Guide To Four Stroke Motorcycle Engines, Part 3
The Cylinder Head
The three most popular four stroke motorcycle engine cylinder head configurations in use today are the pushrod overhead valve, the single overhead cam, and the double overhead cam.
Pushrod Overhead Valve
Activated by pushrods coming up from a case-mounted camshaft, the rocker arms of this type of cylinder head act upon the valves, opening them at the proper time.
Single Overhead Cam
The camshaft in this type of head mounts above the valves so that pushrods are eliminated. Extra drive mechanisms (chains, sprockets, and idlers) are required, but are justified since the performance of overhead cam engines often surpasses that of the pushrod overhead-valve engine. The rocker arms simply ride with their follower end on the cam, and the adjuster end activates the valve.
Double Overhead Cam
Dual Overhead Cam motorcycle engines were once rare because of the extra expense and weight with little gain in performance capability over the single overhead-cam design. There still is precious little real reason for a DOHC motor other than it sells better as it sounds more sophisticated than an SOHC to the uneducated consumer.
The cylinder head contains two camshafts, one for exhaust valves and one for intakes. The valves are adjusted by little spacers placed under the caps which ride over the valve tips. Thicker spacers reduce clearance, while thinner spacers increase valve clearance.
The passages or holes through which the intake and exhaust gases flow are called ports. Most engines have one port per valve all the way from each carburetor to each intake valve, and one exhaust port per exhaust valve all the way to the exhaust pipe. Ports that branch into two ports are known as siamesed ports.
A valve is a very important part of the cylinder head because it is the door that opens and closes at exactly the proper time to draw in the fresh fuel mixture in or to expel the burnt exhaust. Though a one-piece item, a valve has several sections with different names.
Since most heads are made of somewhat soft aluminum, a hardened steel valve seat insert is necessary to absorb the pounding shock of the valve face as it seals the cylinder. These seat inserts are installed with an extremely tight press so they don't fall out during operation. If badly pitted or worn, the seat insert is a replaceable item that requires special tools and training to repair or replace.
The soft aluminum of the cylinder head also requires that a better metal be used to guide the valve. The bore through which the valve stem operates must stay tight for good oil control and accurate valve seating. For these reasons the valve guide, made of an iron or bronze pipe, is inserted into a larger hole with a tight press fit.
The valve is lifted from its seat by the positive mechanical force of the cam applied through mechanical linkage to the tip of the valve. The valve spring is the only force that returns and holds the valve in its seat. Most valve springs are the coil-spring type that are retained between the valve retainer and a valve-spring seat around the top of the valve guide.