I have some R34 GTT caliper bolts here, brand new. Nissan part number 41005-AA000.
Hexagonal head, M14, 1.5 thread pitch.
Total length measures 49mm. Head is 9mm, then 12mm unthreaded, then 28mm threaded.
The last 2mm of the unthreaded section tapers down to the threads so maybe that should be counted as part of the threaded section? I'm no expert on fasteners. 10mm and 30mm are nice round numbers though.