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Plugging incorrect holes

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Peter Howell 112/03/2021 09:08:31
37 forum posts
4 photos

I managed to drill and tap 3 x M3 holes in the wrong place in a cast iron casting. (measured from wrong face !)

I need to plug them and re-drill however the correct holes will slightly overlap the incorrect ones.

My first thought is fix a sawn off screws in the holes with Loctite Retainer, but will they stay put with the new holes overlapping ?

Any other ideas.

Brian Wood12/03/2021 09:12:53
2566 forum posts
39 photos

Hello Peter,

They should do unless the originals are very shallow. Give the Loctite about 24 hours to really get to full strength before you run and tap the new holes.

Regards Brian

Frances IoM12/03/2021 09:51:47
1268 forum posts
28 photos
JBWeld - they are only small holes with plenty of surface area to grip

Edited By Frances IoM on 12/03/2021 09:52:19

John Hinkley12/03/2021 09:55:59
1332 forum posts
426 photos

I have encountered this problem on occasion. One solution, if you have sufficient room, would be to drill the hole in the 'right' place with, say, a 6mm centre-cutting end mill to a suitable depth, such that the resulting hole includes the one already drilled in the 'wrong' place. Plug the resultant 6mm hole as per the Loctite solution above and, when set, mill the repair flush, re-drill the hole in the 'right' place in the plug and tap. Ideally of course, use a cast iron plug, or rely on a painted finished to disguise the repair.


JasonB12/03/2021 09:56:50
22751 forum posts
2653 photos
1 articles

The only problem with JB Weld and to some extent a screw is that if the filler material is softer than the rest the drill will wander into the soft material if the holes overlap. Even if you plunge with a 2.5mm milling cutter to get a straight hole the tap will wander into the softer stuff.

I would drill out the thread and turn up some CI plugs and JB Weld or loctite them in. If you don't have any CI to make the plugs from then Drill out larger and fit a steel or brass plug that is large enough to take the whole of the repositioned thread and also fill the unwanted hole.

Recent thread with the same issues dealt with

Edited By JasonB on 12/03/2021 10:00:16

Dave Halford12/03/2021 10:31:46
2050 forum posts
23 photos

As Jason says drills or taps will wander into the softer material, so plug with steel and you get wandering into the iron, even chineseium screws may be too hard.

Ian P12/03/2021 10:37:44
2590 forum posts
114 photos

They will stay put. Your problem will be drilling and tapping the new holes into two different materials. The steel screw will not cut as easily as cast iron and the drill and tap will get pushed to one side.

Could you make a screw out of cast iron and loctite that in. Alternatively enlarge the existing holes to say M6 and then redrill.

Ian P

Dullnote12/03/2021 12:04:42
94 forum posts
29 photos

Just had to do this yesterday, found the edge then moved in 8 mm, drilled, forgot to take the size of the edge finder into account, so 2mm out.

drilled out bigger , machined a plug for push fit , then drilled in correct place. Pleased as you can hardly see the fix, I did skim the part on the mill.

This was all steel, so my question would be, would it be advisable to push fit a new part in cast, or better to loctite. As Peter issue is with cast and mine steel

duncan webster12/03/2021 12:15:12
3987 forum posts
65 photos

If the idea of enlarging the existing holes to M6 is to get the correct holes all in plug material the plug would need to be 9mm dia. If you have a bit of scrap CI I'd try drill a 3mm hole, loctite or JB weld a bit of CI in, then drill and tap a new hole say 2.8mm away and tap that. The new thread will be 318 degrees in the base metal, 42 degrees in the plug, so plenty strong. The only doubt is whether the plug will try to turn as you tap it. That's why you should try it out on a bit of scrap

If it's only very shallow it won't work, but say 5mm plus feels OK

Edited By duncan webster on 12/03/2021 12:17:23

not done it yet12/03/2021 13:22:40
6812 forum posts
20 photos

If the above is not acceptable, maybe putting in a threaded plug which will cover both the correct and wrong holes and drill into that might be OK.

Duncan is spot on re that and the angle of thread in new and old. Possibly use a longer screw (deeper hole) in the correct place if this is a blind hole?

Peter Howell 112/03/2021 14:23:43
37 forum posts
4 photos

Thanks for all the suggestions.

I've been looking at the job this morning. Most things look better in the light of day.

The simplest solution , in this case, is just to remake the bracket that it fixes to to suit the incorrect holes. It's only made from a piece of angle. Hopefully no one will notice they are not on the centre line.

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