Yamaha TD1s - CLASSICS REMEMBERED

The production road racers that democratized the sport

The idea of a production racer is as old as manufacturing. English makers who ran special bikes in the Isle of Man TT races between the wars saw that by offering for sale race bikes built to the previous year's specification, they could make a bit of money, put their name about, and possibly foster future champions on the brand. AJS, Velocette, and Norton took this path.

When the new postwar Yamaha Motor Co. surveyed the world, they saw a need for two separate racing programs. One would be aimed at the rich US market and would because of AMA Class-C rules be based on production bikes. The other would enter European Grand Prix road racing with designed-for-the-job prototype machines.

Once singles were established in production, Yamaha sought a starting-point for twin-cylinder bikes and found it in the German Adler twin. Yamaha engineers studied this motorcycle but did not copy it, finding some features which conflicted with ease of manufacture. This is an important difference between Japanese design and traditional European design. European design, because it began at the beginning, was a natural growth that tended to carry with it antique features “because we’ve always done it that way.” Japanese engineers, starting amid the rubble of the Pacific War, were obliged to set aside much of tradition in favor of designs that could be quickly, economically, and reliably made on modern equipment.

German Adler racing twin magazine layout
The German Adler racing twin (bike which Yamaha engineers studied, adopting some ideas but rejecting others as they developed their own twins). From the January 1963 issue of Cycle World.Cycle World

There persists the idea that Japanese goods in the 1950s and ‘60s were low-priced because of cheap labor; labor may have been cheap but as little of it as was possible went into manufactured goods that were being made on the brand-new production machines. Why? Because most pre-existing factory equipment had been destroyed by war.

While Yamaha's GP program developed rotary-valve two-stroke racing engines, first air-cooled and then water-cooled, the bikes to be raced in the US had to be based on production engines. That meant the D1, a parallel 180-degree-firing twin, built on a vertically-split aluminum crankcase with a 5-speed all-indirect gearbox (that is, with power coming in on one shaft, passing through one of the meshing pairs of gears to another shaft carrying the output sprocket). This gearbox was shifted by three shift forks guided by two rails underneath the gear shafts, moved by a horizontal shift plate whose milled upper face grooves engaged pins on the forks.

Like the Adler, this engine’s clutch was on its crankshaft (Tohatsu did the same with their LD twins). There were two problems with this. First, sure, because torque is less on the crankshaft than it is after a 2 1/2-to-one primary gear reduction has multiplied it on the gearbox countershaft, you’d think a crank-mounted clutch could be smaller, lighter. Wrong! When you make a racing start on a 250, you pin the throttle and control the drive by slipping the clutch. That means some power goes to the rear wheel, and some goes into frictional heating of the clutch plates. The smaller and lighter the clutch plate stack, the higher its temperature goes during the start. With the TD1 you basically got one start and then had to disassemble the clutch to replace warped, burnt, or cracked plates. Owners learned to perform this work quickly, efficiently – and often.

That wasn’t the worst of it. In order for clutch plates to disengage freely, they can’t be tightly, accurately centered. That meant the clutch was never balanced, so at 10,000-rpm its vibration constantly wiggled the crank extension on which it was mounted. Until that 20-mm solid steel shaft developed a crack, broke off, and the energy of the spinning clutch cleaned everything off the left side of the engine – everything. As it says in service books of that time, “Fit new”.

TD1-A had the melon-shaped red gas tank, which is shown. Is carb float bowl bracket was bolted to the rear of the engine. The top fork crown was made of thick flat sheet steel.

Yamaha did the right things – increasing the fillet radius where the shaft joined the crank flywheel, then increasing shaft diameter to 25-mm, then increasing the fillet radius a second time. It still broke – just not as soon. And so the crank-mounted clutch was used on TD1-A (1963-’64) and TD1-B (1965-66), but for the much-improved TD1-C (1967-’68) it was replaced by a much heftier clutch in the conventional place – on the gearbox countershaft. End of problem.

