In an ideal world, the combustion chamber in an engine would be completely airtight during the compression stroke. However, due to basic mechanical inefficiencies, there will always be blow-by somewhere. But by keeping this to a minimum, your engine is able to produce good power.
Possible ways for the air/fuel mix to escape could be via the piston and rings, through a failed head gasket or the valves in the cylinder head. Because we're dealing with valve clearances today, we'll stick with those.
In modern engines, the poppet valves in a four-stroke engine are either machined perfectly to form a seal against the 'seats' in the head, or lapped in with grinding paste to match them together (older engines). Springs then hold them tight against the seat and, voila, no air leaks.
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| Suzuki GS550E. In this pic you can see the two camshaft lobes above their respective bucket and shim. The cam cap is removed because some clown (previous owner) stripped a thread — in fact they mullered a few of them and decided to replace them with some coarse-threaded bolts! Marvellous. |
