1962/2017 MATCHLESS G50
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’