Valve clearance: origin
The purpose of valve clearance is to compensate for heat expansion, right? Wrong. This hoary, tenacious assumption may be intuitive, but it is very wrongly so. Actually, as the valve in an overhead cam engine heats up, it expands more than the castings around it, resulting in the head of the valve increasing in size and the valve consequently being pulled into the combustion chamber, increasing valve clearance. 1
Why then, clearance? Valve clearance is wholly a product of cam lobe design (shape). It's tied to the specifics of a cam's intricate geometric design. A cam lobe is not just an ellipse machined onto a shaft. It's a complex shape with several distinct radii, each with its own job to do, and one of those dimensions is where valve clearance comes from, as we'll see. Let's start with the "base circle", also called the heel. The base circle or heel is that part of the cam lobe that looks mostly perfectly round, a constant radius. It's the datum line, home base, the point of departure from which everything else on the cam is measured. It is also the valve's fully-closed point, giving the valve its cooling time. Opposite the cam's base circle is the "nose", the point at which the valve is fully open. The nose affects valve open spring pressure, an important consideration in engine tuning. Either side of the nose are relatively flat areas called "flanks". They open and close the valve. On some cams the opening and closing flanks are symmetrical. But not on all of them. In addition, shim type engines, rocker arm engines, and pushrod engines each have their own distinct flank shapes, most flattish but a few more rounded.
But the most interesting of the cam lobe's parts and one with many secrets is the "ramp". Detectable only with a dial indicator, the ramp is the tiny transition zone at which the radius of the base circle becomes the broad, almost flat surface of the flank. Though complex in design, the ramp's job is simple: to cushion the valve. For a handful of crankshaft degrees, the ramp eases the valve into its sudden climb up the lobe's opening flank. Then after the valve has opened and is skiing down the closing flank, it comes to an identical ramp on the closing side of the lobe that gently decelerates the valve so it doesn't crash onto its seat.
Paradoxically, there isn't actually room enough on the cam for as much transition as the valve really needs. Here's where valve clearance comes in. Valve clearance supplements the cam's built-in cushion, augmenting the always-minimal ramp. Different motorcycles have different valve clearance specifications because they have different cams with different amounts of ramp. Modern engines run three to five times the valve clearance of engines of 30 years ago because their cam's larger lobes cut into the ramps, making them smaller. Less ramp means less opening and closing transition-- big cams need more valve clearance. Small ramps go with big valve clearance, large ramps with small clearance. Heat has nothing to do with it. 2
And neither does noise. Valve clearance has nothing to do with noise control. 3 Many seem to believe that should an engine develop a slight ticking noise this means the valves are out of adjustment. Actually, there is little direct connection between the two. I have known customers who wouldn't adjust their valve clearances until hearing this ticking, as if the whole reason for adjusting the valves is to ensure a quiet engine, and conversely, who believed no noise means all is well, no need to adjust anything. If the former is ignorant, the last is dangerous. It is not for nothing that factory valve clearance adjustment intervals are spaced just 3,000 miles apart on 70s and older Hondas. It's a very important part of maintenance, second only to ignition service. In another post we'll expand on the kind of wear that makes valve adjustment necessary.
Valve clearance: usefulness
I knew a mechanic who obsessed over getting valve clearances exact, to the point of using a dial indicator in place of a feeler gauge. This is silliness. Valve clearances are not akin to carburetor jetting or piston-to-cylinder clearance, a specification indelible, sacrosanct, unchangable without marked consequence. Have you ever noticed that, much like spark plug gap, many engines' clearance specifications are given as ranges? Valve clearance is all about valve cushioning, remember, ramps, and has nothing to do with heat or noise or demandingly exact valve timing. 4 Knowledgeable mechanics have learned to be relaxed about clearances, opting in most cases for slight looseness, and nearly every old timer has long since discovered that increased clearances helps the engine in numerous ways. It forestalls the valve's eventual burning due to insufficient clearance. It increases the valve's ability to shed carbon. It adds valve cooling time. It lengthens the engine's compression phase, boosting cylinder compression and by the same token shortens valve-open time for increased intake air velocity that improves low rpm performance. These are real world benefits long understood and valued by career techs. Don't minimize them.
Cam measurement
The official manual describes a camshaft measurement routine that is often misunderstood. It has you measure the entire cam lobe, heel (base circle) to nose. While having a purpose, the actual outcome of this measurement is not to gauge the potential of your cam or compare it to another cam. You can't do those things by this measurement; it's too gross, it spans too many of the cam lobe's important regions all at once, areas of the cam that need to be separated to get meaningful specifications. The goal of the manual's measurement is maintenance, not performance. It's not intended to tell you anything about the cam's specifications. It is simply a crude and easy way to track cam wear, and you'll discover an astonishing rate of wear doing this. Japanese standard model motorcycle cams wear very quickly.
Some feel that measuring heel to nose and then measuring again ninety degrees to that across the heel, then subtracting, is a way to derive the cam's valve lift specification. As with many things, this sounds good but ignores reality. Three problems make this idea unworkable. First, the cam's heel or base circle is not necessarily continuous for 180 degrees, in fact in most cams seldom is. Remember the ramps? So you're not really measuring the base circle with your micrometer at ninety to the nose. In fact you're not measuring anything of any consequence. Second, when the cam is from a rocker arm equipped engine, the cam's movement profile does not match the valve's; the engine sees different movement due to the rocker arm's ratio. Therefore valve movement, both duration and lift, on rocker arm engines is most accurately measured at the valve itself. Engine builders actually do so on all engines, whether equipped with rocker arms or not. It's that critical. Third, even on a given model engine, the cam's base circle diameter changes slightly from year to year, making the heel to nose measurement of absolutely no use in comparing cams. You can tell almost nothing significant from measuring the size of a cam lobe. The manual's reason for doing this is to track cam wear and nothing more.
