OK, someone called SAAMI, and SAAMI said?
http://www.saami.org/glossary/display.cfm?letter=H
I did not call SAAMI, had I called SAAMI I would have upset them and I would have ask to talk to someone that knew what they were talking about.
Search SAAMI for head clearance, head space, while you are close like up head space gage.
Clark said something about "What was that sound, sounded like a crack in my skull, something like an awaking, or dawn, it just dawned on me, case head clearance and head space"
Clark, forgive, I took a few liberties.
F. Guffey