There is a situation that is paradoxically the most significant to vintage Honda owners and yet at the same time the least appreciated and understood, especially on forums. Namely, the fact that Honda put very soft valves in their motorcycles from the late 1960s through the mid 1980s. These valves are in fact plated (actually plasma-coated) for wear purposes. Honda’s use of soft valves is very significant. Because of it, all of these engines suffer low cylinder compression until the valves are replaced.

Honda published a 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 Stellite wear surface 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 clear 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 ZX600 (and EX250) more dramatically even than the Hondas. But by 15,000 miles vintage Hondas also 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 Honda's rocker arm engines.

All of Honda’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. However, interestingly, this extreme recession due to softness is slower on the DOHC 450/500 and four-cylinder Goldwings because of another little-known Honda fact: Honda put softer valve seats in those few engines. Which is odd. Honda traditionally used very hard valve seats, so hard in fact that the Titanium valves in their V4 Interceptor-based factory roadracers wore out precipitously, according to the man who built them, Nigel Patrick. The 450 and Wing softer than normal seats wear along with the valve, which results in slower valve recession. But the valves still recede. Whenever possible it is best to use stainless steel replacement valves in vintage Honda engines.

Compression Chronicles part 3