Identifying Internal threads

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.

Screw a suitable diameter of wood or plastic into it

... unscrew, and measure the pitch of the male thread

MichaelG.

Use some polymorph, it's cheap, and reusable

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

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.

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

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.