Yamaha RD350 - CLASSICS REMEMBERED

In the beginning, YA meant 125 single in Yama-code and YD meant 250 parallel twin, as in YA-6 and YDS-3 Yamaha two-strokes. Autolube throttle-proportioned oil metering pumps meant no mixing oil into fuel at the gas pump. Just make sure there's oil in the oil tank. As Britain learned from her 500cc imports into the U.S. in the late 1940s, Americans always want more. So Yamaha repurposed its YDS line to crank out bigger 60 x 54mm two-stroke twins with 305cc and a claimed 29 to 31 hp, with the model number YM1.

Such a half-measure was okay for a couple of years, but then came the full-sized YR1 twin of 1967, at 61 x 59.6 it displaced 348.35cc. That year, Tony Murphy and Mike Duff finished 18th th and 19th in the Daytona 200 on race-kitted YR 350s, and the next year, Yvon DuHamel and Art Baumann came 2nd and 3rd behind Calvin Rayborn’s new-tech flat-head Harley KR750 (with its Cal Tech fairing, free-breathing heads inspired by Neil Keen and C.R. Axtell, and dual carburetion).

For four-stroke lovers, the worst was yet to come; for 1969 Yamaha offered TR2 350 production racers for sale to all comers. No need to “know somebody” at Triumph, BSA or Harley to be given the “privilege” of ordering racing parts. Just buy a 55-hp production racer at any dealer and go race.

In 1972 and ’73, Yamaha 350 twins left the four-strokes behind. A four-stroke would not win the Daytona 200 again until rules wrote them back in for 1985.

Between YM1 and the R-series, Yamaha made the leap to horizontally split crankcase, speeding assembly and cutting costs by allowing crank, both gear shafts, and kick start shaft to be set into the upper case. Because it was inefficient to produce vertically-split 250s and horizontally split 350s sharing no common parts, Yamaha engineers soon replaced those designs with a same-stroke modular system. Both would henceforth be horizontally-split, the 250 having 54 X 54-mm bore and stroke and the 350 64 X 54. Because crankcases, gearboxes, cylinder stud pattern, &c were shared, both could be assembled on a single line. Low production cost is the key to low market price.

The first of these modular engines was the R5, with piston-port intake system. At the same time, Yamaha were upgrading the drivability of their off-road single-cylinder DT models with reed intake valves (rumor traces the origin to Dale Herbrandson), so it was natural to convert R5 to reed valve for 1973, resulting in the 250-cc RD250 and 350.

These were very popular bikes because they looked good and ran with bigger heavier bikes thanks to good power and moderate weight. Production economies made their price low and RDs became for a time the top-selling bike in the US. They also instantly became popular production-class road racers – older readers who spent time in club racing will remember the elaborate hanging-off style adopted by riders in stock RD classes as a means of keeping the low stock pipes off the ground. The inside foot on the peg, the outside knee hooked over the seat, and the rider’s body as far inside as his/her reach to the bars would allow. Anything to keep that inside pipe from jacking the wheels out from under you!

There was a vigorous aftermarket of pipes, reeds, bigger carbs, and cycle parts, and I must admit to having built a few souped-up RDs myself. If a rider wanted closer-ratio gears, there they were in the TZ parts book – a drop-in fit for at least 2nd through 6th (to use the TZ first gear you had to convert to the long countershaft and dry clutch).

From the R5/RD came the line of first air-cooled (TA) and then water-cooled (TZ) 250 and 350 racers. Many a rider owned just one race bike but carried both sets of cylinders, and would juggle parts during a race weekend to compete in both 250 and big-bike classes. At the time, it was said that during the annual conversion of production lines for new models, Yamaha would use the tooling for a few days to make the racing parts for the following season.

Wrecked your crankcase? A friendly dealer could sell you a set for $105, or you could pull a set from a written-off streetbike (yes, they weren't exactly the same, but they worked fine) . Ruined your nice chrome-bore twin cylinder block? Ditto, for $140. At those prices we could afford a lot of racing ("We're on safari to stay…"). RD needs a fresh crank and the two of you plan a weekend trip to the Cape? Got tools and a manual? Two or three nights after work did the trick – there was nothing difficult about working on those bikes. RDs were affordable fun and transportation.

Preserving the same 54-mm stroke and crank and rod forgings was the 185-mph TZ750A 4-cylinder racer of 1974, starting a lineage that would win the Daytona 200 1974-82 inclusive. That A-model had the same 64 X 54 bore and stroke as an RD.

Nothing is enough for long, so in 1976 the RD350 was replaced by the stroked 64 X 62-mm RD400, giving more bottom punch. The end of days was coming for street-going two-strokes in the US.