A reciprocating engine has intake and exhaust valves for intake of air-fuel mixture and exhaust. At the time of intake and exhaust, the intake and exhaust valves are opened and closed. A cam shaft controls the opening and closing of the intake and exhaust valves.
Types of operation of opening the intake and exhaust valves by cams of the cam shaft are classified into a direct acting type in which cams directly press valve lifters for opening and closing valves, and a rocker arm type in which cams press valves via arms called rocker arms for opening and closing valves.
The intake and exhaust valves thermally expand because of heat from combustion chambers. When the valves thermally expand, intake and exhaust timings change accordingly. Thus, in order to absorb such thermal expansion, in a condition in which a cam does not press a valve lifter or a roller of a rocker arm, a clearance (valve clearance) is provided for adjustment between the cam and the valve lifter (or the roller of the rocker arm).
In assembly of an engine, usually, after assembly of the valves to a cylinder head, corresponding valve clearances are automatically adjusted by an adjusting apparatus. Subsequent to the adjustment, valve clearances are actually measured for confirmation.
Conventionally, a worker manually measures and confirms the valve clearance; specifically, he/she inserts a clearance gauge called a shim between the cam and the valve lifter (or the roller of the rocker arm). Such manual work is employed for the following reason. Since cam shafts, valve lifters, rocker arms, etc. are forgings, design accuracy on the order of μm is difficult to attain, and assembling accuracy differs among engines. Therefore, basically, automation is difficult.
According to a known valve clearance measuring apparatus for automatically measuring a valve clearance between a cam and a cam contact member of a valve, an attachment is brought into contact with a surface of the cam contact member, with which the cam of a cam shaft comes into contact, so as to use the surface as a reference plane; and, in a condition in which valve clearance VC is present between the reference plane and the lower end surface of the cam shaft, a valve clearance is measured from the difference between a cam shaft height D1 from the reference surface and a height D3 of the cam shaft itself (refer to Patent Document 1).    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2005-54728