® The Myth of Line Boring

Image courtesy of Dan's Motorcycle Repair (www.dansmc.com)
There is a misconception that is feverishly perpetuated on motorcycle related user forums that says since Japanese motorcycle cylinder head camshaft holding recesses are "line bored" (in itself a misnomer, the correct term is "align-bored"), these engines' cam holders are practically inseparable from their host cylinder heads, i.e. can't be moved to another head, and any attempt to do so results in mechanical disaster. It just isn't so.

There is nothing mysterious or exotic about line boring. It simply means machining a round recess longitudally. That the removable holders or caps are usually (though not always) in place while this is done is only logical. But what those who have not spent a lot of time around Japanese motorcycle engines don't seem to understand is that the Japanese do two very interesting things. First, they are masters at repeatable tolerances in mass machining. In areas where they want to hold to close spec, they do so astonishingly well. The plain bearings for their forged, one-piece plain bearing multicylinder crankshafts are fitted with less than 0.001" oil clearance. On every single example produced, in some cases resulting in 30,000 copies exactly the same, with no two bearings varying by a more than 0.0001". That's 1/10,000 inch -- one-thirtieth the thickness of a piece of college ruled notepaper! The Japanese are very very good at this, and have been for a very long time. The second thing seemingly unknown to all the Internet forum "experts" is that despite this ability the Japanese at times choose for whatever reason to not exercise it, and this includes and is exemplified best in the area of cam holders, whose typical oil clearance is 0.005" and is never less than 0.003". Quite a bit looser, in other words.

The end result is that, one, the Japanese could make every head and cam holder excruciatingly exactly the same, in which case you would be safe. Nowever, they take the opposite tack and make the caps very loose. Either way, you're home free. And just as career Japanese motorcycle engine rebuilders have known and practiced for generations, you can indeed move cam holders from one head to another exactly like it and have no fallout. None. Honda at one time even promoted this, by making for a limited time a previously unavailable cam holder as a separately obtainable part.

This does not mean you shouldn't Plastigage both the old and new setups, that's simply good practice. But it does mean you can stop listening to all the nannies on user forums. Because as with so much else of what they say, they are wrong on this one too.

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