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Cylinder Head – Description
The engine is fitted with an aluminium alloy cylinder head, which carries the camshafts,
valves and spark plugs. The cylinder head is cast as a single entity and various
components are permanently added after machining.
The camshafts run directly in the cylinder head without separate bearings. Valve
clearances are adjusted by changing variable thickness shims which sit between the valve
tappet bucket and the valves.
The camshafts are driven by a silent-type chain. The chain is guided by two blades and is
tensioned by a hydraulic tensioner.
The hydraulic tensioner is fed oil via a gallery in the cylinder head. The combination of oil
pressure and spring pressure pushes the plunger against the tensioner blade which
tensions the camshaft drive chain. The hydraulic tensioner has an oil pressure relief valve
located in the plunger. It is set to open between 19 and 23 bar. When open it sprays oil
through a drilling in the tensioner blade onto the camshaft drive chain.
Oil is supplied to the cylinder head by an internal gallery. Once supplied to the cylinder
head, the oil is distributed along internal drilling's within the head casting and camshaft.
Single valve springs are used to close both the inlet and exhaust valves. These valve
springs have close wound coils at one end to assist in the prevention of valve bounce at
high engine speed and to give a smooth valve actuation. When assembling the cylinder
head it is important that the close wound, colour coded ends of the springs are fitted
downwards (towards the piston). The tip of the inlet and exhaust valves are hardened to
give a long service life.
Due to the methods used to assemble the valve seat and valve guides to the head, these
parts cannot be replaced.
In any of the following operations which necessitate the removal or disconnection of
the camshaft drive chain, NEVER turn the engine without the camshaft drive chain and
tensioner correctly fitted and adjusted. In the disassembled condition, the pistons will
contact the valves if the crankshaft is turned, causing severe engine damage.
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