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Just a small problem

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Joe McKean01/09/2019 09:45:09
25 forum posts

I have just drilled the first of four blind holes in a ductile casting with a 2.05mm drill and then went to tap it with a 7BA tap. I now find that there is not enough material left to cut a decent thread.

Upon checking the drill bit to see if it was the right size I found that it was bent which I didn't notice before using hence the oversize hole.

So do I now drill the hole out even larger and fill with a metal epoxy then drill and tap or has anyone got a better solution.



Brian H01/09/2019 10:06:57
1592 forum posts
104 photos

Sorry to read of your problem Joe, that's really annoying.

Is the any chance of going up to 6BA and using the one-size-down nuts and bolts that are available?


Chris Evans 601/09/2019 10:10:12
1631 forum posts

Is there enough metal around the area to go even bigger? Drill/tap and plug off using a dab of studlock to hold the threaded plug in place then redrill to suit 7BA? Remachine the plugs flat before redrilling.

Andrew Johnston01/09/2019 10:13:41
5425 forum posts
629 photos

It would be quite difficult to completely fill a small blind hole with epoxy. Two simple solutions are to drill out slightly and tap 6BA, if that is acceptable. Or alternatively drill out and tap much larger, say 2BA, and fill with a metal plug which can then be drilled and tapped 7BA.

A 2.05mm drill seems a bit small for 7BA, I'd use 2.1 or 2.15mm.


Edit: Must learn to type faster embarrassed

Edited By Andrew Johnston on 01/09/2019 10:14:54

JasonB01/09/2019 10:22:21
17877 forum posts
1954 photos
1 articles

How much metal is beyond the blind hole's bottom? Could you simply go deeper

Depending on load and if the holes are to take studs you could drill out to say 2.5 and set studs in with JBWeld, saves having to retap close to an epoxy joint.

Edited By JasonB on 01/09/2019 10:28:17

Neil A01/09/2019 12:45:49
55 forum posts

The usual salvage in industry is to drill and tap to a larger size, plug with a suitable material fitted with either a retaining compound or just peened into place. Then re-tap the hole the the correct size. This keeps all the fasteners the same size.

Another fix that was sometimes used, if the fastener was a stud, was to make a stepped stud with a oversize thread at one end and the other end would have the same thread as the other fasters. This requires a special fastener which you may not want to do.

My preference would be to plug and start again, it depends how much metal you have to play with.


vintage engineer01/09/2019 13:57:48
249 forum posts
1 photos

I would go with a 6BA stepped stud. That way you are not removing anymore material.

Phil Whitley01/09/2019 15:43:05
1153 forum posts
145 photos

Helicoil it? can you get them that small, I don't know, but I use them all the time for thread replacement.

Joe McKean01/09/2019 15:52:19
25 forum posts

Sorted, many thanks for all your ideas.

not done it yet01/09/2019 16:00:18
4513 forum posts
16 photos

Sorted how? Of the different options supplied, which did you choose? It would be good for readers to know what worked.

Harry Wilkes01/09/2019 16:25:01
875 forum posts
61 photos
Posted by not done it yet on 01/09/2019 16:00:18:

Sorted how? Of the different options supplied, which did you choose? It would be good for readers to know what worked.

Agreed it would be good to know !


Joe McKean01/09/2019 20:58:32
25 forum posts

More or less did what Chris Evans 6 said in the second reply.

Drilled the hole out with a 4.2mm drill then tapped to M5, only reason for using this size was that it was the largest I could safely use without weakening the casting I then cut a small plug out of some mild steel rod and threaded it and used a small dab of loctite 603 to hold it.

I have now machined the plug level with the face of the casting, drilled it and tapped it.

not done it yet02/09/2019 09:12:04
4513 forum posts
16 photos

Thanks for the update. You have been lucky that this is the first time you have encountered a problem such as this in 3 1/2 years! smiley I have to make modifications, repairs or other changes to items on a fairly regular basis. Welding, helicoils, over-size tappings, stepped studs are all legitimate fixes as far as I am concerned. Sometimes, even, making a new part is the best option.

BTW, I would have used a 5mm screw/bolt/stud instead of making a piece from scratch. Just easier and quicker.smiley

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