Identifying and repairing screw threads

Reviewed Oct 8, 2026 by Site editor.

A damaged or unfamiliar thread usually raises the same chain of questions: what size and pitch it is, what drill to use before tapping, how deep the thread needs to go, and what to do once a hole has stripped. These seven discussions follow that order, from measuring and identifying a thread, through tap drills and thread depth, to thread inserts, epoxy and very small screws.

They are community discussions from a metalworking newsgroup, most of them more than a decade old, so read each one for the reasoning rather than as a specification. Product and brand names in them are examples, not recommendations. Sizes quoted from memory in a reply are not always right, and some replies disagree with each other. Check any thread size or tap drill against a current reference or standard before relying on the fit for anything load-bearing, pressurised or safety-related.

  1. 1. Why a bolt measures smaller than its nominal size visited done

    Nominal thread size

    An M12 bolt measures about 11.8 mm, and threads cut on a lathe from 12 mm stock only take a nut once cut too deep. One reply says nominal size is what the bolt started as; others bring in the rounded root and crest of real threads, class of fit, Machinery's Handbook and a table of maximum and minimum major diameters. A last reply offers a reading of the ISO definition that nobody confirms.

  2. 2. Identifying an odd thread: the tyre valve and Schrader valve visited done

    Does anyone know what thread size a tire valve stem and the Schrader valve are?

    A broken waterer fitting uses a valve-stem thread and no tap or die can be found. Several replies cite references for about 0.302 to 0.305 in (7.7 mm) by 32 tpi for the outside thread; others give different figures from memory or measurement, and some say a 5/16-32 die fits, if loosely. Practical answers include valve thread chasers sold at auto stores and soldering in a scrap valve stem.

  3. 3. Working out the tap drill for a metric thread visited done

    Tap drill size for metric threads?

    For an M14 x 1.25 spark-plug tap missing from an old handbook, replies give the rule nominal diameter minus pitch (12.75 mm, with tables listing 12.8). One reply challenges a simpler rule of thumb, and another lists table sizes that differ slightly from the formula, apparently rounded to drills that exist.

  4. 4. How deep the full thread has to go in a tapped hole visited done

    "Full Thread Depth" blueprint definition

    What 'full thread depth' on a drawing means. Replies define it as the depth a bolt or thread gauge will screw in, short of the drilled depth to allow for the tap's lead and chips, and disagree on whether thread plug gauges measure it properly. Two replies say to get the meaning clarified, and one shows how a standard hole note states it.

  5. 5. Fitting a Helicoil-type insert without the installation kit visited done

    "Helicoil" installation without the insert tool? Need advice

    A 1/4-28 insert won't start in a hole tapped with an insert tap. Replies describe making an insertion tool from a notched 1/4-28 bolt or a slotted shaft, or using a prewinder; one says friction winds the insert tighter onto the tool as it enters. Others note insert brands and taps are not interchangeable and that a special insert tap is needed.

  6. 6. Can epoxy rebuild a stripped thread you can't drill out? visited done

    repairing stripped threads with JB Weld

    A stripped 1/4 in hole in an aluminium transmission case can't be reached with a drill, so the poster plans to rebuild it with JB Weld. Most replies are against it, saying epoxy is too weak for the thread load, and suggest an insert, a glued-in stud (called a bodge) or a self-tapping stud. The poster ends up gluing in a stud with a nut, for a bracket that carries little load.

  7. 7. Tiny threads: a stripped screw hole in a glasses frame visited done

    stripped screw hole in glasses frame

    The threads came out of a metal frame with the screw. Replies suggest taking it to an optician, some saying it was done free, or tapping the hole to the next size up with a small screw and lock nut, or a longer screw and two nuts. Others mention thread-locker or clear nail polish, and solder.

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