A sin­gle-cylin­der en­gine tuned for en­durance rac­ing

Classic Bike (UK) - - CONTENTS -

The noisy and oily cen­tre of a Thrux­ton racer, pulled to bits and given a good coat of look­ing-at

The Venom Thrux­ton, sold from 1965 un­til Ve­lo­cette’s Hall Green fac­tory closed early in 1971, had a long ancestry. It evolved out of the fac­tory’s first pushrod sin­gle, the 250cc MOV of 1934, which then spawned the pre-world War II 350cc MAC and 500cc MSS. In the mid-’50s, a new gen­er­a­tion of sin­gles with en­gines on sim­i­lar lines in­cluded the sporty 500cc Venom.

In 1960 Ve­lo­cette in­tro­duced the Venom Club­man, and in 1961 a fac­tory ma­chine set a 500cc world record at an av­er­age of 105.21mph over 24 hours, still un­beaten by any sin­gle-cylin­der ma­chine. Sub­se­quently Ve­lo­cette of­fered the 41bhp Venom Thrux­ton to ho­molo­gate a 500 for production rac­ing. With a big­valve cylin­der head and Amal GP car­bu­ret­tor, its de­sign re­vi­sions were based on tuners’ ex­per­i­ments in sev­eral coun­tries.

Ve­lo­cette deal­ers en­tered UK en­durance races, dom­i­nat­ing the 500cc class in the mid-1960s. One was Ge­off Dod­kin, whose shop at East Sheen on Lon­don’s South Cir­cu­lar Road be­came a mag­net for Velo fel­lows. Dod­kin’s sec­ond at­tempt at the Barcelona 24 Hours race in 1965 saw his riders El­lis Boyce and Tom Phillips fin­ish sec­ond in the over-250cc class. The en­gine seen here was built specif­i­cally for that ’65 Span­ish marathon.

All Ve­lo­cette ohv sin­gles have a su­per-slim crank­case and nar­row, very rigid crank­shaft. Plac­ing the pri­mary-drive chain in­board of the clutch and fi­nal-drive sprocket en­ables the one-inch (25.4mm) di­am­e­ter drive-side main­shaft to be short and stiff, with ad­e­quate sup­port from a sin­gle main bear­ing. Main bear­ings on both sides are taper-roller units with an out­side di­am­e­ter of 2¼in (57mm).

Slow-ta­pered ends on the main­shafts press into the fly­wheels with an in­ter­fer­ence fit and are pinned by ax­ial ¼in screws. The crankpin also has taper fix­ing, which is strong enough to dis­pense

with se­cur­ing nuts, so re­cesses for them are not needed to clear the con­rod; con­se­quently a high pro­por­tion of the pin’s over­all length is sup­ported in wheel metal. There is an ob­vi­ous Dod­kin rac­ing mod­i­fi­ca­tion to these fly­wheels, which have bev­elled outer edges to re­duce crank­case oil drag and cut weight. Less ob­vi­ous is that they are from an ear­lier MSS model, heav­ier in stan­dard con­di­tion and made from a higher grade of steel than the Thrux­ton’s wheels.

The steel con­rod, orig­i­nally of H-sec­tion, is light­ened and pol­ished. It runs on the stan­dard big-end’s sin­gle row of caged

9/16in x 3/16in rollers, while the small end has a plain bush with drillings in the rod and bush to ad­mit oil. The long-skirted three­r­ing pis­ton has a domed crown with front and rear flats for valve clear­ance. The com­pres­sion ra­tio is set dur­ing build-up by mea­sur­ing oil poured into the com­bus­tion chamber, with the cylin­der tilted so the spark plug hole is at the high­est point. Com­pres­sion is var­ied by fit­ting shims of dif­fer­ent thick­ness un­der the base flange of the cast-iron cylin­der bar­rel.

The sand-cast al­loy cylin­der head fits on a shal­low bar­rel-top spigot, with a plain cop­per gas­ket. Four long 3/8in studs pass­ing through the head and bar­rel screw into steel sleeve in­serts threaded into the top of the crank­case and nuts with wash­ers hold the head down. The bar­rel’s lower fins are shaved back on the right side to clear the ‘map of Africa’ tim­ing chest formed in the right-side crank­case half, which ex­tends well above the bar­rel joint. Inside it, the four tim­ing gears have silent-run­ning fine he­li­cal teeth.

