ENGINE STRIP: 1938 TRIUMPH SPEED TWIN
The parallel twin that set a pattern to be followed by Triumph for decades
Landmark British bike pulled to pieces in front of your very eyes
The Triumph 360° parallel twin unveiled in 1937 broke with convention to become the most influential design in British motorcycling history. The overhead-valve 500cc engine powering the 5T Speed Twin launched for 1938 was conceived by Edward Turner. Installed as managing director and chief designer when Triumph Engineering was set up in 1936, Turner arrived from Ariel where he had developed his unorthodox Square Four engine under senior engineer Val Page. Their programme included testing a two-cylinder version, which inspired both men with its sweet running.
Page had left Ariel for Triumph in 1932, to design the 1933-1936 6/1 650cc parallel twin, a weighty sidecar tug. In contrast, Turner’s compact and less costly Speed Twin offered brisk acceleration with a 90mph-plus maximum speed. Easy to kickstart, it felt smooth compared to a single and was just as light. Importantly for Triumph, the
engine could be produced at relatively low cost. A preproduction test machine covered many miles, 10,000 of them with a loaded sidecar attached.
Produced for two seasons before WWII intervened, the 5T resumed production with minor changes in 1946 – and six other British makes followed suit in offering a 500cc vertical twin. Triumph had increasingly full order books for 25 years, became one of the world’s best-selling makes, and Speed Twin-based engines were winners in almost every branch of motorcycle sport. A 650cc variant appeared in 1949 and the 750cc twin derivative launched for 1973 was in production until the mid-1980s. Vibration caused by the parallel twin’s out-of-balance forces worsens at capacities above 500cc, but introducing balancer shafts and adopting a 270° firing interval has seen the format remain successful to the present day.
The engine seen here is the 1938 original. Its crankshaft is in two halves, each having a mainshaft integral with an
outer counter-balancing web, a big-end journal and a fullcircle inner flange with six holes for the high-tensile bolts that fix the two halves to a central flywheel. The big-end journals, disposed side-by-side, carry conrods with parallel sides. Unusually for a pre-war engine, the rods are forged in heat-treated Hiduminium RR56, an alloy developed for aviation use with the same tensile strength as mild steel, but a third of the weight. Their steel big-end caps have integral studs for attachment to the rods by slotted nuts secured with split pins. The centrallyribbed caps have a coating of white metal on their bearing surfaces, while the rod metal bears directly on the journal. Pressed-in phosphor bronze bushes provide the rods’ small-end bearings. The vertically-split crankcase is shaped to avoid wasting space around the crankshaft. The two main bearings, both ballraces, are an interference-fit in the crankcase side walls, where the section of alloy is thickest. The slightly larger drive-side bearing is retained by a circlip against its outer face, with a steel washer to impede the exit of oil into the primary chaincase. Later twins have a more effective oil seal. The engine output sprocket has a spring-loaded ramp cam for shock absorption.
The cast-iron one-piece cylinder barrel, with a curved external profile repeated on all Meriden’s twins, has a base flange held down to the crankcase by six studs. Breakages here led to a thicker flange and eight-stud fixing being adopted for 1939. Deeply spigoted into the crankcase, the barrel also has shallow spigots at the top of the bores to engage with recesses in the iron cylinder head. Eight bolts hold the head to the barrel, with a copper gasket at the joint.
The pistons, a split-skirt type here, each have two compression rings and one oil control ring with flat tops indented for valve clearance. The combustion chambers are hemispherical with the valves disposed at an included angle of 90°, a layout Turner chose for efficient breathing and optimal heat dispersion. Valve operation is by separate inlet and exhaust camshafts, set transversely ahead of and behind the crankshaft axis in the upper
crankcase, bearing in phosphor bronze bushes. The lobes contact pairs of followers with radiused hard-faced feet. They slide in cast-iron tappet blocks, housed in circular apertures in the barrel’s base flange and prevented from turning by set-screws. Ball-ends on the tops of the followers fit steel cups on the lower ends of the Duralumin pushrods, while cups at the top of the rods engage with the rockers. As on 1930s Triumph singles, the pushrods are enclosed by chrome-plated tubes, in this case set at the front and rear of the barrel. Their lower ends rest on the tappet blocks, with synthetic rubber sealing rings, and their tops fit into the undersides of the two separate rocker boxes, also sealed by compressible rings.
The rockers, cylindrical with arms at each end, swivel in pairs on pressed-in spindles running across the alloy rocker boxes, with Thackeray spring washers and shims to locate them laterally. Their outer arms carry lock-nut clearance adjusters that contact the valve stems, accessible by removing threaded inspection caps on the rocker boxes which have raised hexagons for spanner contact. The valves, sliding in cast-iron guides, have double springs seated on steel collars, while the top collars are fixed to the valve stems by split collets.
The curvaceous timing cover that became a Triumph trademark conceals drive to the camshafts by steel gears. A pinion keyed to the crankshaft engages with a halftime gear, bushed and running on a spindle fixed in the crankcase and supported by a boss in the timing cover. It drives the two gears pressed, keyed and retained by large nuts with left-handed threads onto the ends of the camshafts. Difference between the numbers of teeth on the gears creates a ‘hunting
‘UNUSUALLY FOR A PRE-WAR ENGINE, THE RODS ARE FORGED IN HEATTREATED HIDUMINIUM RR56, AN ALLOY DEVELOPED FOR AVIATION USE’