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Replacing lost threads

Rethreading where the thread is stripped

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David Haynes16/11/2011 21:49:10
168 forum posts
26 photos
Hi,
I have gunmetal horn blocks with 6BA threads to locate the horn keeps, but one of the threads has stripped. Whilst I could re-tap as 5BA without increasing the bolt head too much, I wondered there is another solution I have not seen. I have already tried filling the hole with epoxy filler and re-tapping, but the thread soon stripped and the cost of the thread repair kits is too expensive for just one hole. I wondered if anyone had experience with soft solder; that is maybe not as heat intensive as silver solder.
Thanks
Dave
Steve Withnell16/11/2011 22:08:38
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858 forum posts
215 photos
Could you tap it out bigger, and thread a piece of gunmetal and screw that into the hole with loctite. Do it up as tight as you can, then file flush to the original. Once the Loctite has gone off (might not need the Loctite), then drill and tap 6BA... This should be an invisible repair...
Clive Hartland16/11/2011 22:39:33
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2929 forum posts
41 photos
Plugging with a bigger threaded bush is the way I repair damaged theodolite bodies in Alu.
It hasnt failed me yet. I do use Loctite to assist the bush stability and normally I pre-thread the bush to the original thread.
For some reason the stainless steel screws and the protective anodising on the bodies do not like each other and sieze the threads.
Clive
Ian S C17/11/2011 01:32:28
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7468 forum posts
230 photos
Clive, am I correct in thinking that in older instruments they used blacked brass screws instead of stainless. Stainless and aluminium have quite a stronge electrolitic effect.
Keep away from the epoxy, or solder, and as the others say plug and redrill and tap the origional size. Ian S C
Chris Trice17/11/2011 02:16:08
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1376 forum posts
10 photos
My recommendation would be to tap larger and then soft solder in a threaded plug made of the same gunmetal before drilling and retapping. Adhesive bonds with Loctite or epoxy are fine for fixing in sturdy pieces but the forces involved in tapping and the fragile nature of the insert once it's drilled will have it stripped out in no time by the tap. Flux the thread first with plumbers flux (Powerflux in the yellow tub is good) and then tin/lead solder. It'll easily flow into the joint if you've fluxed it properly and the thread isn't 'tight'. Then you can redrill and tap with confidence.
Robert Miller 117/11/2011 04:43:46
19 forum posts
Although they aren't made in BA, Helicoils are a quick and easy - no heat or adhesives required -, but somewhat expensive, fix for stripped threads. They are particularly useful for spark plugs in aluminum (or as you lot would have it aluminium ) cylinder heads in which the spark plugs seem to have a habit of welding themselves to the aluminum resulting in the thread often being stripped when the spark plug is removed.
 
I have no personal connection, but have found them to be useful and reliable,

Clive Hartland17/11/2011 08:20:03
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2929 forum posts
41 photos
Hello Ian, Yes, nearly all the old type Theos. and Levels used Cupronickel screws that that over time became brittle and the result was heads snapping off whilst in tension on the instrument.
Then the game of removing the siezed threaded portion with out mangling the bodywork!
This coupled with the fact that the base castings were ductile anyway.
We made jigs for certain places and were then able to drill it out and plug it as described.
 
Helicoils, I use helicoils quite often as we have kit that has to be opened and closed with multi screw covers and the threads get stripped.
Once you start using them many applications arise where they save the day.
 
Clive

Edited By Clive Hartland on 17/11/2011 08:33:07

Ian S C17/11/2011 10:00:36
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7468 forum posts
230 photos
We used Helicoils when repairing the exhaust studs on the Continental aero engines that I used to over haul, that was after the engine had gone through two over size studs, or a stud had broken and damaged the thread beyond repare. Only time I heard of anyone in the aviation field having problems with Helicoils was a spark plug thread being repared with a stainless insert, the thermal resistance caused the spark plug to burn out, a bronze insert should have been used, result from the accident report, engine failure in flight. Ian S C
Bogstandard17/11/2011 13:19:44
263 forum posts
David,
 
I used to do this a lot when people were making from bronze and brass castings and drilled holes in the wrong place, and I had to reclaim the castings.
 
Don't try to make your own bronze inserts, the threads on the outside and inside are liable to crack into each other due to built up internal stresses..
 
I always drilled out the hole, in your case to about 5/32" or 3/16" if you have room, and silver solder a slug of bronze into the hole. Dress it off flush and then drill and tap for your original size and position.
 
If you do it correctly (and it is easy) you will have a joint almost as strong as the original and will never be noticed.
 
This first post on the topic shows what I am on about, you should if possible drill all the old thread out.
 
 
 
John
Stub Mandrel17/11/2011 20:50:58
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4318 forum posts
291 photos
1 articles
I have used the soldered plug method. the plug dioesn't even need to be a brilliant fit.
 
For a hirn keep I'm sure soft solder would be fine top keep the plug in as the stress on the screw is very low - I bet it was overtightening not service that stripped the thread.
 
Neil
David Haynes17/11/2011 22:58:17
168 forum posts
26 photos
Thanks folks, plenty to go on. I seem to remember the the Helicoils would be in the region of some £10s as I would have to buy various other bits as well. Iwill let you know how it goes.
Dave

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