® Japanese Engine Top End Assembly

A typical Ammco straight cylinder hone. A great tool when used right, which unfortunately, it often is not.
Whether because of ignorance or laziness or a misunderstanding of modern Japanese engine technology, there are many in books and on the Internet promoting a Japanese motorcycle engine assembly ethic that is completely wrong.

First, the "break in" myth. If properly assembled, with a straight and round bore, correct piston to cylinder clearance, the correct fitment and type of rings, the correct type cylinder hone, and with a proper valve job, the newly assembled engine will have the tightest, purest, best cylinder specification, function and efficiency it will ever have, and therefore the most compression it will ever have. Compression will not increase with miles and time. The idea that it will is a myth. There will be no "bedding in" or "breaking in." It is only when an engine is assembled incorrectly that breaking in in the top end takes place. In fact, there really is no such thing as top end break in the real world. Transmission parts, yes, linkages, sure, roughly cast and forged pieces, yup. But machined to fit parts? No. The concept of break in as used by some folks is actually borrowed from another place altogether, and that is manufacturer's factory warranty policy. That is where the term came from. Break in is a warranty term, that is, a concept the factory uses to cover its corporate backside, while waiting for something to go wrong during the warranty period. It is not a valid technical term. In other words, break in is not for the engine (at least not the top end) but for the manufacturer. All one has to do is look at the kinds of clearances Japanese manufacturers use for the fitment of their cylinders and valves to see the truth of this. They are, compared with American V8 car engines of fifty years ago, excruciatingly tightly put together, with air-cooled engines containing their pistons at less than 0.001" and liquid-cooled ones just a smidge more, not the 0.002-0.005" common in our parents' era.

And this brings us to honing. User forums promote a related fallacy with their frequent recommendation of using a straight type hone when simply fitting new rings, and their fairly consistent promotion of removing rust or oxidation in cylinders by, again, the use of the straight hone. Straight hones, whether portable or floor mounted, are intended as the final step in the process of boring a cylinder. They have no other legitimate use. A straight hone actually continues the machining scenario by removing significant amounts of metal, as much as 0.003" in many cases, to ensure a straight and round cylinder and one with a smoother than bored surface. Such an operation is never to be done to fit rings, and is never the correct solution to repairing cylinder rust, precisely because metal is removed, thereby upsetting the careful proper piston to cylinder clearance the factory calls for. The correct type hone for new ring fitment is called a flexhone, once available in both a ball end and string configuration, though it seems the ball type has in recent times superceded the string. The flexhone is very gentle, being just strong enough to break the glaze on a cylinder without appreciably removing metal.


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