resulted in considerably more power but at much higher revolutions with a narrower power band that required a new multi ratio gearbox as used by the European works machines to be effective. Once again the rotary valve engine was removed and put on a display stand until Rex and Dorothy Tilbrook arranged a function for the late English journalist Vic Willoughby during his visit of Australia. Rex organized a display of classic motorcycles, including a range of Tilbrook tourers and racers, at their Encounter Bay property and the rotary valve model was restored to working condition, although it was never restarted. It was appropriately in a prominent place at the funeral service for Rex on 6th. December 1997.