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Your Exhaust Valve worn out? » Read more about common malfunctions • Repair • Replacement manual
Releasing the exhaust: the exhaust valve
In four-stroke engines, the exhaust valve (exhaust valve) is responsible for the exit of burnt exhaust gases. The valve is located under the camshaft behind the rocker levers. Its replacement being a complicated repair creates an occasion for additional maintenance. The result of a successful repair is a total engine overhaul.
Exhaust valve: function
Three out of four engine strokes the exhaust valve remains closed. Especially at the combustion stroke, i.e. the moment of fuel explosion the valve must keep the combustion chamber tightly shut. Only when the burnt exhaust must be released, it opens. The camshaft controls the exhaust valve and the valve springs. The tappet of the camshaft presses the exhaust valve into the combustion chamber, allowing the exhaust to escape. The raising piston pushes the exhaust gas outwards. When the tappet has passed the valve, the valve spring presses the valve back into its original position, closing the combustion chamber.
Exhaust valve defects
A valve can get damaged in four ways:
- defective valve control
- annealing as a result of engine overheating
- valve disc fracture
- wear of the valve seat or valve stem.
The most common engine valve defect occurs as a result of synchronisation failure between camshaft and crankshaft. The cause is a skipping or ruptured timing chain or belt. The valve still hanging in the combustion chamber is hit by the rising piston, causing the valve stem to bend; the valve can no longer close. An engine running too lean can overheat. The excessive heat developing in the combustion chamber affects the valves. An overheated valve anneals and loses its shape. Annealing, mechanical stress or an alien body can damage the valve disc. When a piece breaks off, the valve no longer fully seals. As a result of a leaking valve the explosion in the combustion chamber reaches the cylinder head. Compression no longer develops and engine damage is imminent. Inadequate lubrication can ultimately damage the valve stem and the camshaft.
Exhaust valve replacement
Replacement of the exhaust valve requires removal of the cylinder head. The valve can only be pulled out downwards which makes this extensive disassembly necessary. On this occasion the camshaft must be checked. Often a complete overhaul of the cylinder head is unavoidable. When removing the cylinder head from the engine, the gasket and the cylinder head bolts must be replaced. Prior to reinstalling the cylinder head, it must be measured and possibly resurfaced. Once the cylinder head is removed, the valve control should be renewed as well. This applies to timing chain as well as the belt. These are quite expensive repairs. Therefore it makes sense to do it "right" in one go. Instead of only replacing a defective valve, this is an excellent occasion for replacing all valves. This makes the repair only slightly more expensive and provides a totally overhauled engine. The same applies to the camshaft. When still undamaged, at least its bearing cups and the hydraulic tappets should be replaced. The additional costs are minimal; meanwhile this contributes considerably to extended engine performance. Replacement of the valves always implies the replacement of valve stem gaskets.
Purchase of exhaust valves
The exhaust valve is a very cheap spare part. Branded quality parts cost ca. 15 EUR (± £13). These costs are out of proportion compared to the complexity of their replacement. Therefore, total costs for a correct repair could amount to 300 – 400 EUR (±£260 - £340). This implies a total renewal of all components in connection to the exhaust valve. In addition to the spare parts comes investment in several special tools. These are often required, and in all cases make the work considerably easier. These tools are:
- torque spanner
- Valve spring compressor
- precision straight-edge
For this tools, another 200 EUR (± £180) should be added. Resurfacing the cylinder head is available in professional garages, starting at ca 100 EUR (± £90).