1962/2017 MATCH­LESS G50

Classic Bike (UK) - - News -

Still raced in anger to­day, the 500cc over­head-cam sin­gle-cylin­der Match­less G50 en­gine has a long his­tory. Although As­so­ci­ated Mo­tor Cy­cles (AMC) only sold the G50 from 1959 to 1963, its ori­gins are in the 350cc AJS 7R of 1948. When AMC, own­ers of both AJS and Match­less, went into re­ceiver­ship in 1966, side­car racer Colin See­ley re­vived G50 and 7R en­gine pro­duc­tion for six years. And since the 1980s, the en­gines have en­joyed a re­mark­able after-life in his­toric rac­ing.

In the 1940s and ’50s, AMC was one of the few Bri­tish fac­to­ries to sup­port road rac­ing. The 7R ‘Boy’s Racer’ was con­ceived by ex-racer sales boss Jock West and en­gine de­signer Philip Walker. It suc­ceeded in its in­tended role as a com­pet­i­tive yet easy-to-main­tain pri­va­teers’ ma­chine, and there were also top per­for­mances by fac­tory rid­ers, while a three-valve 7R3 vari­ant won the ’54 Ju­nior TT. In that year Jack Wil­liams be­came AMC’S chief de­vel­op­ment engi­neer and for ’56 changed the bore and stroke from 74 x 81mm to 75.5 x 78mm and upped power from 37bhp to 42bhp. AMC’S G45 twin, sold to 500cc pri­va­teers from 1953 to 1957, was less suc­cess­ful. Since its track-test times were no bet­ter than the de­vel­oped 7R’s, West and Wil­liams con­cluded that en­larg­ing the 350 en­gine would make a bet­ter 500. The re­sult­ing G50 en­gine was cre­ated by bor­ing the 7R’s cylin­der to 90mm and mod­i­fy­ing the cylin­der head. First seen as a pro­to­type at the 1958 Isle of Man TT and on sale the fol­low­ing year, the Match­less was lighter than its Nor­ton Manx ri­val and pulled bet­ter out of slow cor­ners, but took time to make an im­pact in UK rac­ing.

In Amer­ica, the G50 sparked a bit­ter row. For 1962 Match­less dis­trib­u­tor In­dian Sales cat­a­logued the road-le­gal G50CSR, purely to ho­molo­gate the en­gine for AMA cham­pi­onships. Dick Mann raced it in 7R and BSA chas­sis with ex­cel­lent re­sults, but the in­flu­en­tial ‘iron tri­an­gle’ of Har­ley-david­son, BSA and Tri­umph feared an ohc ri­val and had the G50 banned in 1963. Mike (now Michelle) Duff lapped the TT course at over 100mph on Tom Arter’s G50 in 1962. But the best re­sults came later in the 1960s after Colin See­ley, who had be­gun build­ing rac­ers with 7R and G50 en­gines from AMC in his own light­weight chas­sis, bought out the fail­ing fac­tory’s race shop. There were many See­ley wins, no­tably for Derek Min­ter, Dave Crox­ford and John Cooper. But in the 1970s, when the 500cc

class was un­der­mined by Yamaha’s more mod­ern 350cc twostroke, See­ley di­ver­si­fied, of­fer­ing the ex­clu­sive road-le­gal G50pow­ered Con­dor. Mean­while, Jack Wil­liams’ son Peter and the Tom Arter equipe per­se­vered. The Arter Match­less was the only Bri­tish sin­gle to get a top-five fin­ish in the 1973 GP sea­son, when Peter fin­ished sec­ond in the Se­nior TT with a 102.74mph lap.

But the aged G50 wasn’t fin­ished. When the Clas­sic Rac­ing Mo­tor­cy­cle Club formed in 1978, its pre­mier class was for pre1973 500cc ma­chines, thrust­ing See­leys and some AMC G50s back into ac­tion. De­mand led to parts be­ing made and in turn to whole en­gines be­ing pro­duced by spe­cial­ists like Mick Taberer, George Beale, Mick Rut­ter and Andy Mol­nar. New en­gines, mostly with mod­ernised in­ter­nals, are still avail­able from Mol­nar or Min­no­va­tion, who took over from Beale and make only 92mm-bore short-stroke units.

