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Manged to pull the threads on some 2.5mm holes - tapping Aluminium

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Steve Withnell15/05/2017 21:12:34
796 forum posts
215 photos


As per title. Being a bit clumsy, I've managed to pull the threads on a couple of holes I was tapping in aluminium block - blind holes. How can I repair the damage so I can tap the holes correctly?


Emgee15/05/2017 21:32:50
1359 forum posts
212 photos

Hi Steve

You could use a Helicoil thread insert or if your hole is now 2.5mm that'sM3 tapping size, or if you want less TPI go to 6-32 UNC, that's 2.8mm tapping size hole.

Or if you can drill a deeper hole and retap for the designed screw, but longer than designed.

Drill oversize and fit a plug with Loctite or similar, drill and tap to suit.  I'm sure there are other ways but a lot depends on what the fixing is retaining.


Edited By Emgee on 15/05/2017 21:36:15

Michael Gilligan15/05/2017 21:34:13
14786 forum posts
635 photos


IF there is sufficient wall-thickness available, then a Helicoil repair would typically be 'better than the original design'.

... No doubt there are good reasons why you consider this a non-starter sad


Henry Artist15/05/2017 21:38:48
68 forum posts
46 photos

Can you use a bigger fastener? The tapping hole for M3 is 2.50mm.

Drill out the hole much, much bigger, tap it and fit a plug. Then try again to tap M2.5.

Start making the part again from the beginning.

David Standing 115/05/2017 21:43:11
1287 forum posts
48 photos

Depending on the circumstances, another option could be to TIG weld the holes up and start again?

Jon15/05/2017 21:43:38
989 forum posts
46 photos

Holes probably 2.9mm now, if can get away with it don't bother enlarging just run a few M4 taps down, some will cut better than others plus need the flat bottomer better off ground flat.

Check screws not to long to bottom out.

Emgee15/05/2017 21:58:22
1359 forum posts
212 photos
Posted by Jon on 15/05/2017 21:43:38:

Holes probably 2.9mm now, if can get away with it don't bother enlarging just run a few M4 taps down, some will cut better than others plus need the flat bottomer better off ground.

Jon, M4 tapping drill is 3.3mm so it may be difficult to start a tap if the hole is at 2.9mm.


duncan webster15/05/2017 22:31:40
2345 forum posts
38 photos

Tap M4 and make a stepped stud, short length of M4 one end, the rest 2.5mm dia

Ian S C16/05/2017 11:13:25
7468 forum posts
230 photos

Drill and tap 4 mm, make a plug of the same material with a 4 mm thread, fit the plug in the hole with thread lock (Loctite) and cut it off flush, redrill and tap to the original size.

Ian S C

Edited By Ian S C on 16/05/2017 11:14:32

Steve Withnell16/05/2017 19:31:55
796 forum posts
215 photos

Thanks guys, out of the armchair and into the shed...

Steve Withnell19/05/2017 15:02:54
796 forum posts
215 photos


So, I had just enough clearance to drill and tap 4mm, loctited in a brass screw, skimmed flush, tapped out 2.5mm. Perfect!

What is it?  23cm Bandpass filter to stop those cell towers wiping out 23cm receive front ends!


23cm Bandpass Filter


Edited By Steve Withnell on 19/05/2017 15:09:34

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