screw size problem

[pgrbffParticipant @pgrbff]

I’m in Italy so finding imperial screws not very easy. I am trying to determine the size of a threaded hole on my tablesaw, an old Startrite before ordering some screws.

I bought a range of screw size checkers recently but as luck would have it none fit.

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?

[Nigel Graham 2Participant @nigelgraham2]

I think Startrite is American, so the screws are likely to be of inch dimensions but AN / UN standards.

[pgrbff]

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.

[David JuppParticipant @davidjupp51506]

This web site might possibly help

https://startrite-spares.co.uk/

[Martin ConnellyParticipant @martinconnelly55370]

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

[David JuppParticipant @davidjupp51506]

Have you tried M8 ?

[Martin Connelly]

On Martin Connelly Said:

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

6mm goes straight through. I tried M8 dowel but couldn’t get a impression.

[David Jupp]

On David Jupp Said:

This web site might possibly help

https://startrite-spares.co.uk/

I have already looked, no sizes, unfortunately. It doesn’t even show the item I need.

[David Jupp]

On David Jupp Said:

Have you tried M8 ?

too small but sort of matches threads

[Clive Brown 1Participant @clivebrown1]

If you’re refering to the thread pitch then M8 is very close to 20tpi which is common in imperial thread standards.

[Michael Gilligan]

On 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.

[ChrisLHParticipant @chrislh]

Is it possible that the thread in the hole is stripped ?

[Grizzly bearParticipant @grizzlybear]

Hello,

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

Good luck………..

[Martin ConnellyParticipant @martinconnelly55370]

Not getting an impression on the 8mm dowel makes it sound like it is a stripped 5/16″ as Chris is thinking, is it in cast iron? It may be possible to Helicoil it to match the rest of the machine or to cut the next size up thread and fit a larger screw. A quick check on EBay shows reasonably priced 5/16 UNC and UNF Helicoil type kits are readily available. Is there someone near you who works on older cars or bikes with UNC threads?

Martin C

[JohnFParticipant @johnf59703]

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

[larry phelan 1Participant @larryphelan1]

I have a Startrite bench saw, so might be able to help.

Where is that screw located on your machine ?

[mark smith 20Participant @marksmith20]

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.

[Robert Atkinson 2Participant @robertatkinson2]

You could also use the nearest size helicoil* insert for either a metric or imperial replacement screw. This is a good option if there is a chance the existing thread is damaged. You can get low cost tooling if just doing a few inserts.

*other thread repair inserts are available…..

[Yngvar FParticipant @yngvarf]

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

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