These were simple air-cooled engines. Each cylinder had a single exhaust port. A pair of flat-topped, back-aimed transfer ports allowed fresh charge, compressed in the crankcase, to jet into the cylinders, forming a flow loop that passed across the piston to the non-exhaust cylinder wall, up that wall and across the underside of the head, and then down to the yawning exhaust port. Each of the two pistons had a slightly domed top.

Preventing loss of most of the fresh charge out the exhaust was the heart of the modern two-stroke – the "expansion chamber", or resonant exhaust pipe. As the exhaust port began to be exposed by the piston's top edge at about 45% of stroke, the remaining combustion pressure in the cylinder (at roughly 100-psi) rushed out in the form of a powerful sound wave. As that wave hit the diverging part of the pipe it sent back toward the cylinder a reflected low pressure which helped to remove remaining exhaust gas. As it hit the converging part of the pipe, it sent back a positive reflection that arrived just as two things were happening;

  1. The exhaust port was in the last stages of closing and;
  2. The flow of fresh charge had completed its loop inside the cylinder and was beginning to flow out the exhaust port.

That positive reflection now shoved any charge that had entered the pipe back into the cylinder, positively preventing its loss -and increasing power.

Oh yes – power. TD1-A was claimed to make 32-hp. Since this was too much for the stock cast-iron production cylinders to get rid of, and sometimes too much for the easy alternative – aluminum cylinders cast onto iron liners, Yamaha decided the best way to keep pistons cool and not seized was to eliminate everything between the hot piston and the high heat conductivity of the aluminum cylinder itself. No iron liner several millimeters thick, acting as a heat-retaining blanket. TD1-A introduced a first try – to convert the bore surface of the aluminum cylinder into the hard ceramic aluminum oxide by anodizing. If everything was just perfect (jetting, timing, fuel, good luck) this could work, and the late, great and cheerful Don Vesco set a record at Daytona with one of these. Since everything is so seldom perfect (consider marriage, or international relations), the anodized cylinders were a flop. TD1-A was considered the best way to lead the first five laps and watch the rest from trackside.

TD1-B began with sand-cast crankcases and melon tank, but was later switched to die-cast cases and the penis-shaped tank. Float bowl bracket and fork top crown as above. Both A and B had their clutches on the crankshaft's left end, so that's where the clutch cover was largest.

One innovation was a brilliant success – development of aluminum-silicon piston and cylinder alloys displaying both high hot strength (the central problem of two-strokes is controlling piston temperature despite being heated by combustion twice as often as in a four-stroke) and low thermal expansion. MZ in East Germany had pioneered two-stroke basics, but lacking Yamaha's industrial clout they had to make do with thick iron cylinder liners and undistinguished piston alloys. Result? The occasional fabulous win and many DNFs.

The next step in cylinder bore preparation was hard chrome plating, using a process that made the surface porous or textured enough to retain oil (the bore of a new cylinder had a reassuring matte gray look). This worked a treat – with cylinders of this kind the air-cooled factory Yamaha RD-56 250 GP bike of 1963 left 'em for dead at the Belgian GP. Then two World Championships against Honda.

And it worked on the production racer TD1-B, but not perfectly. With that machine, if you weren’t stacking clutch discs you were pulling the heads to see if the new chrome plating had chipped above an exhaust port. Or re-setting the ignition, which went out of time every time you ran the engine. Or replacing the brittle cast-iron piston rings, which could break at any time, wiping out a cylinder. In your spares kit were rows of small-end caged needle rollers, because the stock cages were apparently over-hardened by heating to white heat and then quenching in cold water. At the factory, all these defects were noted and countermeasures devised for the next model.

Yamaha TD-1B magazine layout
Yamaha TD-1B article from the March 1965 issue of Cycle World.Cycle World

The chassis were heavy because at each re-design more metal was added in hope of discouraging the chronic frame cracking that these “vibromaster” parallel twins were famous for until 1980, when sacrificial aluminum flex-plates were put between engine and chassis (replace every 150 miles, or every one million vibratory cycles).

