Valve

Each cylinder has at least one intake and one exhaust valve; the intake valve is usually larger. The valves are opened by the camshaft and closed by springs. On SOHC engines rocker arms sit between them.

• A worn valve stem seal lets oil run down into the combustion chamber; you see blue smoke on start-up.
• Incorrect timing or over-revving can make a valve hit the piston and bend.
• A gas-tight seal between valve and seat is achieved by lapping.

Check the valve clearance regularly so the valves close fully.