Discussing valve train geometry

Question:

In a previous issue a question was asked about how to remove a broken tap. Rutland Tool and Supply in Whittier, California shoprutlandtool.com), sells a system for drilling out broken taps.

Your article on hydraulic valve adjustment in the May 2008 issue didn’t say anything about valve train geometry. On a Ford the lifter is centered when the plunger is .060-inches below the retaining ring. If the plunger is too high the rocker arm will be too high at the pushrod end and will not produce the correct valve lift. It will have less lift. A plunger set too low will also produce less lift because the pushrod end of the rocker arm is too low and rocker arm geometry will not be correct.

Answer:

Ideally, the valve train geometry should be such that the contact point of the rocker arm tip should contact the exact center of the valve stem end at the exact point in time that the valve is exactly at the midpoint of its total lift. This minimizes side-loading of the valve, which in turn minimizes friction as well as valve stem and valve guide wear. It also discourages distorting the valve stem, something that can occur if geometry is way off and the rocker is pushing the valve sideways as it pushes it down.

A lot of factors will influence valve train geometry, including deck height of the block, height of the cylinder heads, position of the valve seats, valve stem length, pushrod length, rocker arm dimensions, camshaft lobe dimensions, and thickness of the cylinder head gaskets. As a result, it is usually pointless to say that the lifter plunger must be at a certain height in order for the valve train geometry to be correct. If every variable that influences the geometry is exactly at its optimum dimension or position then it’s entirely logical to say that the lifter should also be at its optimum height. But given the reality that the variables will vary, a much more sensible approach is to adjust those things that are adjustable until your measurements and observations confirm that the rocker arm tip is contacting the center of the valve stem end at the point in time that the valve is at the midpoint of its total lift.