Push Block
Roller
Cam
Fig. 12. Poppet Valve
[Fig. 12] shows a poppet valve. This type of valve has only one adjustment, called the tappet. The adjustment is made by turning the cap-screw out of the push block until the head comes into contact with the valve stem. The lock nut on the cap screw is then turned down tightly to the push block to hold the adjustment. A strong spring is placed on the valve stem which causes it to close quickly and remain closed until it comes into contact with the cam.
Valves are set and operate in three different positions as shown in [Fig. 13]. The exhaust valve in this case seats on the floor of the combustion chamber and is operated by the stem which extends through the casting to the tappet, while the intake valve seats on the upper wall of the combustion chamber and is operated from over head by a push-rod extending from the tappet to a rocker-arm. When both valves are operated from above and seat on the upper wall of the combustion chamber the motor is referred to as the overhead valve type of motor. In the majority of motors both valves seat on the floor of the valve chamber.
| Rocker Arm | |
| Valve Stem | |
| Valve Open | Valve Seat |
| Combustion Chamber | Tappets |
| Cam | |
| Cam Shaft | |
| Overhead Type Valve | Push Rod |
| Poppet-Type Valve |
Fig. 13. Valve Types, Location and Operation
Valve Timing.—Valve timing is usually accomplished by setting the first, or exhaust valve cam, to correspond with a mark on the flywheel and cylinder (shown in [Fig. 14]).