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Cast Iron query

helping out a chap with his cast iron door

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Ian Skeldon 203/08/2017 12:37:43
489 forum posts
41 photos


A friend who is down on his luck atthe moment has asked if I could help him out, he has taken the front door off his old wood burning stove to give it a good clean and in trying to remove the glass he has sheered or snapped off one of the securing bolts, all of the others are ok.

I looked it over and it really does need something where the old stud/bolt was located to secure the glass to door seal so ignoring it isn't an option. He managed to remove the old stud/bolt himself but in doing so has opened up the hole and left it none uniform in shape and size.

Without wanting to make it a big task and costing me any more than was needed I ground one of my 8.5mm twist drills so that I could use a 3/8 unf tap for the full depth (only 3.5mm) my plan being to now ground a few of my cheaper 3/8 carbon taps flat to get to the bottom and then put a new bolt in.

I would like to ask if this seems the best way forward or am I unlikely to get a good result?

If a good result is unlikely, am I likely to be able to braze a bolt into this hole without distorting or damaging the door further, note that I don't have a brazing hearth just a very good plumbers brazing torch an dsome brass spelter and flux somewhere.

Thanks, Ian

Clive Foster03/08/2017 12:56:22
2375 forum posts
76 photos

For a similar short thread issue, albeit not on wood stove, I restyled the thing and drilled right through to use ordinary bolts in both the damaged hole and in some other the others to give and adequately aesthetic arrangement.

If that won't fly due to external features I'd still drill right through to use either a bolt or stud and operate on the exposed end so it doesn't show.

Wood stove cast iron can be pretty horrible stuff to work with at the best of time. Some is pretty good but the worst is ..... Being well hotted up and cooled down a few 'undred times doesn't help either. Fast'n functional beats engineering nicety every time in this sort of situation.


I.M. OUTAHERE03/08/2017 13:20:38
1468 forum posts
3 photos

I would plunge a 3/8 slot drill in then machine up a threaded plug and braze it in place that way you get back to the original thread size that i'm guessing was 5/16 ?

3.5 mm seems shallow and i would not like to try cutting that thread !

A photo would be helpful.

larry Phelan03/08/2017 14:07:47
544 forum posts
17 photos

Hi Ian,

I too would be inclined to drill right through and use a standard bolt,and drill the other bolt places as well,to make it look right. Remember,when an architect makes a cock up on one side of a job,he makes the same one on the other side,so no one notices ! Works every time !

PS Sometimes he even charges more for it !!

John Reese03/08/2017 20:15:21
847 forum posts


I like your idea. Thread the hole and make a threaded insert. You probably do not need threads all the way to the bottom of the hole. Either use a short insert or turn down part of the OD to the minor thread diameter.

Ian Skeldon 203/08/2017 22:23:01
489 forum posts
41 photos

Ah both good ideas, in trying the insert first I can still resort to drilling through and using a bolt(s) if that doesn't work.

Many thanks for those ideas,


duncan webster04/08/2017 00:46:21
2795 forum posts
41 photos

I'd use a coarse thread for the insert, not UNF. The reason was explained eloquently in ME recently

Paul Lousick04/08/2017 03:21:48
1541 forum posts
578 photos

3.5mm wall thickness is not enough for 3/8" UNC (or other) blind tapped hole. Not enough threads to hold a stud.

As suggested, drill and tap 3/8" UNC all of the way thru the plate. Then make a stepped stud 3/8" UNC to whatever the original stud diameter was. You could turn down and thread the shank of a bolt to the smaller thread size.

Leave the 3/8" end of the stud long enough to pass thru the plate and secure with a nut on the inside of the stove. This will stop the new stud from unscrewing and also give it much greater pull-out resistance.


I.M. OUTAHERE04/08/2017 09:58:33
1468 forum posts
3 photos

Ian a photo of what you are trying to fix would help


Ian Skeldon 204/08/2017 10:32:42
489 forum posts
41 photos

Mmm seems like my initial concerns are well founded, I like the idea of drilling through and tapping, then using a shouldered or stepped thread.

I will try to get a couple of photos up to show the problem in more clarity.

But in the mean time, thanks again to all of you for your help.


Ian Skeldon 210/08/2017 20:32:27
489 forum posts
41 photos

Quick update,

drilled it a little deeper (must have been close to breaking through the outer face, but didn't), then tapped it in have a go style with (Duncans advice about using UNF followed) a M10 Carbon plug tap, ground to be very flat to allow it to go as deep as possible.

I am sure it is more down to luck than skill but it has worked perfectly. If it does fail in the future due to lack of depth then I will have to use some of the excellent ideas put forward.

Many thanks to everyone for being so helpful.

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