Most dimensional disputes on custom fasteners come from a drawing where the functional dimensions were not distinguished from the reference ones. The factory quotes to the drawing it received, not to the one in the buyer's head.
Functional vs reference dimensions
FUNCTIONAL dimensions determine whether the part assembles and carries load: thread diameter, pitch, class and length; the lengths that set the assembled geometry; seat angles and diameters; hole positions and diameters; and the concentricity of any thread relative to a seat.
REFERENCE dimensions describe the part but do not need tolerance control: overall profile radii, fillet sizes that do not carry load, and dimensions duplicated from other views.
The mistake is usually not a wrong number but a missing distinction: a ±0.5 mm general tolerance applied to a seat diameter that needs ±0.05 mm, or a functional length drawn without a tolerance at all.
The ten dimensions worth checking on any custom fastener
- Thread: system, nominal diameter, pitch, class, and length of full thread.
- Thread run-out: where the thread ends relative to a shoulder.
- Overall and functional lengths, with tolerances.
- Seat or contact face: angle, diameter and flatness, since this is where load enters the part.
- Head or body diameter and the across-flats dimension if a wrench is used.
- Hole diameter and position, with position tolerance not just size tolerance.
- Material and class, stated on the drawing and not only on the purchase order.
- Coating and whether it is applied before or after threading.
- Straightness or runout callout, especially for long bars.
- Marking requirement: heat number, grade, or a part number for traceability.
Marking up the drawing for a quotation
Circle the functional dimensions and add a note listing them. Add a general note that unlisted dimensions are reference. State the inspection method for the critical features — a thread gauge, a caliper, a fixture — because a factory can meet a tolerance on paper and fail it in practice with the wrong instrument.
Where possible, send a PDF for clarity plus a DWG or DXF for the geometry, and state that in case of conflict the PDF dimensions govern. This single sentence prevents a whole class of dispute.
What to ask for in the quotation
Ask the factory to confirm, in writing, that they have read the functional dimensions and the inspection method, and to flag any dimension they cannot hold. A factory that flags a tolerance at quotation is offering information; one that discovers it after production is offering an argument.