Valve Stem Geomtery Calculator

[Admiral Akbar]

Here is a simple calculator that calculates what happens to the valve lift and how far the rocker tip slides across the end of the stem both for an ideal valve train geometry setup and one that has valve stem protrusion error.

It starts with the basic assumption that the valve train was setup properly to start.. It totally ignores what the pushrod side it doing. It simply assumes that a correctly setup rocker / valve end will produce a specific rocker rotation.  That same rotation will be available for an valve stem with different protrusion.. This is not necessarily true as of the angle between the pushrod and other side of the rocker changes also if the same rocker arm is used..  This will effect then amount rocker rotation (and lift)  An angle that is different from 90 degrees mid lift on the pushrod side will increase rocker rotation. The calculator expects that the rocker geometry on the pushrod side has been corrected and only looks at the valve stem / rocker side..


For reference here is a picture Ohio used in another discussion about stem protrusion.

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Here is an example of what happens when the stem protrusion is off on a 0.6 inch lift cam.. The calculations assume that the valve is centered on the current lift then examines what happens when protrusion changes including where the tip travels on the valve.  The inputs are pretty self explanatory. The first is nothing more than the length between the center of the rocker shaft to the center radius on the rocker pad / roller.  Lift is total lift.. Stem Protrusion error is the difference between the ideal mid-lift rocker geometry and the setup seated position.  Valve stem diameter at chamfer is the diameter of the valve stem face that is flat for the rocker to rub against.  Center offset allows for calculations where the mid travel (swipe) is not centered.



In this example a perfectly mid lift centered rocker will move about from 0.015" on the rocker side to  0.015 other side of valve center at mid lift then 0.015 back to rocker side at max lift.

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With a 0.1 stem protrusion the total valve lift only decreases  0.002" from 0.600 lift but the swipe the rocker takes increases 77%.. The last table shows how tip movement has changed and the how much the resulting contact line shrinks based on stem being round.

I've run a few test cases and looked at the results to manual calculations and the numbers looks good so the calculations are valid.. If anyone can find an issue let me know..

[HD/Wrench]

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