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Lapping valves
The number one thing people do wrong with cylinder heads is they essentially ignore them during an engine rebuild. For one thing, lapping can’t take the place of an actual “valve job”. During a proper valve job, the valve is removed and inspected. The mechanic is looking not at the stem so much as the head of the valve, because over time a ridge is worn onto the head’s sealing surface called the valve’s “face”. This ridge is termed “recession”, and like it sounds, it means the valve is retracting into the head as it wears. This is normal wear. The problem is this ridge affects the valve’s sealing ability. In the really old days the valve’s ridge was removed on a valve “refacing” machine. But on all Honda valves made until the mid-1980s, this operation is not possible due to a coating on the valve. The coating is thick enough that it can’t be bothered by lapping, but it is thin enough to easily be removed by refacing. Incidentally, this coating is not why lapping is unwise. Lapping is bad practice because it circumvents proper valve seat machining, and because it ignores valve recession--the way valves wear the most--and does nothing to correct it and even makes it worse. This one thing--valve recession and improper corrective attempts--is why so many on forums report unsatisfactory results when "rebuilding" their engines. An engine rebuild in which the valves were lapped instead of the seats machined is not actually a rebuilt engine.
Some talk about replacing Honda valve seats in the cylinder head. Of course, Honda never supplied replacement seats so right away you encounter two challenges. Who will supply those seats, and will they install them correctly. The latter problem, installation, is of serious concern. They're not made to be replaced, so the technique is not well established outside the somewhat archaic automotive head rebuild industry. Recently I had in my shop a CBX head that a car machine shop had replaced the exhaust seats on and among many other problems valve protrusion was so out of spec the required shims went beyond Honda's range.
The valve seat
Once the condition of the valve is known, the surface it seats against in the cylinder head is examined. Honda valve seats typically show rounding and widening on the intake side of the head and severe pitting and some widening on the exhaust side. These things are corrected by lightly machining the seats. There are literally dozens of ways to do this machining, and some are definitely better than others. See my compression related articles and the one that compares valve seat machining equipment. While the mechanic can often get away with using the lesser-quality methods, when valve guide replacement is included in the valve job, most of the older methods are inferior in that they can’t compensate for slightly shifted valve guide position. Even worse and yet more common, many “problem” cylinder heads such as the CBX1000 and early DOHC fours present seat alignment problems even without valve guide replacement. See my valve seat tools article for details. Special tools are required because the machining is done at three angles. The sealing surface is 45 degrees, while the entry and exit sides of the seat have 60 degree and 30 degree transition angles. All of these angles are machined as part of a valve job.
Tipping
In many Honda engines the valve wears quickly on its tip due to the scrubbing action of the valve adjusting screw. Those valves that won’t need replacing due to recession will in many cases need replacing due to this wear at the tip. However, the factory Honda valve's tip cannot be resurfaced like in the old days because it has the same coating on it that the valve’s face does. Aftermarket replacement stainless steel valves can be tipped and refaced however. And where tipping is most necessary is for heads using shim and bucket valve trains. These engines demand that their valves be tipped in order to maintain reasonable valve shim range usability. Beware of a machine shop that does a valve job on shim type engines without taking valve protrusion--and consequent shim availability--into account.
Valve guides
Occasionally, the valve guide must be replaced. Honda’s valve guides are cast-iron. This is actually a very good material and unlike Honda’s cast-iron cylinders the cast-iron valve guides wear very slowly. The classic test for valve guide wear is the valve is put into the guide, then withdrawn about a half-inch and a dial indicator used to measure the amount of side-to-side wiggle. All of the Big Four manufacturers advocate this method, and it is not bad when an experienced person does it. However, many engine builders prefer specially-designed valve guide measuring tools.
Bronze valve guides
Honda’s vintage valve guides are quickly becoming unavailable. This leaves us at the mercy of aftermarket replacements which are almost universally made of bronze. While racers claim a cooling benefit over cast-iron, bronze guides in street use are not as good as the factory cast-iron ones. The valves have to be fitted looser because the guides lack cast-iron’s self-lubricating quality, and they invariably lack provision for the oil-blocking o-rings Honda puts on their valve guides. Yet this is what we have today. Valve guides must be honed after installation. Honing or reaming a valve guide is even more tricky than honing a cylinder. The equipment is very expensive, meaning that most mechanics will use inferior tools, and “hour-glassing” of the guide often results, and/or a guide whose fit with the valve is too loose. When installing cast-iron guides hammering on the top of the guide can result in it cracking, so knowledgeable engine rebuilders try to drive them in other ways. Honda heads are best heated in an oven when valve guides are removed and replaced.
Valve guide seals
Valve guide seals have got a lot of notice lately, due to the emergence of Viton as an aftermarket replacement. It may seem that more durable Viton would be an advantage, and many on the Internet espouse this theory. However, Viton is also a lot less flexible than other forms of synthetic rubber, making it an inferior choice compared with the stock Honda valve guide seal. Although the Honda seal will eventually react to engine heat and get hard and lose sealing ability, the Viton seal starts out relatively hard and knowledgeable engine builders dislike it for that reason. Factory Honda valve guide seals have always been the best in the industry.
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