Retapping Imperial thread to metric

[Richard_Griffiths]

Hi all,

Just a quick question I have a metal part on one of my model cars that has a screw hole for a 4-40 screw.

I was wondering if it is possible to retap this with an M3 Tap I belive this is the nearest metric equivilent.

My reasoning is that M3 screws are more common here in the uk and would be less hassel for me to get hold of.

Thanks

Richard

[j.m.]

4-40 is a slightly larger diameter than M3, so I don't think it will work out well. I don't think the M3 screw will have much bite in the 4-40 hole, though the M3 has much coarser and deeper threads than 4-40.

Going from M3 to 4-40 is another story though, I've done that with great success.

If at all possible, try to test it out on a part you don't need. Ideally you could drill a hole in a new piece of raw aluminum, tap the hole to 4-40, then try to tap it again to M3. But you probably don't have a 4-40 tap....

Jon

[plod]

4-40 is a slightly larger diameter than M3, so I don't think it will work out well. I don't think the M3 screw will have much bite in the 4-40 hole, though the M3 has much coarser and deeper threads than 4-40.

Going from M3 to 4-40 is another story though, I've done that with great success.

If at all possible, try to test it out on a part you don't need. Ideally you could drill a hole in a new piece of raw aluminum, tap the hole to 4-40, then try to tap it again to M3. But you probably don't have a 4-40 tap....

Jon
I think you have it wrong buddy

I've upgraded from 4-40 to M3's on a number of occassions, and M3 is slightly larger diameter than 4-40 so IMO you have it the wrong way round.

If you look on LSTforums, there are pages on the subject of upgrading from 4-40's to M3's for greater strength.

[sosidge]

I think you have it wrong buddy

I've upgraded from 4-40 to M3's on a number of occassions, and M3 is slightly larger diameter than 4-40 so IMO you have it the wrong way round.

If you look on LSTforums, there are pages on the subject of upgrading from 4-40's to M3's for greater strength.

I second this... 4-40's have a slightly smaller diameter and a coarser thread than an M3 screw.

You can try re-tapping, but if the 4-40 has stripped the hole clean there won't be much material for the M3 screw to grab either, unless you can fit a longer screw instead or maybe a nut on the other end.

[ConceptRacing]

I did this recently to some associated shock mounts, as long as you take it easy with the tap and use some light oil to ease it through, it works really well.

http://i29.photobucket.com/albums/c266/teamsnoopyracing/SAM_2060400x300.jpg

[SlowOne]

If the hole you want to re-use is in plastic, then you can simply put an M3 screw in and tighten it up - M3 is larger than 4-40. If it is in metal, then it won't work. Leaving aside the maths and dimension of the two threads...

(see imperial http://en.wikipedia.org/wiki/Unified_Thread_Standard and metric http://en.wikipedia.org/wiki/ISO_metric_screw_thread)

...unless the tap can cut into fresh metal, the thread will be very weak at best, and non-existant at worst. In the example above, it looks to me as though almost no load is going through the screws, and the screws are in plastic, so that would be a good application of the 'M3 for 4-40' exchange. If any load is involved, don't make the swap as the threads will be weak and will fail.

Note also that because a 4-40 thread is coarser than an M3 thread, it will work better in plastic parts than an M3, given that the hole is the right size for a 4-40 thread in the place and is 'fresh'. HTH :)

[Richard_Griffiths]

Thanks for the info guys i think i will give it a go if it fails i might have to make up the metal part and drill and tap fresh.

[TheReferee]

richard
your original question was because it was easier to get m3 than 4-40.
i use MK for my screws top quality and reasonable price, full range of 4-40
http://www.m-k-racing.com/index.php?cPath=5_53&sort=&filter_id=41

[j.m.]

Dang, I'm sorry.

I definitely got it exactly backwards there... :(