On 5 December 2024 at 13:47 pgrbff Said:

[…] A 5/16 18 or 24 will go in but is clearly too small. A 3/8  16 or 24 is too big.

What is there in between these two?

11/32

But they are rare !

… I would recommend taking a cast of the tapped hole, and measuring it carefully before proceeding further.

MichaelG.

I would get a wooden dowel of about Ø6 and see if you can screw it in. A suitably sized dowel will then take the form of the thread and when unscrewed can be checked with a scale or screw gauge to find what the TPI or pitch is. Knowing this will make it easier to figure out what the thread is. It does not have to be a tight fitting dowel, just slightly larger than the bore of the hole. Even a wooden pencil that is thinned down a bit will do or a square section of wood with a diagonal dimension that is about 6mm.

There are some screw forms in the standards that can be made into non-standard but still listed threads, I have recently made something with a 1/2″-16 UN thread (I had suggested a 1/2″ BSF thread but the designer wanted a UN form to match other UN form threads that were used at different diameters including a 2 1/8″-16 UN thread). Out of curiosity I tried a 1/2″-16 UN thread nut on a 1/2″ BSF rod and it fitted however trying the opposite, 1/2″ – 16 UN tap in a 1/2″ BSF hole it did not fit.

Martin C

On 5 December 2024 at 14:06 Michael Gilligan Said:

On 5 December 2024 at 13:47 pgrbff Said:

[…] A 5/16 18 or 24 will go in but is clearly too small. A 3/8  16 or 24 is too big.

What is there in between these two?

11/32

But they are rare !

… I would recommend taking a cast of the tapped hole, and measuring it carefully before proceeding further.

MichaelG.

Not sure what, if anything, this proves … but

https://toolz4you.co.uk/product/11-32-x-22-tpi-whitworth-form-1-5-16-od-die-ed594/

MichaelG.

Hello,

Have you got an identical screw on your machine that you could remove and photograph against a steel ruler?

Good luck………..

Hi Maybe this will help, lots of obscure threads here   https://www.gewinde-normen.de/en/index.html

I like Grizzly’s idea find another screw on the machine to aid identification.

Another possibility is turn a dummy from soft material, alloy or lead slightly over the minor dia with a taper on the end and screw it into the hole — hopefully it will form a thread on the dummy,  the best way is use Woods metal to cast a pattern.

John

Startrite are british as far as i know ,i have a mercury pillar drill. Theres a tablesaw manual on vintagemachinery .org  but doesnt mention the screw types in the parts lists.

I recently got a Startrite 275. There is a lot of metric fastners on it for a british machine from the sixties.