Popular wisdom has it that in every four-stroke powersports engine, valve clearance compensates for heat expansion. This is one of the most tenacious myths in the motorcycle industry, one that is taught by the most prestigous vocational schools, and is even given nodding assent by many of our industry's authorities. It is too bad that it just isn't so. To understand why requires a brief journey into camshaft technology.
The All-Important Ramp
The camshaft may seem like just a stick with bumps on it, but it is actually fairly complex. Each of its seven parts has a specific job to do, illustration number 2. We want to focus on the areas labeled "ramps." Sometimes called clearance ramps, the ramps are at once the least understood yet possibly the most important areas of the camshaft. They're also, paradoxically, almost accidental, being more byproduct than intentional -- what is left over after the lobe's shape is built. Yet the clearance ramp is vital to valve train health. As the vintage Honda illustration (number 3) communicates, the ramp absorbs or softens the shock of the valve train's action. The valve and valve train (rockers, etc.) are at rest during much of the camshaft's rotation. When the cam opens the valve, the valve and its parts seem to be suddenly catapulted into movement. In reality, the ramp first gradually applies tension to the valve train so that all its gaps are taken up, before the valve is shoved open, illustration number 4. The result is longer valve train life.
Consider a mild and a performance camshaft. The two cams have very different flanks, that part of the cam lobe that actually opens the valve. The shallow flank of a mild cam opens the valve gently. The steep flank of a performance cam opens the valve more abruptly. Whichever kind of cam you have, there is only so much territory in the lobe with which the designer can create the wanted shape. The mild cam has a lot of ramp because less flank is there taking up room. The performance cam on the other hand has less ramp because more of the cam's lobe is taken up in making a steep flank, so there is less left over for the ramp. Mild cams have lots of ramp, performance cams have less ramp.
Ramp and Clearance Related
And here's the trick -- camshaft ramp and valve clearance are inversely related. Mild cams such as those on early Hondas have very small amounts (5~6mm) of lift, resulting in very gradual flanks, and their valves need only tiny clearances (0.002"~0.003"), illustration number 5. However, the performance cam, with more of its shape given over to valve movement, has a steeper flank. The added flank cuts into the clearance ramp, so the peformance cam has less ramp, and thus needs more valve clearance to compensate. Make sense? Valve clearance is just another form of cushion. Steep flank = less ramp = more clearance. On the other hand, shallow flank = more ramp = less clearance.
The common but false wisdom that valve clearance is connected to heat expansion comes from the days of pushrod engines. As the valve heats up, it expands more than the castings around it, resulting in the head of the valve increasing in size and the valve being pulled into the combustion chamber. In the OHC (overhead cam) designs which are the vast majority, the rest of the valve train expands less than the valve, thus the valve moves away from the these parts, increasing valve clearance. However, in pushrod designs, the considerable mass and length of the pushrods and associated parts expand more than the valve, and the valve train tightens up -- clearance diminishes. Thus the valve clearances in pushrod engines do get tighter with heat, whereas the valve clearances in OHC engines do not. But, whether pushrod engines whose valve trains tighten with heat, or OHC engines whose valve trains loosen, the function of valve clearance in each engine is the same -- to work with the cam's clearance ramp to "babysit" the valve train. There is only a coincidental relationship to heat expansion, and that only in regard to the pushrod engine. And now you know. Hold the pickles...