Thread Fit Check

A thread size is only half of the answer. What decides whether a bolt turns into a nut, and how much play it has once it is there, is the tolerance class of each part. The Fit Check takes one external thread and one internal thread and works through both tolerance zones for you.

• Verdict: the pair assembles, or it does not, and for a pair that does not, the reason.
• Minimum and maximum clearance or interference: the loosest and the tightest the pair can be when both parts sit at the ends of their limits.
• Tolerance-zone map: the zone of the external thread and the zone of the internal thread drawn against each other, so you see the margin before you read a number.
• Engaged drawing: both threads drawn to scale in contact, with the flanks touching, so the fit can be read as a picture.

1. Pick the standard

The check works inside one standard at a time, because the classes of one standard are only defined against each other. The mating list therefore shows only threads of the chosen standard.

2. Pick the external and the internal thread

Choose the size, the series and the class of the bolt-side thread, then the same for the nut-side thread. Every candidate in the list carries a tag: Standard pair (the combination the standard itself pairs), Tighter, Looser, or Won't mate with the reason.

3. Read the result

Look at the verdict first, then at the clearance or interference range, then at the map and the drawing. Change the class on one side to see how much of the range that single choice is responsible for.

4. Start from a thread page

Every ASME thread page has a Mates section. Its links open the Fit Check with that thread already selected.

The Fit Check covers the ASME thread standards on this site:

• ASME B1.1: Unified inch threads (UN, UNR and UNJ forms)
• ASME B1.5 and B1.8: Acme and Stub Acme
• ASME B1.9: Buttress threads
• ASME B1.10M, B1.13M and B1.21M: miniature, M profile and MJ profile metric threads
• ASME B1.11: microscope objective threads
• ASME B1.20.1, B1.20.3 and B1.20.7: general-purpose pipe, Dryseal pipe and hose coupling threads. For tapered pipe threads the check works with the make-up of the two tapers, and for Dryseal threads with the sealing analysis, in place of a simple clearance.

All limits come from the tables and formulas of the standards, in the same data that feeds the thread pages. The ISO metric tool is not part of the Fit Check yet.

• A positive result is the allowance between the two zones; a negative result is interference. Both ends of the range matter: the tight end tells you whether the parts can still be assembled, the loose end tells you how much play to expect.
• The check describes the geometry of the tolerance zones. Coatings and plating, lubrication, material and the length of engagement are not part of it.
• It is a reading aid for design and inspection decisions, not a replacement for the standard itself.