The Datum

The issue Ask what diameter an internal spline is and you've asked three questions. Major, minor and pitch circle diameter each govern a different aspect of fit — and the PCD, the one that controls load sharing and backlash, is a geometric construction no surface of the part physically

The issue An O-ring is only as good as the gland it sits in, and every sealing-failure investigation eventually arrives at the same unglamorous place: the groove. Its diameter — always larger than the bore that hides it — carries some of the tightest tolerances on the drawing, routinely ±0.

The issue An internal thread has three diameters, and the one that governs fit — the pitch diameter — is the one no ordinary instrument touches. It lives between the crests, on the flanks that actually locate the mating part, routinely toleranced tighter than either diameter you can see. When a thread