Why is it that grinding down valve stem is sometimes needed?

[ttimtucker]

Generic question for the gurus on this forum: I know there are some mowers where the only adjustment related to valve lash is to grind the stem to shorten the valve. My question is this: what is happening in the engine that would make the valve (or something else) "too long", such that shortening it is appropriate? My expectation is that engine operation will slowly wear things down and increase clearance between cams and push rods and valve stems. What mechanism is causing the clearance to decrease?

 

[ILENGINE]

The engines where you have to grind the valve stems are the L or flat heads which the main issue I have seen over the years with those is the valve face wears into the seat. Sometimes to the effect of .010-.015 which shows as a groove in the valve face. And with fewer moving parts less room for wear changes in the Camshaft lobe, to tappet to valve stem, where with an OHV you have the camshaft lobe, to tappet, to lower end of the push rod, to upper end of push rod to rocker arm, the rocker arm pivot, and the rocker arm to valve stem.

So the effect is on L head or flat head the clearance closes to the point of floating the valves, but with OHV the clearances tend to increase.

 

[ttimtucker]

Thanks for the clear explanation. Makes perfect sense.