Cam profile
It's actually a bit more work to properly measure a cam. The most important, most useful, and most widely used cam detailing method is what is called "profiling" the cam. The cam is left in the engine, a degree wheel is affixed to the crankshaft, and a dial indicator is placed onto the valve. As the crankshaft is rotated, the indicator's readings are marked on a piece of graph paper every five degrees, and the result is a right-angle, X and Y, duration and lift depiction of both the intake and exhaust valve movement caused by that cam. This is a tremendously useful tool for getting a meaningful "snapshot" of a cam, and thus for comparing camshafts. Because it requires the same tools and setup, engine builders add yet another layer to this process that measures valve-to-piston clearance at various crankshaft positions, for a very complete picture of cam action. After reading this you might think cam profiling a very esoteric exercise. It's not. No one who properly modifies an engine neglects this.
Lobe center
Now let's combine ramps and measuring. We've learned we have acceleration ramps which gather up the clearances in the valve train before the valve is shot off its seat, and deceleration ramps which decompress those bits and cushion the shock of the valve's closing. And that they are what determine valve clearance. The problem is that these ramps accelerate the valve so slowly that interpreting exactly when it has opened or closed is difficult. Because of this, engine builders have developed a method of ignoring the ramps when profiling a cam. The ramp isn't there to move the valve anyway, its sole purpose is to graduallly take up clearance. Builders disregard the valve's movement until it has moved a certain amount, then start recording it on the graph paper, thus ensuring that the valve is well clear of the ramp. That delayed amount is called the "checking height." A checking height is merely a predetermined starting and stopping point in cam measurement.
This is great. It firms up the process of measuring the cam, makes it more accurate. But the problem is that ramps aren't all the same size. The ramps on a pushrod engine's cam for example are huge, because that engine's spindly valve train demands very gentle opening and closing curves. The ramps on overhead cam engines by contrast are much smaller, because there are fewer valve-related parts flailing about needing cushioning. Each engine design has its own size ramp and correspondingly its own recommended checking height; some 0.020", others 0.040", and still others 0.050". Comparison of these different cams is impossible, as two different checking heights give two very different measurements. What's needed is a system of measuring cam timing which is independent of the checking height used. Is there such a thing? Yes! That's the lobe center method. That is why it was developed. Though it still requires the use of a checking height, the lobe center approach indexes the cam based on an *average* of its opening and closing points -- the cam's "center" -- and not one of those points, then counts the number of crankshaft degrees from there to top dead center (TDC). This means different checking heights can be used on two different cams and this will have no effect on the timing numbers because no matter what the starting and stopping points are, the center of a thing is always the center. Make sense? Lobe center divorces checking height, once an insurmountable hurdle in ecumenticizing cams, from cam measurement, making legitimate cam comparisons possible.
You might argue that no one compares the cam from a Harley Big Twin with that from a Hayabusa and of course you're right. That isn't the kind of comparison we're talking about. Then why do we need the lobe center method to compare two cams for the Hayabusa? Their checking heights will be same, won't they? That's just it, they won't necessarily! Cam makers, each one having their own history and influences, continue to use different checking heights even on cams made for the exact same model motorcycle! Why then isn't the cam lobe's center measured directly? Because it isn't the lobe's *physical* center we're looking for, it's its *mathematical* center -- the average of its opening and closing points. In some cams, the physical and mathematical centers are the same, but not in all cams. The cam lobes in overhead cam rocker arm engines for example are asymmetrical -- their opening and closing flanks are shaped differently to compensate for the change in geometry the system undergoes as the rocker arm slides over the lobe.
So that's lobe center. However, an odd thing has happened. In recent years lobe center has been appropriated by many for use in timing cams to the engine instead of merely comparing cams. They propose that a particular lobe center works best on a particular engine design, i.e. larger lobe centers on Kawasakis and smaller ones on Hondas. An intriguing idea yet I'm not sold on it. I think it's pretty arbitrary. In my view it obfuscates the real goal of adjusting intake valve timing and assumes a kind of wavelength-of-kryptonite-struck-with-a-hammer forensic precision that is unrealistic and unwarranted. In any event, there is nothing wrong with using lobe center for timing, if you want to go to that trouble, other than it unnecessarily adds to and ignores the most important valve timing event, intake valve opening and/or closing.
Firing order
A four-stroke engine's cylinder firing order is largely determined by its camshaft. A couple of examples. Generations ago, Yamaha 650 twin racers used to run cams that were cut in half and welded back together to make the engine fire its cylinders in a different arrangement for improved traction, and presently owners of these vintage bikes in search of enhanced engine smoothness are doing something similar, that is, rearranging the cam's lobes. Also, the exotic six-cylinder Honda CBX1000 had an error in its earliest official manual that when followed made the left side cams 180 degrees out, resulting in such a dramatic alteration from the bike's 1-5-3-6-2-4 firing order that all of this engine's famous silkiness was eliminated and the cam chain failed within 25 miles.
Next post, Installing a high performance cam
1 Except in the pushrod engine. In this engine, the pushrod expands along with the valve, partially or completely eclipsing the valve's growth, resulting in either a constant clearance or less clearance.
2 So inextricably tied together are ramp and valve clearance that engineers often speak of the ramp portion of the cam lobe as the "clearance ramp".
3 Just as a carburetor's float level is not a product of how best to keep the carburetor from leaking.
4 Some on forums like to warn against clearance changes by claiming dramatic valve lift consequences. However, a graph showing the result of loosened valve clearance will illustrate that of the two things relative to valve operation that it changes, lift and duration, increased valve clearance changes duration many times more than it does lift.
