The Missing Link: Yamaha's first 4-stroke thumper “Around 1974 or so, I got a call from the retail dealer in Buena Park [Calif.]. He said there was a guy asking for parts for an unusual engine. A light went on, and I asked the dealer to find out where he had got it. The guy clammed up. That’s the last I heard of it until I saw it at the Hanford meet.” — Bill Stewart, retired, Yamaha Motor USA Testing Department Manager.
The course of engine development is not always smooth. What seems like a good idea on a computer screen does not always translate well to reality. And in the old days of drafting tables, the process of moving from two to three dimensions was even more fraught with unforeseen obstacles. Prototypes demonstrate in a concrete form the process of development of the production motorcycle. But since factories don’t want rival companies — or journalists — to get their hands on a prototype, they’re almost always destroyed.
The occasional prototype does survive, and when it does, it’s usually very valuable. Glenn Bator, a well known California-based restorer and vintage bike broker, found this engine and, through a lucky break, learned its history.
In the early 1970s, the Yamaha testing facility (then in Buena Park) got rid of unwanted pre-production motorcycles and other test machinery twice a year. Bill Stewart remembers they would call in a scrap metal company, who agreed to grind everything they got into little bits in exchange for free scrap. “Somehow, this engine slipped through — someone took it out of the scrap yard or they stole it,” Bill says. The engine Bill is talking about, no. 043-0-001, is the missing link between Yamaha’s offroad 2-strokes and its popular and successful TT500 4-stroke thumper.
In 1973 or 1974, Bill and his crew opened a crate from Japan and pulled out an offroad motorcycle powered by a 4-stroke, single-cylinder engine. The engine, marked with serial no. 043-0-001, had sand cast cases clearly derived from the 500cc 2-stroke SC500 Yamaha was then making.
The valves (45mm intake, 37mm exhaust) were operated by an overhead cam with the cam chain running on the left side. The spark plug was on the right. The piston was a flat top, 3-ring style typical for the era, with small reliefs for the valves. The cam sprocket and crankshaft flywheels were special construction billet steel marked with red layout dye and scribe lines. The connecting rod was a production level forging with “043” embossed on the surface. Lubrication was wet sump. The cam gear and oil pump gear were machined billet steel, while the oil pump was a special sand cast unit.
The clutch, transmission and cycle parts were from the ancestral SC500, as was the frame, although modified to hold the 4-stroke engine. A cable operated compression release was automatically operated by the kickstarter.
Crankcase breathing and oil control seem to have been the principal challenges to the Yamaha engineers. The cases appear to have been repeatedly reworked by welding and epoxy filling to prevent oil ventilation to atmosphere during the piston’s down stroke (which pressurizes the crankcase) and to control oil pickup by the flywheels. There’s also evidence that the cylinder studs and carburetor mounting were moved at least once.
Despite all this work, no. 043-0-001 wasn’t going to win any races. Bill remembers: “I had an old Ariel Red Hunter — it would ride rings around that thing. [The Yamaha] had a lot of problems. It was only 420 or 440cc [the engine as it now exists is actually 478cc, but it might have been bored out at some point], due to having to reduce the cylinder height to fit the 4-stroke conversion in the old 2-stroke frame. The motor was short stroke, and it needed a longer stroke to make any torque. The engineers knew it wouldn’t do the job — it was a Band-Aid until they could design a good motor.“
Meanwhile, someone owed Jack Davis money. Jack ran a business in the Los Angeles area making performance motorcycle and snowmobile parts, and often volunteered to help the Yamaha factory with offroad races. Jack was given this engine (missing rocker box and side covers) to clear the debt. It then sat under Davis’ bench for a number of years. Eventually, he gave it to Bob Gregg, another offroad enthusiast, who operated a foundry.
Bob had C&J Racing make a frame for it, but made the rocker box cover and side cover himself. Bob adorned the chassis with typical period offroad gear including Preston Petty fenders, Malcolm Smith handlebars and S&W shocks. Jack rode the completed bike and wasn’t impressed. Bob eventually gave the bike to Jack, who eventually sold it. It might have ended there, if not for Glenn Bator. On July 4, 2007, Glenn was watching a parade and chatting with a family standing next to him. The husband mentioned that his brother had a prototype Yamaha TT500 he wanted to sell. Glenn went to see it, liked it and bought it. “I used to work for a Yamaha dealership,” Glenn explains. “I used to work on TT500s when they were new.”
In an effort to give the bike more exposure and determine its history, Glenn brought the bike to the big vintage bike meet in Hanford, Calif. Bill happened to stop by, and saw the bike. “You know,” said Bill, “that motor shouldn’t exist.” The rest, as they say, is history.