Yes, these bikes gave hard definition to the term "high maintenance", but we loved them because those B-models were FAST. Privateer 250 single Ducatis, built to the max? Piffle! Accelerate in their slipstream, then pull out to pass while sitting up and pretending to adjust your goggles. The Harley Aer Macchi 250s were tougher, but they too were driven from the field by 1967 (TD1-C). To us, the Yamahas were locomotives – long, low, and powerful. Nothing can stop the Duke of Earl.

Chassis and cycle parts were "of that time". The steel tube frame was backbone-and-cradle, with an external-spring telescopic fork up front and three-tubes-and-a-gusset swingarm at the back. That gusset grew longer with every model change, so someone was definitely paying attention. Stock chrome rear springs were pogo-stick specials with 125-pound-per-inch rate. Being a trendy young man I replaced them with Girlings and 60/90 dual-rate springs. Much better.

Drum (dumb) brakes – cast aluminum full width, with two linoleum-lined shoes per wheel, operated by cam-and-cable. After a couple of years, cracks appeared in the iron drum liners, and if you just kept riding, chunks would fall out. Do yourself a favor, eh? Buy a new bike every season.

TD1-C had a left-hand engine cover with a big bulge for the gearbox-mounted clutch (new with this model), had the penis tank painted a strange pearlescent white. Its float bowls hung from a bracket that hung down from the top frame rail to a position just above the rear top of the crankcase. I believe an aluminum top fork crown was used.

Carburetors were 27-mm 276 Mikuni licensed from AMAL, with a pair of remote floatbowls bolted to the back of the vibrating engine. In 1967 came the TD1-C with a bracket descending from the frame to hold the bowls. The idea of remote floatbowls was to isolate them from engine vibration. Don't tighten those carb mounting nuts too much – that distorts the carb bodies, making the slides stick open.

Finally, the Hitachi magneto, accurately described as “high-tension” because spark voltage was generated right in the magneto’s coil secondaries, traveling down bright red wires to the NGK B10EN spark plugs. Sometimes the voltage difference between coil and rotor caused a black spot to appear on the coil insulation; time for a new coil – the old one’s arc-ing through. The C-model’s mag would be much improved, with felt cam lubricators, and did not need to be re-timed every two minutes. Timing the mag was a relaxing ritual. A dial gage was inserted down a spark plug hole, held by a screwed-in adaptor. This told you the piston’s position. The Okuda Koki meter was clipped across the contact-breaker points of the cylinder being timed. Set the points gap, then back up the crank to around 2-mm BTDC and rock it to see where the meter’s needle swung to indicate points opening. Correct any errors. Repeat until satisfied.

To ride it. The vibration, putting my hands to tingling sleep. The sudden two-stroke rush at 7500 to 8000, caused by the pipes’ wave action coming into step with the fast-moving pistons, cramming charge into the cylinders. All very pleasant. So I never learned golf. Price for a TD1-B was $847 to the dealer, $1147 retail.

1969 Yamaha TD2 studio side view
The 1969 Yamaha TD2 has the chassis we yearned for, the big double-sided front brake, and four real transfer ports per cylinder.Ytak171 at en.wikipedia [CC BY-SA 3.0) or GFDL, via Wikimedia Commons

Power? Not so much. The B-model was supposed to make 35, the C-model 38. The two-stroke revolution was launched and every succeeding model would make more power, have a better chassis, better brakes, better reliability. In 1969 would come the Big Step – the 250 TD2 with what was basically the RD-56 GP bike's chassis and giant brakes. Water cooling in 1974 and real (disc) brakes soon after. Street guys loved those huge double-sided drum brakes. I helped them load that heavy drum on their truck.

When two-stroke 250 GP bikes sang their final song some five years ago they were making an easy hundred horsepower. Fun while it lasted.

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