Valve Jobs Done Right
A close-up of a Honda CB750 SOHC valve seat. Though the upper 30 degree and the lower 60 degree shapes have been rough cut using Neway cutters, the 45 degree cut in the middle, the one that counts because it is the actual sealing surface, has the benefit of a stoned surface, the best possible with hand tools.
Ah, valve jobs. One thing that has come to characterize the inevitable changes happening in the powersports service industry is that very few OEM dealers are doing them any longer. Not only have the cost of OEM replacement valves, when needed, always been prohibitively high, but when added to the labor cost, the job takes on price proportions that effectively knock the average repair shop out of the game. When you can pick up a low mileage cylinder head complete with as-manufactured decent valves and seats from a salvage yard for a tenth of the repair order estimate, there's no contest. Today only non-Japanese dealers and speciality shops are active in cylinder head work, the nation's largest tech school has stopped training the skill, and one Big Four OEM discourages their dealers from doing it as well. The valve job, at least everywhere but in race shops and Harley dealerships, has become a lost art.
What is a Valve Job?
Owning a vintage motorcycle naturally means you're going to at some point need to get after its cylinder head. This may be due to simple smoking issues, which if on deceleration points to valve guides and/or seals. It may instead be because cylinder compression is low and a cylinder leakdown test result points toward the valves. Either way, cylinder head work is something many owners tackle themselves; it's part of the vintage ownership experience. However, many would-be engine rebuilders aren't aware of something, namely this -- done correctly, cylinder head work involves a lot of uncommon knowledge and expertise, and not a few specialized tools of a sort not likely to be found in the average garage. It is not simply yanking valves out, putting abrasive paste on them and burnishing the valves into their seats in hopes of improved sealing, despite what you find on the Internet, where there is more folklore than fact assumed to be proper cylinder head rebuilding practice.
In fact, a number of pitfalls await unwary do-it-youselfers. For example, the valve lapping technique referred to above, although thought by many to be the major part of a valve job, is not in reality a part of anything professional technicians do, let alone an important part of cylinder head reconditioning. Another, hmm, well if not exactly a fallacy, at least very poor practice, is the use of Neway valve seat cutters. Again, the industry pro either does not use them at all or uses them only during the initial reconditioning stages on unusually heavily worn valve seats, usually the exhaust seats which errode the heaviest, and then only very sparingly, followed by the correct tools. Let's expand on these two things, valve lapping and Neway seat cutters, and also visit a few related issues.
First, valve lapping. Valve lapping, despite a few manufacturers' adherence to the procedure, is bad practice. It actually comes from the very early automotive mechanic's world, the turn-of-the-century period in which cars had much in common technologically with tractors. Lapping is related to interference valve angles. Interference valve angles means the valve faces were, for example, at 46 degrees and the seats (in the cylinder head) at 45, for example, the thinking being the difference ensured instant valve sealing. Of course this did not ensure sealing by itself -- a proper valve job did -- unless the valve job was sloppily performed. In that case, interference angles forced a knife-like edge on the valve seat. Unfortunately, this produced a built-in valve *recession* situation, so much so that Harley-Davidson, one of the practitioners of this technique, warned in early Sportster manuals about the loss of valve clearance merely miles after "valve jobs" are done in this fashion. Valve recession is a subject all by itself, but as one of the worst things that happen to a powersports valve, it isn't something you want to create on purpose! This knife edge is not a good way to do things. A half millimeter to one millimeter is standard spec in multicylinder powersports engines today. The interference angle method produced an effective valve seat that was far narrower than proper, and much narrower than mere numbers suggest, taking into consideration that today's valves are roughly half the size of their predecessors. Here's where valve lapping comes in. What valve lapping did was, as a final step, it broadened the interference angle's knife-edge contact point, by prematurely wearing part of it away, i.e., by rounding it off. Strangely, lapping has remained the final step in a few manufacturers' recommended technique long after interference angles have disappeared from powersports, mainly through inertia. That is, these manufacturers see little reason to abandon it, especially since it "covers a multitude" of cylinder head reconditioning sins at the dealer level. Lapping also smoothes the rough finishes, sort of, resulting from poor choices in valve seat cutters. So that is probably its claim to fame today. But best practice it is not. Best practice is to recondition the seats perfectly, at the correct angles and finishes, not using interference angles but the right, matching ones, and then not having to rely on any artificial "wearing in," by lapping or whatever. This also means using good valve seat cutting methods in the first place.
