Identifying Internal threads

[VicParticipant @vic]

Anyone got any tips on how I might Identify an internal thread on a machine? Its not metric and I don’t have any imperial bolts of a likely diameter to try. I’m not in the workshop at the moment but I think it’s likely 5/16 or 3/8 something or other! I’ve got some thread pitch gauges but not too easy to use on internal threads.

[VicParticipant @vic]

[Michael GilliganParticipant @michaelgilligan61133]

Screw a suitable diameter of wood or plastic into it

… unscrew, and measure the pitch of the male thread

MichaelG.

[Pete.Participant @pete-2]

Use some polymorph, it's cheap, and reusable

[Tony Pratt 1Participant @tonypratt1]

Okay not easy with your limited 'equipment',what m/c is it & how do you know it's not metric? Give us the manufacturers name that will be a good clue.

Tony

[VicParticipant @vic]

Update. I’ve seen reference to the size in the manual for a later model so I have a staring point now. Apparently it’s UNC.

[Tim StevensParticipant @timstevens64731]

I have had success using a simple wooden toothpick. Break off the sharp end, and slide the pick around the thread while keeping it in line with the axis, using pressure from a finger (etc). This should result in a series of grooves or dirty lines (or both) along the pick, so you can measure how many fit to an inch or to 25mm. Then a moment's calculation should reveal the threads-per-inch, or the pitch in mm.

Cheers, Tim

[David Marks 2Participant @davidmarks2]

If you have a set of Letter series drills you should be able to get a good idea of the core diameter of the thread by inserting the shank of the drill into the hole. Regarding the thread profile and TPI, a small amount of car body filler placed into about 30 – 40% of the thread works. Due to the thread shape it lifts out very easily.

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