It really is a fascinating thing, the four stroke motorcycle cylinder head. In a space the size of a twenty-five cent coin, you have the engine distilled down to its essence, the most important part of the air pumping character of the powerplant: the valve seat.
So very much depends on that steel sliver, that keeper of passageways, together with its mate the valve itself. The absolute seal so elusive, and degrees approaching perfection dearly sought, each offering yet another height of performance satisfaction, the added pound of pressure so earnestly sought. Its physical beauty like Saturn's rings, like the diamond's facets, its charm ever more potent due to its essential utility.
Valves and valve seats
Valve job work is focused on valves and valve seats. The two actually wear very differently. Valves show their wear mostly as recession, that is, a groove that is worn into the valve's face as the valve gradually recedes into the cylinder head. The softer intakes recede more than the exhausts, but regardless, recession results in two bad things. One, it gradually ruins the valve's seal, and two, it moves the valve toward its lifter, which reduces the valve's clearance. Both lower cylinder compression. If left unchecked, valve recession results in burned valves. Before the early 1970s, it was common for receded valves to be corrected by machining their faces flat again. But this has long been inadvisable due to a plasma coating found on major powersports manufacturers' valves. The valves are replaced instead. Valves can also wear on their tips, with certain models more prone to this than others.
The valve seat, for its part, is the part of the cylinder head the valve rests on. Valve seats on vintage Japanese bikes wear by widening, getting out-of-round, and by pitting or cratering. Interestingly, intake seats exhibit mostly widening and rounded edges, whereas exhaust seats are heavy on the pitting. Valve guides also wear, of course, more on rocker arm engines, and usually the exhaust more than the intake.
These then are the reasons for a valve job on a Japanese cylinder head: receded valves, and worn and pitted seats. And frankly, lapping valves -- the term used to describe putting abrasive paste on a valve and spinning it against its seat -- fixes neither. Although some OEMS legitimize it, valve lapping is not best practice. A valve-to-seat interface that needs to be lapped is one whose parts are faulty. The valve job itself is inferior from the outset. I haven't owned a valve lapping stick since Carter was president.
Valve seat angles
A cylinder head's valve seat has three basic angles. The sealing surface, where the valve will reside, is usually a 45 degree angle. This is the heart of the valve seat, the holy grail. Valve seats also have two additional angles, a 60 degree transitioning from port to seat, and a 30 degree transitioning from seat to combustion chamber. These angles are sort of overgrown chamfers that ease airflow across the valve seat. So critical are these transition angles that the Superflow corporation, makers of the most widely-used cylinder head airflow measuring equipment in the world, holds that these three angles constitute the most important place in the port, flow-wise, and thus the most impactive on performance. Do a careful valve job and you will reap at least 80 percent of what you might hope to gain by "porting". I've proven it.
Manipulating the angles
A proper valve job looks like this. First the head is inspected for damage that would make spending time and resources on the valves and seats inadvisable. Once the head is deemed usable, it is ultrasonically cleaned and painted. The next step is to machine the seat's 45 degree to square it up relative to the valve guide and dress away pitting and unevenness. Then Prussian Blue is carefully smeared on the seat and the valve inserted, to "register" where on the valve's face the seat is contacting it. It's important that the contact be centered on the valve's face. If it is off center, then the 30 and 60 degree angles are machined as necessary to move the contact where it belongs. If for example the valve-to-seat contact is off center toward the combustion chamber, then the 30 degree angle is machined slightly to move the contact the other way. If on the other hand the contact is off center on the opposite side, toward the valve guide, then the 60 degree angle is dressed a bit to move the contact appropriately. Once the seat is smooth and centered respective to the valve, you're still not done. Now that 45 contact zone needs to be either widened or narrowed, as needed and more or less to factory spec, for good sealing and long life. Narrow seats seal the best, but wider ones last the longest, so a compromise is sought between these two goals. The 45 degree is narrowed by machining both 30 and 60 an equal amount. It is widened by machining the 45 alone.
If the cylinder head us from a shim valve train type engine, the work involved in a valve job is potentially doubled. Many extra steps are needed due to the constraints of available shim sizes. The tech must be sure that the end result does put the installed shims near the small end of their range. Even more work is required to prepare the head for aftermarket cams.
First, lapping. Why do it? If you have done a good valve job, there is no need to lap. In fact, you will be degrading, undoing, much of your hard work. Lapping at its very best is not part of a valve job but a very poor substitute for it. If you understand the what and why of valve recession, why then would you do a procedure that hastens it, and actually produces it where there was none to begin with? Makes no sense to me. Now though I avoid lapping, my friend and colleague Nigel Patrick (five-time consecutive ProStar champion) very lightly and faintly laps to check contact and concentricity. Just a few strokes. And that makes sense, in a way. He doesn't do it to improve the seal, only to confirm it. But even there I have to ask -- why? Why substitute the traditional Prussian Blue to do this job, with lapping? With him I think it's just habit born of many many years. No harm. Third answer: 70s Japanese valves are plated. Why defeat or harm that plating by abrading it? I can't say this strongly enough: though OEMs countenance it, lapping is a hack procedure best avoided and with virtually nothing to commend it.
Valve seat tools