I had been trained on, had used extensively, and had significantly invested in, Neway cutters before I realized what they were. So I am well-versed in them, and have rebuilt many engines with them and many since I stopped using them. There are a number of problems with Neway cutters. The first and foremost is that Neway is a "dead pilot" system. Dead pilot means the critical indexing point of the whole job is a tapered pilot pressed into the valve guide, and it is this pilot that the cutter revolves around. The dead pilot's taper practically ensures eccentricity because it will not center exactly in the guide, meaning the cutter will cut the seat off-center. Then add in the inevitable valve guide wear (guides wear oval and hourglassed) and there is even further difficulty and hence even more reason to be wary. The best valve seat reconditioning systems, whether hand or powered, are "live pilot" type wherein the indexing pilot is one with the cutter and rotates with it. There is no taper involved, and guide wear is less an issue because the pilot is no more influenced by the guide's wear than is the valve. That is, it "floats" as does the valve, in other words, in the guide, thus finding its own center.
Another big problem with Neway cutters is they cut very aggressively, half a thou (0.0005") with each revolution, to be exact. With a light touch. More if you bear on them. This cannot be over-emphasized. They have to be used carefully or they can trash a cylinder head in no time. (Training MMI students included managing their well-intended efforts before they cut the seats down to China!) More importantly, no matter how carefully used, that is, even in the hands of a pro, Neway cutters leave a finish on the valve seat that is just too rough for good sealing, which is why pros use Neways only for correcting very bad seats, following with stone wheels for the final shaping.
Shim Type Engines
Shim type valve trains, which means most today, are the second most important reason for the disappearing art of the valve job, after cost. Shim engines present serious obstacles (in the way of specialized technique) to would-be valve seat reconditioners. The problem is valve protrusion. Valve protrusion is the amount that the valve sticks up above its spring's seat, as measured with a caliper. Every 0.001" cut off the valve seat moves the valve an equal amount upward toward the shim. This is corrected with a quarter turn of the valve adjusting screw on non-shim engines, but on shim engines a smaller shim must then be found. The problem is shim engines have only so many shim thickness choices, and it is almost inevitable that any cutting on the seat is going to put the resulting valve clearance into the smallest end of the available range. This is a problem because there are no shims left under the factory's smallest for future valve clearance adjustments. There is a reason OEMs ship their bikes with shims at the large end of the range. Putting an engine, through valve work, into its shims' smaller end is not acceptable. And machining shims, as some people resort to, is no solution either (not to mention the mark of a hack). A new valve will not help because it is the seat that is the problem, not the valve. There is only one way to handle this. The valve must be "tipped," the name given to the method of shortening the valve stem to correct the excess protrusion. Tipping is a time-honored solution as there are very few engines in which the valve seat itself can be replaced (Harleys, mainly). If fact tipping must be considered part of a valve job -- that is, not an option -- on every shim type engine.
But now we have another problem. Before the 1970s when all engines had two-piece valves (welded together) made lf durable materials, a valve could be tipped no problem. However today, virtually all valves are one-piece, made of much softer materials, and plated to make them durable. In most of today's engines you cannot tip the valves because of this plating.. Tipping would remove the special anti-wear coating on the tip of the valve's stem, with the result the valve would wear extremely rapidly after being put back into service. You cannot fool around wtih this plating, technically known by its tradename Stellite, literally a hard metal plasma layered onto the stock, soft metal valve. This Stellite coating is merely a couple thousandths of an inch thick, not enough to allow removing of some of it during the tipping operation.
