In the spring of 1967, I bought a secondhand Yamaha TD1-B production roadracer for $500 from a respected Cambridge, Massachusetts, alternative culturist whose friends called him "Gee Chief."

I would never get this bike to push-start properly because it didn’t occur to me to read the tiny alphanumerics stamped into its carburetor metering needles. If I had, I’d have seen they were of two different types, only one being the correct 8F3 type. As a result, at a Canadian Junior event at Mosport, when the flag dropped the others pushed, bumped their bikes to life, and accelerated away. Not I. I pushed and I bumped, but there was no answering crackle from my engine. When it did finally pop and snivel half-heartedly into life, I was halfway around turn 1. But that’s not my main narrative here.

Although that bike’s claimed peak power was only a miserable 35 hp at 9,500 rpm, it was equipped with a 525 drive chain—a heavier chain than the 520 now used on 250-hp MotoGP bikes. What struck me as curious was that between the engine sprocket and the gearbox-output shaft seal was a quite thick felt washer, part number 156-17465-00.

I wondered why that washer was there. What was its purpose? When I asked my betters, I was told, “Uh, it keeps dirt off the shaft, you know, where it comes out of the seal.” Hmm. But there was no felt washer behind the ignition rotor to keep that shaft clean where it emerged through its seal.

Understanding came to me in a flash two years later, as I stood staring at Kel Carruthers’ mysteriously fast Yamaha TD2. It had three small screws, arranged at 120 degrees around that gearbox seal. Under the head of each of them was a wide washer that projected over the outside diameter of the seal. Carruthers had put those screws there because that seal sometimes eased itself out of place, allowing gearbox oil to be flung out into the path of the rear tire. The thick felt washer had been a production engineer’s quick, economical fix for the same problem: keeping that seal in place.

Kel Carruthers
The description written on the back of this black-and-white print of Kel Carruthers from Daytona 1972, reads, “Kel is just as meticulous at midnight in his van as he is at noon in a well-equipped workshop.”Cycle World archives

In later seasons, at least one of Yvon Duhamel’s celebrated get-offs in the Laguna Seca Corkscrew had been caused in just the same way, by seals that had gradually squidged their way out of place to unleash a grip-destroying deluge of gear oil.

The next step was revealed to me in 1969, as with soft hammer and dial gauge I trued a new Yamaha snowmobile crank. Its outer seals were still in place and each of them had a narrow rib around its OD, obviously intended to fit into a corresponding groove machined into its horizontally split crankcase. Positive seal retention! Such a self-retaining seal had not been possible on Yamaha’s TD series engines because, with vertically split crankcase halves, their seals had to be pressed in. Ultimately, such OD-ribbed seals became industry standard, positively preventing gearbox or crank seals from being pushed out in service.

As a footnote, I must mention that even today there are people building engines who are sure that O-rings, seals, and gaskets cannot possibly work unless drenched in case sealer. My favorite was a BSA Bantam, whose owner had used a whole tube of sealer. Its inward extrusion as the crankcase halves were drawn together glued the crankshaft to the cases.

Gaskets and seals are best installed dry as, under pressure, case sealer acts as a high-viscosity lubricant, actually making it easier for them to be squeezed out of place (now ask me how I know). O-rings need only a light coating of oil or grease to let them assume proper position in their grooves. For detailed information on O-ring groove dimensions and desirable degree of rubber compression, see the digital version of the Parker O-Ring Handbook. My printed version is dated 1967.