This late in powersports history, when new motorcycles have advanced so incredibly and technical knowledge has become so accessible, I often find myself surprised that riders still don't understand oil consumption, as demonstrated each time I am presented with the argument, common in many quarters, that since the spark plugs aren't oily and the exhaust isn't smoking, then oil consumption isn't happening. This unfortunately is fallacious thinking.

Certainly, how an engine is machined, either the first time or during a rebuild; that is, the quality of the fits and surface finishes resulting, play an important role in oil consumption. In rebuilt engines especially, the lingering (and malingering) adherence by many mechanics to a basically 1940s engine rebuild ethic, wherein clearances were large, cylinder wall finishes coarse, and the word "break-in" actually had meaning, is troublesome. This is not however the way Japanese motorcycle engines are made or how they are remade. 1 Much of the bike engine rebuild community is adrift in a storm of misinfomation and confusion resulting in bad rebuild practices. I deal with it constantly. Open-eyed observation of the design of Big Four engines and a little careful study of their service manuals promise real enlightenment. But you'll not find discussion of this on forums. One of the scandals that saddens me. 2

Obviously, all powersports engines consume oil, and the 50-year old ones we hold dear likely lead the way, for various reasons, not the least reason due to their rock-hard half-century-old valve guide seals. But new engines consume oil also, for reasons not yet considered. Because a telling piece of data and one of the unfortunate ways in which the powersports industry and the public are on different sides of a vast technical chasm 3, is that the Big Four in particular have historically "hedged their bets" for warranty purposes in regard to factory-approved levels of oil consumption, with the infamous "quart per thousand miles" policy. 4 This startling potential is no doubt present in only a random small number of mass-manufactured engines, certainly not many. And it happens by virtue of "tolerance stacking" during production. That is, the target spread of several interconnecting machined internal clearances sliding toward a sum total representing the extreme end of the acceptable range. And by establishing the 1000-mile reference the factory isn't saying they expect all their engines to be like that, only that they will accept the one in several hundred (or whatever the figure is) that might, because it is just too costly to establish manufacturing measures that would prevent its occasionally happening. 5 And hey, most of the production is to spec, and even the few out of spec are "to spec". They say so. -)

But this is not news. Much more importantly, and what you should know, is given the right combination of conditions, it is possible to make even a well-manufactured and well-maintained engine use oil excessively. Because the above-mentioned cases are not the only, or even the greatest, sources of oil consumption. The most significant source of oil consumption in powersports engines is and has been as long as most of us can remember, oil vaporization. Vaporization, that is, the oil frothing and misting and exiting the crankcase as an ether, goes on in all engines no matter their size, type, wear or condition. But none can compare with the phenomenon in the powersports industry's relatively small engines, whose power is necessarily developed at large throttle openings and whose crankcases hold surprisingly small volumes of oil to begin with. 6 So critical in these engines is oil vaporization that motorcycle motor oil developers take pains to rate the vaporization characteristics of their products, something car motor oil technical people don't even think to do. And so dramatic are the possibilities that there are documented cases of brand new single-cylinder and twin-cylinder Hondas virtually running out of oil at the end of a single, long-distance sustained high-speed run. Think Barstow to Vegas. The oil was not burned; it did not appear at either exhaust or spark plug. It simply disappeared. Consequently, I have for most of my career warned the owners, particularly those of smaller bikes, to watch their oil levels closely. But even larger motorcycle engines such as the peculiarly hot-running CBX1000 are observed to vaporize their oil noticably. Oil vaporization is something every rider must keep in mind.

1 This is the biggest problem in the powersports engine repair industry, this fixation on rebuild techniques outdated generations ago, and some that were never legitimate in the first place. There is no such thing as "break-in" of a properly assembled Japanese engine's top end, either at original manufacture or remanufacture. The valves are sealing their best before the engine is ever started, and the rings follow immediately after, by the time the first test-ride is completed.

2 Forums unfortunately universally perpetuate bad rebuild practices.

3 Not just a chasm, but veritable galaxies that place them in different edges of the motoring universe.

4 And some Euro OEMs prescribe to half that, i.e. a quart in 500 miles.

5 In other words, the cost difference between knowing one in several hundred will be almost out of spec and one in several thousand will be is exponential-- it isn't simply spending five percent more in production. As with many things, the cost graph curves upward steeply for only tiny improvements in production processes.

6 High rpm whips the oil more and smaller volumes heat it more.