[rudidelange]

The stem heightt is measured between the tip of the stem and the surface of the head - my 83 model is specced to between 13.65 to 14.5 (I hope I remember correctly), the installed spring heightt is measured between the seat of the spring and the retainer on top of the valve spring (the space that the spring occupies). If a seat is machined too much then the stem heightt will increase. The installed spring heightt will also increase but can be corrected by shims underneath the seat of the valve.
The installed spring height is different for the inlet and exhaust (earlier models) but was later corrected for later models were they are the same. Porsche machined the head where the shims and seats are seated.

I found a good explanation how to measure the installed spring heightt in Rennlist, somewhere in 2002.

The questions that I still have is:
How important is the valve stem heightt and the installed spring heightt?

I guess that an incorrect installed valve spring heightt will affect the "pressure of the spring" and a wrong valve stem heightt will affect the proper functioning of the lifters.

If a seat has been machine too much and the valve stem heightt is more than the 14.5 mm, can a person get the heightt correct by removing some metal from the tip of the valve stem?

I hope this helps
Let me know if you learn more