There is a situation that is paradoxically the most significant to vintage Honda owners and yet at the same time the least appreciated, the least understood, and the least communicated in the vintage world. Career powersports mechanics as well as countless DIY owners have for a long time observed the rapid extreme valve wear in Hondas and other Japanese four-stroke engines. Honda valves in engines designed before roughly 1980 are made of a softer material than that used in later valves. Consequently, they recede (wear upward into the cylinder head) much faster. A 1971 Honda service bulletin explains that these valves were treated with a special hard coating to slow wear. But they needed that coating because the valves were soft, and unfortunately the valves wore quickly despite it. The coating merely allowed these valves to survive as valves at all. It did not result in their having the lifespan observed in the valves of other engines.

Honda's dealer bulletin, Service Letter #84, that warned that these valves cannot be refaced on the traditional automotive valve refacing wheel because the risk of going through the hardness-providing Stellite coating is very real. At factory Honda tech school we cut a valve in half lengthwise, then dipped it in warm battery acid, and it was easy to see (due to dissimilar oxidation) the two metals. And just like the drawing in the Honda bulletin, we could see how thick the coating was. It is on the tip, and it is on the face (sealing surface) of the valve. And it is about 0.020" thick, that is, roughly half a millimeter. Too thick to be bothered by lapping, if that is your thing, but also too thin to risk removal when the valve is being refaced, which of course is the subject of the bulletin.

Though speaking only very briefly about the coating and warning against refacing, this official notice let the proverbial cat out of the bag. It's plain that Honda's supplier changed their manufacturing method and began making softer valves. This proved to be the case in the practical world. Career Honda and Kawasaki mechanics can tell you stories about this. We called it, "Ninja syndrome", because it affected the early ZX600 (and the Ninja 250) even more than it did most Hondas. But by 15,000 miles vintage Hondas suffer significant compression loss due to a ridge worn onto the valve face, technically known as recession. Recession is actually a normal occurrence. But rapid recession quickly reduces cylinder compression and increases valve heat as the valve sinks into its seat, reducing clearance. And strangely, the Stellite coating doesn’t help much. The valve face recedes incredibly fast and the valve's tip gets pocketed very consistently on rocker arm Honda engines. All of the company's 1960s and 1970s production four-stroke engines and a smattering of their 80s bikes are included in this problem, from the tiny 50s and 90s through the GL1200 Gold Wing. Whenever possible I use stainless steel replacement valves in vintage Honda engine rebuilds.