Consider Neway valve seat cutters, "make do". The next step (and a significant one) up from the Neway is the current Honda diamond stone, at a little more cost—about $150 per angle—with the total including pilot/driver combination coming in between $700 and $1000, again depending on whether one cutter will service both intake and exhaust. Honda’s then is the best choice at the low end of the market. Unfortunately Honda has discontinued their system and has already run out of certain stone sizes, though China is now making their own diamond stones at a mere $15-20 per stone, so this technology is still available. The next step up in DIY valve seat tool expense is the classic, motorized abrasive stone of the Sioux and Black and Decker variety. However, despite tradition, these tools are hard to use accurately and are actually inferior to the hand-turned diamond stone. Ultimately, the very best in the home mechanic range of hand-operated valve seat machining tools is the Serdi and its look-alikes.

The weakest part of any valve seat machining method is the valve guide. The guide is relied upon to provide two important things: location and loading. The location part is indispensable; you cannot hope to have machining that is concentric to the guide without actually locating on the guide. Logical. However, loading, the other consideration, presents a problem. Valve guides have between 0.001 and 0.003 inch clearance in them. The valve during engine operation naturally self-centers in this clearance. However, a seat machining tool does not self-center, instead shifting significantly with cutting (or grinding) load and thereby resulting in a cut that is not concentric to the guide and thus a machined seat that isn’t either and will not seal optimally. It was probably the German Ludwig Hunger who first came up with the solution. Hunger devised a valve seat cutter that like all cutting systems located on the guide but as an innovation loaded on the cylinder head casting, by being attached to the head’s gasket surface. The French company Serdi refined this quite a bit and became famous for it. Head loading valve seat machining tools--whether hand or power operated--are at the high end of the available solutions and provide the very best end result.

Italian company PEG approached the loading problem in a completely different way, by making their machine cut the seat’s profile and not its plane. That is, the high speed stone slowly traverses the valve seat instead of plopping down onto it, thus eliminating the possibility of a bad seat influencing the machining operation. American manufacturer Hall-Toledo also went this way in their VIP model grinder. Both are called “orbital” designs. However, both companies, though their loading exerts less pressure, still rely on valve guide loading.

And really, though technically superior, the advantage of the head-loading system is in many cases very small in practice. Most riders can get away with using the lessor seat cutting methods. In that regard, the Honda diamond stone is an excellent all-around choice, offering as it does not only an accurate seat with a good finish at an approachable price, but also a live pilot design wherein the pilot is not stationary but rotates with the stone, automatically improving tool centering as it is merely acting in the same way as the valve does. This is the best that can be had in the guide-loading arena.

At least when the valve seat is flat and undisturbed. And therein lies the issue. The head-loading choice in tooling becomes virtually mandatory in the case of certain “problem” cylinder heads, and none more than those in which the seats have become tilted, that is, shifted slightly in the head casting. Honda’s hottest-running vintage engines in fact have this problem, with the CBX1000 perhaps the most extreme example, and the similarly engineered first-generation DOHC fours (750, 900, 1000 and 1100) following close behind. The valve seats in these heads classically present shifted out of plane. In the CBX head the seats exhibit varying levels of shifting out of square with the head beginning at the outer combustion chambers and getting worse in the middle two, and with the exhaust seats being twice as bad as the intakes. When doing a valve job on these heads, a valve seat machining system that, like most, loads on the guide, will not accurately cut these seats but only perpetuate their defect. The cutter simply deflects in the guide until it conforms to the bad seat, resulting in a newly-machined but still tilted seat, and thus not the best work, and not the best seal. Without tools such as the Serdi and its copies the Hunger, Mira, Manek and FM that through head loading and live pilot completely eliminate the influence on the machining operation that both the clearance in the guide and a tilted seat will have, problem heads can be extremely difficult to do good valve seat work on when you have the inferior tools.

Obviously, there are many ways to do a valve job, with methods ranging from archaic 1940s tech to state of the art computerization, and much in-between (and I own several of them). But as I show here, some are definitely better than others. See a fairly thorough comparison of these various types of valve seat machining tools in the form of an illustrated article on my website. It is useful to have the information to make good choices. However, I regret that this won’t be very helpful in choosing a machine shop to do the work for you as any shop will ensure you that they know what to do, when in fact they may be working to very old theories and using tradtional but not very accurate machining methods. But forewarned is forearmed I believe and if nothing else you now have a better appreciation of the complexities of proper cylinder head servicing. More...

Compression Chronicles part 5