DELV­ING IN­SIDE THE RACE EN­GINE

Our ex­am­ple is an ac­tive rac­ing en­gine com­pris­ing a mix of orig­i­nal and re­pro­duc­tion parts. The dis­tinc­tively-shaped in­ner and outer tim­ing cov­ers are ap­par­ently orig­i­nal. They are cast in weight-sav­ing mag­ne­sium al­loy, as were orig­i­nal AMC crankcases. How­ever, See­ley and sub­se­quent mak­ers have opted for stronger alu­minium, seen here. AMC coated cor­ro­sion-prone mag­ne­sium parts with chro­mate and gold paint.

Line-bored steel ‘top hat’ hous­ings for the main bear­ings are held in the crank­case halves by screws; the items here are a type fit­ted by 7R/G50 spe­cial­ist Ron Lewis. There are dou­ble-roller mains on the drive side, while on the tim­ing side a ball­race out­board of a roller gov­erns crank­shaft side-play. While not orig­i­nal Match­less, this crank­shaft fol­lows the orig­i­nal in hav­ing the main­shafts and crankpin pressed into full-cir­cle fly­wheels, which have holes for light­en­ing and bal­ance. A mod­ern Car­rillo steel H-sec­tion con­rod runs on an INA nee­dle-roller big-end, where Match­less had caged rollers. A plain bush is pressed into the rod’s small-end, where the gud­geon pin car­ries a Cos­worth forged pis­ton with two plain rings and an oil scraper ring.

The al­loy cylin­der bar­rel is mod­ernised with a nickel-sil­i­con plated bore. Ad­van­tages over the orig­i­nal cast-in liner in­clude

weight loss and re­duced pis­ton-to-bore clear­ance to main­tain high com­pres­sion and min­imise oil con­tam­i­na­tion of com­bustible mix­ture. The bar­rel is deeply spig­oted in the crank­case by 1½in (38mm) for rigid­ity. At its top, a rel­a­tively high ½in (13mm) spigot fits into the head cast­ing, which al­lows for an ex­tra head fin, which can dis­perse heat bet­ter than a bar­rel fin. Vi­bra­tion can caus­ing crack­ing in the large hor­i­zon­tal head fins, which is why See­ley and oth­ers added ver­ti­cal stays. The up­right fins on top of the head are an­gled so that cool air is drawn over them by an aper­ture in the tim­ing cover.

There is no gas­ket at the head joint. Shims un­der the bar­rel are used to ob­tain squish ef­fect above the pis­ton and sound seal­ing. Four studs threaded into the crank­case pass through the bar­rel, with nuts at their up­per ends hold­ing down the cylin­der head. Ob­long re­cesses are pro­vided for hair­pin valve springs, but ma­te­ri­als tech­nol­ogy has al­lowed dou­ble coils to be used for many years. Ti­ta­nium spring col­lars are re­tained by split col­lets in valve stem re­cesses. Stain­less steel valves are used here, with beryl­lium cop­per seats mostly used with ti­ta­nium valves for its heat dis­pers­ing prop­er­ties, as ti­ta­nium is a poor heat con­duc­tor.

Off-set­ting both the in­let tract and the ex­haust port to the right cre­ates swirl in the com­bus­tion cham­ber. The 1½in Amal GP car­bu­ret­tor mounts to the head at a down­draught an­gle, and three screws se­cure the flanged ex­haust pipe to the head.

IT’S ALL DONE WITH A SIN­GLE CAM

While the ri­val Manx en­gine has twin camshafts, the 7R’s de­sign­ers be­lieved that lighter weight, sim­plic­ity and lower cost jus­ti­fied a sin­gle shaft. A mag­ne­sium al­loy cam­box cast­ing is fixed to the head by ¼in socket-head bolts. Rather than thread­ing di­rectly into the head, 11 outer bolts screw into cylin­dri­cal trun­nions with screw­driver slots to steady them dur­ing tight­en­ing. Us­ing re­place­able trun­nions, like those seen on self-as­sem­bly fur­ni­ture, means worn or stripped threads are eas­ily dealt with away from the work­shop.

The one-piece, twin-lobe camshaft, ro­tat­ing in the op­po­site di­rec­tion to the crank­shaft, is sup­ported in two ball­races, one

‘THE MAIN­SHAFTS AND CRANKPIN ARE PRESSED INTO FULLCIRCLE FLY­WHEELS’

pressed into the cam­box on the tim­ing side and the other in a de­tach­able al­loy hous­ing to al­low for as­sem­bly. The rock­ers – steel stamp­ings with plain buses in their pivot bosses – are forked to carry ro­tat­ing ¾in rollers to bear on the cams. The ends di­rectly con­tact­ing the valve stems have hard-coated ra­diused pads. The rocker spin­dles (not in­ter­change­able) have ec­cen­tric jour­nals for valve clear­ance ad­just­ment. The cam­box has rec­tan­gu­lar in­spec­tion cov­ers over the rock­ers and a slot at the drive-side end of the camshaft en­gages with a rev-counter drive box.

