I vaguely remember reading that some company, back before WWII, was slightly reducing the diameter of the intake valve stem so the tar couldn’t get into the small clearance between guide and stem as easily.
Imagine that the valve is all the way open. Make a mark on the stem just above the guide. Remove the valve and chuck it in a lathe. Turn down the area between the mark and the valve’s “head”.
I’m pretty sure it’s in here:
but I don’t remember where.
And yes, I have referenced this report in the past.
I just looked at several pages of that report and didn’t find it, so don’t beat my up if you don’t find it either.