Vintage Honda owners have little recourse than to use aftermarket pistons when rebuilding their fifty-plus year old engines. Several years ago I had to wait for months for a factory piston to show up on CMSNL, and then the ordeal to find rings ensued. Today, even that is no longer possible and while OEM pistons occasionally appear on ebay, put you'll pay a princely sum for them and in many cases you still have to find the rings, somehow. And never will.

So the aftermarket has forced its presence into the picture. Right away, this is a concern. Very little that's positive can be said of the powersports replacement parts aftermarket. Those who think otherwise just don't know their business. Become knowledgeable about factory parts over a long period and the consistently substandard nature of the alternatives will shock you.

Elsewhere I have compared cast and forged pistons. The bottom line is that the two types do not serve the same market. One lives in the restoration/maintenance world where pistons are engine-specific and famously high quality, the other in the hot-rod realm where pistons are more generic, vary widely in quality, and must be forced into compatibility with the engine. There are exceptions. The odd motorcycle model enjoying high popularity does have cast pistons available for it on the aftermarket. The 70s 450/500 DOHC twin is an example. But that is rare. Lately, cast pistons have all but disappeared and forged has become king. While not necessarily a bad thing (my own bike contains them), the forged piston adds complexity to engine rebuilding.

The first thing that comes to mind is that the forged piston tends to need hand work before being used. Lots of burrs and sharp edges result from the forged piston's need for much more machining during manufacture than does the cast piston. Major forged piston supplier JE at least runs some Scotchbrite over the piston's crown, but you can practically cut your fingers on other spots. Next, as is well known, the forged piston's inferior expansion control makes them need much more clearance in the cylinder. This impacts longevity, sealing, and engine quietness (they can clatter when cold). This is not so bad, but it results in more blowby. Execess blowby can freeze up the rings, cause oil seal failure, and over time really trash the piston. Forged piston's rings invariably need gapping, a thing unheard of in the OEM piston world. And, in many cases, though made for a specific engine, aftermarket forged pistons nearly always present negative deck. Which is peculiar. Also, their makers go overboard on the valve reliefs, in the same way that aftermarket cam makers produce conservative spec camshafts, to anticipate unknowledgeable installation. These hugely oversize reliefs result in more pronounced piston dome to maintain compression, with that dome potentially impairing combustion efficiency. The aftermarket piston pins are occasionally so inferior as to be chrome plated, are often too short or too long, and in rare examples (notably, CruzinImage) actually fit looser in the connecting rods than do the factory pins. Not good. Forged pistons are frequently made with less provision for pin oiling, and the pin retaining technology gives one pause, with clips that seem less secure and even in some cases the complete absence of clips in favor of short-lived plastic buttons. If it's not yet clear, in general, forged piston kits demand more of a knowledgeable, careful, racing-fraternity ethos than do cast piston kits. But that's not insurmountable. You just have to know what you're dealing with.

Neither in my original piston article in which I explored the history, manufacture and application; or in this one where I emphasize the practical outcome of small-scale engineering, am I denigrading forged pistons. But the public has seemingly little understanding of the differences between the cast and forged pistons and few if any voices in online media address them.