Valve guides

[cmfalco]

At the risk of going somewhat off topic, while they are called "3-point" gauges, it's not irrelevant to emphasize that their probes have finite lengths, not points. Also, this type of gauge has variations in design that can have important consequences. Both of the gauges in the photograph would be fine for an open cylinder, like that of a, um, cylinder on an engine, but only the one on the right would be able to measure the diameter of the flat-bottom pocket being machined in a head for a new valve seat insert.

[cmfalco]

Returning to the topic of this thread, clearance is determined by the difference in diameter of two cylinders, while the distance a valve stem moves from side-to-side in a valve guide depends on the tilt of one cylinder inside another. To what extent do these give the same results?

The height above the valve guide of the gauge I used to make the measurement in my first post in this thread was approximately 1/16".

The gap on the following diagram is equivalent to a clearance of 0.010" with a ⅜" valve stem, the length of the longer of the (green) valve guides is 1.6" (vs. 1.7"–1.8" for real ones), and the height of the arrows above the guides is approximately 1/16", so the following is a reasonably-accurate scale diagram of a heavily worn guide.

For a long-enough guide, and a small-enough clearance, the diagram shows the answer to the question in the first paragraph is that the difference would be negligibly small, because the tilt would be negligible. However, in the case of the finite length of an actual guide, but with a clearance 5–10× larger than it should be, the next diagram shows there is a difference between the d(b) measured by rocking the valve stem, and d(a) measured with appropriate instruments, and that this difference would be greater still if the valve guide were shorter (d(c)).

Readers who have been paying close attention will note that the diagram shows that d(b) > d(a), whereas the measurements I reported in my earlier post showed the opposite. However, those same readers will have noted that the differences were only a few ten-thousandths of an inch, and accurate measurements at sub-0.001" are quite difficult. In any case, the scaled difference from the drawing between d(b) and d(a) for an actual clearance of 0.0020" would be just ~0.0003".

It is relatively easy to rigidly mount an indicator to a head to measure the side-to-side movement of a valve stem immediately above the top of the guide. As the diagrams in this post show, for the dimensions and clearances encountered on an engine, such a measurement will be close enough to the actual clearance that, for most purposes, it can be used to determine whether or not a guide needs to be replaced.