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

Old Bike Australasia - - OUT ’N’ ABOUT - Story Alan Wallis O.A.M. Pho­tos Robin Lewis

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.

Alan Wallis with the Til­brook Ro­tary Valve.

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