In modern engines, the pushrod system is being replaced by the simpler overhead camshaft arrangement.
The overhead camshaft is located in the cylinder head. There can be 1 or 2 camshafts. Let’s look at a single overhead camshaft arrangement.
Single overhead camshafts can use rocker arms. The cam can lift one end of the rocker arm, or it can press down on the rocker arm.
On double overhead camshaft systems, the most common arrangement is to use a bucket tappet or lifter. It operates in a guide that protects the valve against side thrusts which it would receive if the cam operated directly against the valve.
The adjustment of valve clearance is usually done by changing accurately machined spacers. Spacers are available in a range of thicknesses, and they’re exchanged to obtain the correct clearance.
Some overhead cam engines use a hydraulic lash adjuster to reduce lash in the valve train. They have zero clearance at the valve stem so there’s no need for tappet adjustment.
It can be put in the valve end of the rocker arm. Like the hydraulic valve lifter, it has a body with plunger held against the valve stem by a spring.
Oil supplied to the adjuster keeps the plunger in contact with the valve and eliminates lash.
Lash adjusters can be put in the cylinder head at the end of the rocker arm. The lash adjusters are stationary and have a pivot for the end of the rocker arm. The plunger in the adjuster holds the rocker up against the cam.
In the lash adjuster inside the bucket tappet, the plunger’s hydraulic action holds the bucket body against the cam on the camshaft and also against the tip of the valve stem so that there is zero clearance.
