Hello, I am an instructor at a motorcycle technical school. Some of my fellow instructors and I had a debate. I told some folks that valve tappet clearance gets larger as an engine heats up, while others thought that it would get tighter. We checked a screw and locknut motor, and the valve clearance was larger on the hot engine (.002” cold .004” hot). We then did a shim under bucket motocross engine and it too was slightly larger when hot. Does this match what you have seen? What is the engineering purpose for valve tappet clearance? Thank you. Terry J


The one thing we never want to happen is for one or more valves to stand slightly open when they should be closed. If this were to be so, the engine might lack compression sufficient for starting, and when operating, the leakage of hot gas past the valve(s) would quickly heat them to the point of damage.

Therefore there must be a clearance at all times when the valves are closed to be sure this can’t happen.

The heads of most motorcycle engines are made of aluminum today—a material which expands with temperature three times as much as does iron or ordinary steel. Austenitic stainless steel, from which exhaust valves are often made, expands about 85% as much as do aluminum alloys.

So here’s the problem. As the engine starts and begins warm-up, its exhaust valve temperature rises much faster than does the head temperature, so exhaust valve expansion at first reduces the valve clearance. As warm-up progresses, the head takes longer to reach operating temperature. The aluminum expands, tending to lift the camshaft away from the valve a bit—this increases clearance.
This is a bit more complicated in air-cooled engines, whose operating temperature is not constant, but rises or falls with air temperature.

The cold clearance is therefore chosen to allow the valve to close completely at all engine operating conditions.

In some cases, exhaust valves are friction-welded together from two different materials—a heat-resisting austenitic steel head and a more friction-tolerant conventional steel stem—which expands less than does the stainless head. In this case the valve will expand less than an all-stainless valve as the engine warms.

If the cylinder head is iron, as it was on many Triumphs of the late 1940s and ‘50s, it expands less than aluminum, so it would increase valve clearance less as it warms to operating temperature.

Everything changes again with a pushrod-and-rocker valve train. Are the pushrods high-expansion aluminum? Or are they steel, expanding only 1/3 as much?
Valve clearance is chosen such that at no time will any valve be held open when it should be shut (let’s call this negative clearance). This clearance is determined experimentally.

To keep tappet noise to a minimum, either self-clearancing hydraulic tappets may be used (as on H-D Big Twins) or a clearance ramp is made a part of the cam profile. The clearance ramp is there to take up the valve clearance gradually, rather than just starting rapid valve lift immediately.