Guide To Four Stroke Motorcycle Engines, Part 4
The camshaft is geared to rotate at only one-half of engine speed since the valve needs to be opened only every other revolution. The profile or "lobe" on the cam determines how high and how long the valve stays open. Mild, reliable street engines need moderate lift and duration, while racing jobs need higher lifts and longer durations. These sacrifice reliability for performance. Like the valve, the cam is a one-piece unit with several sections that have different names.
As previously stated, cams may be mounted in the engine cases or in the cylinder head. There must also be some provision to drive the cam. In overhead cam engines, a chain-sprocket system is generally used to drive the cam, while pushrod overhead engines use a gear drive for the cam. Overhead cam design incorporates a spring-loaded tensioning device also, which keeps the cam chain operating properly.
A cam may be mounted in several places, some requiring more linkage to the valve than others. The pushrod type overhead valve engines requires the most cam-valve linkage, which includes a tappet or lifter, pushrod, rocker arm, and adjuster.
In order to allow for expansion of these metal parts, a certain amount of clearance is required between the parts in this system. Clearance is obtained by adjusting a nut and stud assembly at the end of the rocker arm or by adjustable length tappets as in traditional Harley v-twins.
Overhead cam engines require much less clearance than overhead valve engines, partially because there are fewer parts to expand in the system.
Spark Plug Hole
A threaded hole must be provided in the cylinder head so a spark plug can be installed. Spark plug holes may be 10, 12, or 14 mm in diameter and vary in depth according to engine design. Threaded into aluminum, these fragile holes are often abused, resulting in damaged threads. Fortunately, this damage can usually be repaired with an insert.
Usually there are internal passages for the delivery and return of oil to the moving parts of the cylinder head. These passages must be sealed properly or oil leakage will result.
You should be able to isolate top-end problems in a four stroke either to the head and valve area or to the piston and cylinder area by noise isolation and a compression test. Let's consider some problems that occur in the piston and cylinder area first. Note that these problems are identical to those that occur in two-stroke upper ends.
Even the best air filters available for motorcycles cannot filter out all the dirt from the incoming air. Eventually the abrasive action of dirt and grit will wear away metal from piston rings and cylinder walls. With meticulous attention to air filter condition, engine lubrication, and ignition and valve adjustment, a carefully ridden street machine can get 50,000 miles or more from its original upper end.
See those velocity stacks on your buddy's retro cafe racer? They sure do funnel in plenty of air, don't they? Well, they're sucking up plenty of dirt and grit to grind away at the innards of his engine. He can expect rapid wear to pistons, rings, and the cylinder wall. Most likely his bike will lose power and have excess "blow by" past the rings in just a few thousand miles of riding. One trip down a dusty road with no air cleaner can spell disaster for a formerly healthy engine. Use air cleaners and maintain them.