# Finding the right size of nut for a bolt

I have a bolt with a thread pitch of .8mm and a diameter of 3.3mm. I am trying to find the correct size of nut for it, however the ISO thread chart says that a .8mm thread is M5, but should have a diameter of 4.2mm leaving me confused.

• What kind of screw is this? How do you know that it's metric? Go to the specialty fittings section of a home store and use their screw/thread identification plate. Commented Aug 6, 2018 at 10:34
• That is great that you could measure the pitch! How did you do it? Commented Aug 6, 2018 at 12:16
• Do you have a bolt or a screw? Usually, you only need nuts for bolts. Commented Aug 6, 2018 at 12:43
• @DarrickHerwehe "(machine) screw" is used to refer to a fully-threaded bolt, especially one with a slot/philips/socket/torx head (perhaps more commonly in British English, and the OP is apparently here in the UK). Commented Aug 6, 2018 at 15:51
• It is metric, M3.48x0.793. Dirt cheap in the US where they are marketed as UNC 6-32. Commented Aug 6, 2018 at 22:39

Judging by the pitch and diameter you've measured, what you probably have is an American UNC 6-32 screw.
There are many conversion charts on the interwebs, but if you refer to this one in particular you can see the diameter for a #6 thread is 0.138" or 3.5mm.
The 2nd number in the UNC scheme is the number of threads per inch, so taking 25.4mm per inch and dividing by 32 gives you 0.79375mm - suspiciously close to your measured 0.8mm.

Big box stores (Home Depot, Lowe's, etc.) usually have a board hung up in their hardware aisle that you can use to identify threads/sizes of bolts and screws. You can just take your screw in and check it on the board. It looks like this:

• I have a flat plastic gauge that has both metric and standard sizes under 10\$ U.S. since I used to work at a company that made equipment for world wide shipment and we used equipment from around the world these were a valuable tool, I searched , screw bolt gauge pitch and several different brands popped up. Commented Aug 6, 2018 at 17:08
• McMaster Carr has the device pictured as part number 20375A27. Searching their website for "thread checker" turns up several similar products ranging from simple plastic cards to actual thread gauge sets. Commented Aug 7, 2018 at 15:26

The great thing about standards is that there are so many of them to choose from.

Context is key,

If you are looking at modern electronic/IT equipment then the screws are most likely to be either metric (which you have ruled out) or UNC. From the other posts it seems that your screw is a good match to the 6/32 UNC thread. 6/32 UNC is a very common size used for mounting stuff inside computers (infuriatingly so is M3 which is almost the same size but with a much finer thread pitch, so your often have to test-fit a threaded hole to figure out which size it is supposed to be).

On the other hand if you are dealing with vintage gear then there are other possibilities, for example whitworth also has a 6/32 thread which is similar but not quite the same as the UNC one (similar as in with a bit of brute force you can get them to fit).

• sorting 6/32UNC vs M3 by thread pitch isn't that bad (it's an 8:5 ratio and relatively obvious as a result, on the screws themselves at least) compared to the fun of sorting both types by length where the former mostly come in 3/16, 1/4, and 5/16 lengths with varying sizes and styles of head so you can't just stand them all on their heads and sort by overall lengths. Wikipedia claims more grotesque creations like metric length 6/32 screws also occasionally show up, which might help explain why sorting everything out is so much fun. Commented Aug 6, 2018 at 18:39
• @DanNeely also tolerances on lengths can be quite loose (easily +/- 0.5mm within a batch of metric screws, more if you account for different manufacturers) Commented Aug 7, 2018 at 8:15
• Obligatory: xkcd.com/927 Commented Aug 7, 2018 at 11:52
• @FreeMan I'm not aware of anyone trying to standardize all case screws on a single new size. There're N competing standards (self tapping fan screws, and various CPU cooler mount and water cooling radiators that have very precise length requirements to achieve proper pressure on the CPU or to avoid a puncture in the tubing), where N is almost certainly much larger than the 14/15 in the XKCD. For that matter the needs of self tapping screws for fans and tiny screws for SSDs mean we couldn't consolidate down to a single size even if we wanted to even if M3 could fully replace 6/32 (maybe?). Commented Aug 7, 2018 at 13:05
• @ChrisH I didn't realize spreads could be that large, and within a single case's screw kit (generally presorted by use) I'm pretty sure I'd've noticed a 1mm spread within supposedly identical screws. That much slop between manufactures though would have the various 6/32 screws nearly overlapping and would help explain why sorting them out is so painful. Commented Aug 7, 2018 at 13:08

• Inner diameter
• Outer diameter
• Thread angle (if it is triangular)
• pitch
• direction (left / right)

All of them are arbitrarily chosen and described in standards. Therefore we have pipe thread, withworth thread, UN thread, UNC thread, ISO thread, fine thread etc. The only bolt-nut compatible pair is the pair of very same standard and very same name.

Some standards have geometries close together. For example UNC and metric have same thread angle but they are derived from imperial units (diameter, pitch) and metric units, respectively. There are nut-bolt pairs that fits up tu three turns. Screwing deeper, it locks.

If it is possible, throw the bolt away and get yourself new set with diameter that fits.

• +1 for "Throw it away and replace it". Though if you're doing this and there's a set of them in use, make sure to throw away the others too. Commented Aug 7, 2018 at 8:27
• @Sneftel Good point. Best practice is throw away anything unknown and replaceable and replace it with known. The fewer different "parts", the better. Commented Aug 7, 2018 at 11:13