The first stage of camshaft drive is a pin­ion keyed to the tim­ing­side main­shaft and se­cured by a nut. It drives a gear sit­u­ated above it with twice the num­ber of teeth, and that in turn drives an­other gear of the same size to the left and slightly above it. Both the larger gears are keyed to ro­tat­ing shafts sup­ported both in the crank­case and an al­loy oil pump hous­ing, screwed onto the in­ner tim­ing cover. The lower shaft, run­ning in two ball­races, car­ries the 17-tooth cam chain-driv­ing sprocket be­tween its gear and the pump hous­ing. Two key-ways, at 180° to each other, of­fer sprocket po­si­tions with a dif­fer­ence of less than one tooth, for ac­cu­rate cam tim­ing.

The 3/8in-pitch cam chain has a Weller ten­sioner bear­ing against its slack for­ward run and a rub­bing pad riv­eted to the in­ner tim­ing cover ad­ja­cent to the down­ward run. Shims un­der the cam­box are used to ad­just chain ten­sion.

The top sprocket’s Vernier cou­pling per­mits fine tim­ing ad­just­ments. It has 18 small peg-holes, and a wheel be­hind it keyed to the camshaft has 17. The sprocket has a large cen­tral hole for a threaded sleeve on the wheel to project through. A washer with a peg to fit through two aligned peg-holes is held to the sprocket by a nut on the sleeve, while an­other nut holds the as­sem­bly on the

‘OIL IS PUMPED TO THE BIG-END VIA A QUILL FEED INTO THE MAIN­SHAFT’

camshaft. With the Vernier and the lower sprocket’s key­way op­tion, tim­ings can be set to within a de­gree or two. An in­spec­tion plate on the tim­ing cover gives ac­cess to the up­per sprocket.

KEEP­ING IT ALL WELL OILED

There are two sep­a­rate gear-type oil pumps. The scav­enge pump is driven by the outer end of the shaft to which the tim­ing gear and camshaft sprocket are keyed, while the feed pump with slim­mer gears is turned by the sec­ond large gear’s shaft, run­ning in plain bushes. The feed pump draws oil from the tank via a hose union on the in­ner tim­ing cover, drillings in the cast­ing and a springloaded ball valve to pre­vent drain-down when the en­gine is idle. It is then pumped to the big-end via a quill feed into the main­shaft, while an­other feed goes to a union on the crank­case be­hind the bar­rel. A hose takes oil up to the cam­box and into the hol­low rocker spin­dles, which have holes to dis­perse it.

Some oil re­turns to the bot­tom-end through the tim­ing cover, but there are also ex­ter­nal drains from the front and rear of the cam­box that lu­bri­cate the cam chain, tim­ing gears and tim­ing-side main bear­ing. Oil en­ters the crank­case via a hole un­der the bear­ing and col­lects, along with oil flung from the big-end, in a de­tach­able sump with a mag­netic drain plug. From there it is drawn up to the scav­enge pump through a pas­sage vis­i­ble on the crank­case ex­te­rior for re­turn to the tank. A sec­ond pas­sage seen on the crank­case was a drain on the early 7R, not drilled on later en­gines. Some tim­ing cover de­tails are also relics of an ear­lier oil­ing sys­tem.

Ig­ni­tion is by mag­neto, a Mol­nar elec­tronic type in this in­stance, driven from the feed pump’s gear with a 12/13 hole Vernier cou­pling on the driven gear, ac­ces­si­ble un­der a de­tach­able plate.

‘OUR EX­AM­PLE IS A RAC­ING EN­GINE WITH A MIX­TURE OF ORIG­I­NAL AND RE­PRO PARTS’

Weld­ing re­paired cracks in this head

Mod­ern crank­shaft is built-up like the AMC orig­i­nal

Outrig­ger oil pump mount­ing plate car­ries ball­race to sup­port the camshaft drive sprocket’s shaft

Above:camshaft sprocket Vernier cou­pling al­lows pre­cise tim­ing

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