How to identify a metric thread?

[Lobster]

I want to replace some damaged bolts. I know they are metric thread, but don't know what size they are. What dimension do I need to measure to determine this please?

Thanks David

[Grunff]

Measure the diameter of the bolt, to the outside of the thread. 10mm is M10.

[Lobster]

Thanks - looks like 4.5mm, which I don't believe! (don't have anything other than a ruler on me to measure it tho). I've just found a nut which matches it - this takes an 8mm spanner. That should define a specific thread size too, shouldn't it?

Thanks David

[Dave Fawthrop]

|Lobster wrote: |> I want to replace some damaged bolts. I know they are metric thread, |> but don't know what size they are. What dimension do I need to measure |> to determine this please? | | |Measure the diameter of the bolt, to the outside of the thread. 10mm is M10.

Or a bit of undamaged shaft, 10mm is M10

It is also a good idea to get another bolt of the thread you thing it is, and check that the thread pitch is the same, by matching the two threads together.

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Roughly like that, works OK even on mangled bolts

I have lots of Whitworth, Unified, BSF, BA etc. bolts which I keep in case I need them ?sometime?

[Grunff]

Most likely M5.

No, it doesn't define it - it just hints at it :-)

You can have different head sizes for a given screw size. M5s commonly have 8mm heads, but not always.

[manatbandq]

How is that different to measuring the outside of the thread?

If you're going to go getting, I'd rather get a nut and try it for size.

MBQ

[Peter Parry]

Foolish boy, everyone knows the keeping of such bolts is to ward off the need for them. If you have them you will never need them, if you don't you certainly will.

[Guy King]

The message from Dave Fawthrop contains these words:

Only on cut threads. Rolled threads will be about 9mm for a 10mm thread.

[Dave Fawthrop]

| |Dave Fawthrop wrote: |> On Wed, 22 Mar 2006 14:50:04 +0000, Grunff wrote: |>

|> |Lobster wrote: |> |> I want to replace some damaged bolts. I know they are metric thread, |> |> but don't know what size they are. What dimension do I need to measure |> |> to determine this please? |> | |> | |> |Measure the diameter of the bolt, to the outside of the thread. 10mm is M10. |>

|> Or a bit of undamaged shaft, 10mm is M10 | |How is that different to measuring the outside of the thread?

All of the thread may be damaged and be less than nominal.

|> It is also a good idea to get another bolt of the thread you thing it is, |> and check that the thread pitch is the same, by matching the two threads |> together. | |If you're going to go getting, I'd rather get a nut and try it for |size.

Nut may not may not run down damaged thread.

[Chris Bacon]

Measure the diameter, which will tell you something - then the pitch. If you've really got 4.5mm then the pitch should be 0.75mm (coarse) or 0.5mm (fine). Measure a number of threads to get a more accurate figure. You could go to a hardware shop, if you know of a good one, and compare the thread of what you've got with the ones they stock.

[Dave Fawthrop]

|The message |from Dave Fawthrop contains these words: | |> |Measure the diameter of the bolt, to the outside of the thread. 10mm |> is M10. | |> Or a bit of undamaged shaft, 10mm is M10 | |Only on cut threads. Rolled threads will be about 9mm for a 10mm thread.

I just thought of that as well, so a bolt with a thoroughly mashed up rolled thread is difficult to identify.

[Dave Fawthrop]

|Lobster wrote: |> I want to replace some damaged bolts. I know they are metric thread, |> but don't know what size they are. What dimension do I need to measure |> to determine this please? | |Measure the diameter, which will tell you something - then |the pitch. If you've really got 4.5mm then the pitch should |be 0.75mm (coarse) or 0.5mm (fine). Measure a number of |threads to get a more accurate figure. You could go to a |hardware shop, if you know of a good one, and compare the |thread of what you've got with the ones they stock.

Better to buy a huge box of various Metric nuts, bolts, washers etc. next time Lidl have them, then you always have something roughly right in stock.

[Lobster]

Yep I've got a reasonable collection, for that very reason (I found a matching nut in there!); however this is actually a specific need for countersunk socket screws rather than bolts, and I don't have those. And as each bolt in my 'collection' isn't stamped with its size, that makes me none the wiser as to which I need!

As it is, I wanted this info to include in a Screwfix order today, and as I see they don't go below M6 for these, that's unfortunately a non-starter. So a trog down to my local nuts'n bolt emporium, ie with the old bolt, is called for anyway.

Thanks David

[Guy King]

The message from Dave Fawthrop contains these words:

Still reckon the best way is to take it to a shop and mesh threads with other bolts till you find a match.

Or, better yet, an industrial fastener specialist. There's one in most towns if you know where to look.

[Dave Fawthrop]

|The message |from Dave Fawthrop contains these words: | |> |Only on cut threads. Rolled threads will be about 9mm for a 10mm thread. | | |> I just thought of that as well, so a bolt with a thoroughly mashed up |> rolled thread is difficult to identify. | |Still reckon the best way is to take it to a shop and mesh threads with |other bolts till you find a match. | |Or, better yet, an industrial fastener specialist. There's one in most |towns if you know where to look.

Ours local Bolt and Nut specialist is *very* amiable

[Dave Plowman (News)]

Get a cheap set of taps and dies from a local market or Ebay, etc. They may not be much use for what they're meant to do but are ok for identifying threads.

[Holly, in France]

I saw a 'thing of purpose' for this recently, I used it in the shop to check that the thread of an elusive ceramic tap fitting I was buying was the same as the original. It was just a metal sheet with mabye 20ish threaded holes in it. Designed for plumbing fittings but there were alot of holes, I imagine it had some ordinary metric threads in there. And did I buy one - no! I'm going to regret that one day.....but now at least I know where to get one from should the need arise.

-- Holly, in France Gite to let in Dordogne, now with pool.

[Guy King]

The message from "Dave Plowman (News)" contains these words:

Never kick a camel turd at all if the SOE have been in the area.

[Derek ^]

Our local Industrial Fastener Specialist is very expensive.

Best go there and get them to identify the thread and order the screws from Maplin/Farnells/RS etc.

DG

[Harry Bloomfield]

Lobster brought next idea :

Some have their size stamped upon the head of the bolt.