Hex Bolt M20×2.5 (real thread)

An M20 bolt with a genuine helical ISO thread, not a decorative one: 2.5mm coarse pitch per ISO 261, cut to the ISO 68-1 basic profile — 18.376mm pitch diameter, 17.294mm minor, 0.313mm crest flat (P/8), 60° flanks. 30mm across the flats, 12.5mm head, 40mm shank. It runs on the matching M20 nut below. The fit is where this departs from the standard, deliberately and by a lot: ISO 965's 6g/6H tolerances are a few hundredths of a millimetre, which is a machining tolerance and useless here — printed at 6g/6H a bolt and nut fuse into one lump. The form is ISO 68-1; the fit is not ISO 965. The bolt's whole external profile is offset 0.2mm off nominal, giving 0.2mm of clearance on each flank and each crest — 0.4mm on the major diameter. Print it head-down, standing. The thread's lower flank is about 70° off vertical, which sounds unprintable and is not: it advances only dz/tan30° ≈ 0.35mm of radius per 0.2mm layer, so each layer of thread lands almost entirely on the one below. The undersides come out rougher than the tops. Lie the same bolt on its side and there is a genuine unsupported spiral under every thread, and that really does fail. A printed bolt is for a jig or a fixture where a metal one is overkill, not for anything carrying a real preload. PETG.

from £6.26example, in PLA