The camshaft, high up on all Velo ohv sin­gles to min­imise pushrod length and re­duce flex, is driven from a pin­ion on the crank­shaft via an in­ter­me­di­ate re­duc­tion gear, which has a ‘hunt­ing tooth’, to spread wear across all teeth. The ig­ni­tion mag­neto’s gear is driven by the camshaft gear. The camshaft, a taper-fit in its gear, also turns on a fixed spin­dle and here Dod­kin runs both gears on nee­dle roller bear­ings. He also placed ra­di­al­roller thrust bear­ings on ei­ther side of the in­ter­me­di­ate gear to cope with lat­eral forces gen­er­ated by he­li­cal teeth. The gears’

spin­dles are sup­ported and lo­cated at their outer ends by a steel steady-plate bolted to the crank­case. Above and slightly to the rear of the camshaft axis, two tri­an­gu­lar cam fol­low­ers – called ‘bot­tom rock­ers’ in Ve­lo­cette par­lance – pivot side-by-side on a spin­dle pressed into the crank­case wall and held by the steady plate. They have hard ra­dius pads to bear on the lobes and cups to carry the pushrods, while a dished Belleville washer be­hind the in­ner­most in­let fol­lower con­trols side-play. The Du­ra­lu­min pushrods’ steel lower tips have ball-ends to fit the fol­lower cups and smaller steel cups at the top that nor­mally en­gage with threaded ad­justers in the rocker arms. Here, Dod­kin welded up the ad­justers and set valve clear­ances for Barcelona by ma­chin­ing the pushrods to length. Tele­scopic plated tubes en­close the rods.

Not on spin­dles, the case-hard­ened rock­ers have ex­ter­nal jour­nals be­tween their arms run­ning in half-bear­ings formed by the rocker box al­loy and al­loy caps screwed to the box. Hard but­tons on the left-side arms make lin­ear con­tact with the valve stem tops. Two-piece hair­pin valve springs are an­chored at their open ends in blocks sur­round­ing the tops of the valve guides. Shims are placed un­der the blocks to set spring preload. Hold­ers re­strain­ing the springs’ top loops are held to the valve stems by col­lets, via re­cessed col­lars that al­low the valves to ro­tate in the guides and re­duce the like­li­hood of heat dis­tor­tion.

A smoothly-flowed in­let tract leads mix­ture from the 13/8in (35mm) Amal GP carb to the Thrux­ton head’s huge two-inch (50.8mm) in­let valve. The com­bus­tion chamber is shal­lower than a com­plete hemi­sphere. Dod­kin fit­ted a Viper ex­haust valve – lighter than a Thrux­ton’s, it eases some load in the valve gear.

A spi­ral gear, se­cured at the tim­ing-side end of the crank­shaft by a nut on a left-hand thread, en­gages with the driv­ing gear of the high-out­put oil pump, which is pressed into a hous­ing at the rear of the tim­ing chest. Oil is pumped from the tank into a gallery in the tim­ing cover with four out­lets. The first is a quill feed to the

end of the crank­shaft to sup­ply the big-end from a drilling that de­liv­ers oil to the rollers at an an­gle from the side, rather than through the crankpin. The next two out­lets above lu­bri­cate the camshaft spin­dle and sup­ply an oil jet in the steady plate aimed at the cams. The up­per­most out­let is to a pipe lead­ing to the rocker box, where oil is fed via a banjo joint to the in­let rocker and thence to the ex­haust. To en­sure ad­e­quate lu­bri­ca­tion at Barcelona, Dod­kin added ex­ter­nal pipes on top of the rocker box to dis­trib­ute oil evenly to both rock­ers. Two drain pipes run from the side of the head to a sin­gle union on the pushrod tube, so that oil falls onto the cams, fol­low­ers and tim­ing gears as it de­scends to the lower crank­case. From there, it is drawn through an in­ter­nal pipe to the pump’s scav­eng­ing gears and re­turned to the tank.

For Barcelona, Dod­kin de­vised an ex­tended pump to pre­vent build-up of ex­cess oil in the crank­case. A sec­ond set of scav­enge gears is added ex­ter­nally to the stan­dard pump’s mount­ing plate to draw oil through an ex­ter­nal pipe from a union tapped into the drainage chan­nel from the tim­ing chest to the base of the crank­case.

An ex­haust-valve lifter aids start­ing. A short shaft with a re­turn spring, sup­ported in the front of the tim­ing chest, has a cam at its in­ner end to lift the ex­haust fol­lower when the lever at its other end is pulled by a ca­ble. Ig­ni­tion sparks are by a flange-mounted Lu­cas K1FC mag­neto. Its driv­ing gear (not shown) in­cor­po­rates a cen­trifu­gal auto-ad­vance de­vice.


Bev­elled edges of fly­wheels are a Dod­kin rac­ing mod to re­duce oil drag and cut weight Ex­tended Dod­kin oil pump (right) and stan­dard item Oil sup­ply is via tim­ing cover In­let tract feeds two-inch valve

Rock­ers are se­cured by al­loy caps screwed to the rocker box

Ex­tra rocker oil feeds were added ex­ter­nally to this en­gine for 24-hour rac­ing

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