Which imperial nut size?

M8 is 5/16", and depending on where it was made, it could be BSF, BSW, UNC or UNF.

BSW and UNC share the same pitch (18 threads per inch at that size), but have different shape threads, so are interchangeable as long as ultimate strength is not an issue.

Thanks. It is old British furniture, coarse thread rather than fine (by my eye) so I think I will get some 5/16 BSW nuts from ebay. I am assuming they are described by their thread size rather than the AF spanner size?

Correct. As confirmation, count the threads per inch on the stud, as it is vaguely possible that if the chair is old enough (Centuries, not decades), it may be a non-standard thread. If you're passing a car spares shop, they should have 5/16" UNC nuts in stock, which will probably be good enough.

Try a 3/8ths nut of whatever thread they might be

Why? So it will be too big??

It might be, I don't know. The op said 8mm was too small, as it binds up. 3/8ths is the equivalent of 9.52mm, so that's about 0.75mm bigger all round, and may well not bind up on the threads.

Anyways, what do I know as I only had 40 years experience in engineering.

From the OP's description of the problem, the size is more or less correct, but the thread pitch is wrong. If the nut was too small, he wouldn't even get it on to the end of the thread, unless it was a tapered one, which is unlikely.

I'd hazard a guess that "binding up on the threads" is the last thing it'll do.

I'll tell you for a fact that an M8 nut on an M7 thread will just happily slide up and down without any threads getting even close to touching, let alone "binding". I'd guess M9 (smaller, let's not forget than 3/8") on M8 would be very similar.

And you never came across the concept of "thread pitch" in that time?

Maybe he's a chemical engineer? Tighten it up finger tight, and throw a bucket of salty water over it to corrode it and stop it moving?

This is one of the unseen bonuses of owning a lathe. You can simply make one up to the exact specifications; handy because by no means all threads are 60 degrees and some have different profiles of crests and roots. No big deal perhaps for a sofa stud, but invaluable for classic cars/bikes etc.

If the OP can get it on and run it up a few threads, then most likely it's a TPI issue.

Type Size Dia Inch TPI Dia mm Pitch mm

BA 0 0.2362 25.38 6.00 1.00 MF 6 0.236 33.87 6 0.75 M 6 0.236 25.40 6 1 UNEF 1/4 0.25 32 6.35 0.79 UNF 1/4 0.25 28 6.35 0.91 BSF 1/4 0.25 26 6.35 0.98 BSW 1/4 0.25 20 6.35 1.27 UNC 1/4 0.25 20 6.35 1.27 MF 7 0.276 33.87 7 0.75 M 7 0.276 25.40 7 1 BSF 9/32 0.2812 26 7.14 0.98 UNEF 5/16 0.3125 32 7.94 0.79 UNF 5/16 0.3125 24 7.94 1.06 BSF 5/16 0.3125 22 7.94 1.15 BSW 5/16 0.3125 18 7.94 1.41 UNC 5/16 0.3125 18 7.94 1.41 MF 8 0.315 25.40 8 1 M 8 0.315 20.32 8 1.25 M 9 0.354 20.32 9 1.25 UNEF 3/8 0.375 32 9.53 0.79 UNF 3/8 0.375 24 9.53 1.06 BSF 3/8 0.375 20 9.53 1.27 BSW 3/8 0.375 16 9.53 1.59 UNC 3/8 0.375 16 9.53 1.59 MF 10 0.394 33.87 10 0.75 M 10 0.394 16.93 10 1.5

I once worked at a company that had a Standards Room. One of the guys was a real expert on Screw Thread Gauges (which we used to make) and was consulted by the National Physical Laboratory on many occassions. I gleaned quite a lot about the finer points of threads. There is a lot to them when you get into precision matters - especially when you get into wear allowances on gauges.

Actually, I spent most of my working life in steeelwork engineering, with only a minor part or time in fitting intricate parts together.

So the nut either went on or didn't. (I won't mention that if it didn't, warmed it up a bit, oops!)

Thanks! Something I'd been looking for! But what are those H and HF threads ? Never heard of them before.

Jim Hawkins

Do you mean M and MF? That would be Metric and Metric Fine. I look after a huge selection of nuts and bolts for antique machinery, and get given thousands of nuts and bolts that have to be identified and stored. Since nobody on the Net seems to have done it, I collected data about lots of thread types and sorted them in ascending order of diameter. I have a table for 1/16 BSW to 6 inch BSW. Perhaps it could be hosted somewhere.

Yes ! Should have put the glasses on!

Jim Hawkins

Thanks, I learned a bit about threads from this thread.

A 5/16 UNC was the correct TPI and solved the problem as an interference fit.

That possibly means that the original was 5/16 whit (BSW) but unless both male and female are made to very tight tolerances BSW and UNC will be interchangeable.