The working surface on a valve is its face, where it seals. Spring forces, combustion's heat and pressure, not to mention its chemical byproducts, and the camshaft's hammer -- all combine to wear this face as the valve pounds against the seat in the cylinder head. This wear takes the form of a ridge in the valve face. In our grandfather's day this ridge was simply ground out every 40,000 miles on a special machine that spun the valve against a super-smooth grinding wheel. Neway sells a hand-held tool for doing this same ridge removal by hand, but as this tool is marketed indiscriminately this is a renegade procedure, one I would classify pure hack, for the reasons already given. Today's Japanese valves and that of most other manufacturers cannot be reclaimed this way, but rather must be replaced. The correct way to handle valves that have recession type wear, or any appreciable wear for that matter, is to replace them. In all engines I prefer as replacements the aftermarket long-wearing and machinable stainless steel stock dimension valves. On any shim type cylinder head on which valve seat reconditioning is being undertaken, stainless valves are actually mandatory in my view, because, as we have already explored, unlike the stock valves they can be machined on the other end too, that is, tipped, so as to put shim sizes back into the middle ranges.
Now, this doesn't mean I can't appreciate that for many folks making do is the objective. That is, that they just can't afford new valves or don't have access to the right equipment, or perhaps simply are bucks down. Understood. But what I have described here is best practice, the right way, as the article's title says.
In the world of modern Japanese powersports engines, replacement OEM valves are not always the best option, either because they are upwards of $70 each or because they are discontinued. Plus, very few aftemarket sources exist for engines. Other than the aforementioned stainless steel valves, which are disappearing and in many cases no longer available, there is mainly just the Japanese-made valves marketed by wholesale supplier K&L. They are virtually identical to OEM, at a much lower cost, and as a bonus, are made in lengths consistently 0.001"-0.002" shorter than stock. They are pre-tipped, im other words, a smart move on their maker's part. Unfortunately, in my experience, they are not tipped enough. The intakes are fine as-is. The exhausts however should be tipped at least 0.005" to be truly useful, because exhaust valve seats get cut more than do intake seats, resulting in more valve protrusion. Another solution is to use thinner than stock valve shims, enabling the completed valve job to live many useful years without running out of available adjustment shims. Undersize shims are available.
Here is a picture of an automotive cylinder head being pressure tested. You have to understand one thing, no one who goes to the trouble of pressure or vacuum (my preference) testing their valve seat sealing bothers to use Neway cutters in the process of getting there. Not more than once, anyway. Believe it.
I'm a believer. The head you did for the 750F has now been in service for 2 years. The bike makes just as much (seat of the pants ) HP as the heavily ported head, and gets 44 mpg commuting to work to boot (the other head was good for about 33-5).
John Goulet, MA
The classic valve recession syndrome. Slight recession is normal. Accelerated recession is common on some Japanese engines. If allowed to go unchecked, that is, uncompensated for with regular valve clearance adjustments, the final result is a burned valve.
Valve job services
Folks are advertising on eBay and elsewhere valve job services. Beware. The heads I saw showed much wider than normal valve seats. Also, glass bead blasting and the use of race spec fast-wearing bronze valve guides are specifications that are very far from best practice in this industry. Finally, and most troublesome, the mention of machining the stock valves should have potential customers worried. I find it incredible. Stunning.
A valve job is inherently a highly skilled undertaking. Additionally, cylinder head work necessarily involves a number of things besides valve and valve seat focus, such as gasket surface prep, oil jet cleaning, thread repair, etc. -- even, if warranted, port mods. But the valve and its seat are the really critical things and the reason the head was likely removed in the first place. There is a lot more than most people think to the valve job, and we haven't embarked on spring set up and the like, so there is even more to consider. Don't let the inexpert opinions on forums and elsewhere lead you into complacent ignorance. The valve seat often gets the worst treatment at the hands of mechanics. To sum up, reconditioned valve seats are potentially at odds with shim engine designs, valve lapping is a huge compromise, Neway seat cutters are crude and even dangerous, Japanese valves are plated, recession is the normal sinking of the valve into the head, the fitting of stainless steel valves is best practice, pre-tipped aftermarket valves are available for some engines just as thinner than stock valve shims are also. A valve job is not a valve job. Drop me a line.
Cylinders Done Right
Cylinder head porting
More on cylinder heads
Valve job service
Valve seat tool technology