The Truth About Heads Pt 1 Lapping valves The number one thing people do wrong with cylinder heads is they essentially ignore them during an engine rebuild. However, almost as neglectful is to abrasive-lap the valves. Lapping won’t significantly hurt either the valve or its seat, but neither will it help it. More importantly, it 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 too easily be removed by refacing. The 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. 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 dozens of ways to do this machining, and some are better than others. See my compression related articles. 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. Moreover, many “problem” cylinder heads such as the CBX1000 and early DOHC fours present seat alignment problems even without valve guide replacement. Honda valve seats are machined at three different 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 this is considered and dealt with as part of a valve job. 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. The tip cannot be resurfaced 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. Valve guides Occasionally, the valve guide must be replaced. Honda’s valve guides are all 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, pro engine builders do it a little better by using specially-designed valve guide measuring tools. 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, they invariably lack provision for the oil-blocking o-rings Honda puts on their valve guides, and mechanics find bronze guides to be more difficult to hone. Yet this is what we have today. Speaking of honing, 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 tools that are not as precise, and “hour-glassing” of the guide often results, and/or a guide whose fit with the valve is too loose. When instaliing 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 supposed to be heated in an oven when valve guides are removed and replaced. 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. Honda valve guide seals have traditionally been very high quality.