84 :OLD BIKE AUSTRALASIAOUT’N’ABOUT
A different spin on things
TILBROOK ROTARY VALVE FOUR STROKE ENGINE
During this eramost 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 draw-
ings, 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 over-
seas 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 built-
up flywheel assembly replaced the Villiers bobweights 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. 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 other competition events. StoryAlan Wallis O.A.M. PhotosRobin Lewis
Alan Wallis with the
Tilbrook Rotary Valve.