What causes changes in Valve Clearances?
• Typically clearances decrease due to valve seat/face recession. this is erosion cause by high head, metal transfer, etc. once a valve clearance tightens up to zero, it stops transferring heat across this boundary (cause the valve never really touches the head/seat when “closed”) and erodes much faster. Clearances increase due to a number of issues, one obvious one is if you bend a valve, and it can no longer fully close. Intakes don’t usually bend, it’s the exhausts. This is because intake valves open as the piston drops, the exhausts are open as the piston rises. Another reason a valve will loosen up is if you switch a motor running and built for unleaded to a leaded race fuel. the build up of lead on the seat prevents full closing (but not enough to prevent heat transfer) and increases clearance. I suppose if a valve became carboned up enough, it could also exhibit this behavior. mtiberio (cugino pegaso). • Other causes are: Valve train wear and slop, tappet/adjuster movement (not
