Re-tapping damaged thread

Hi, I have some alloy type metal with a damaged M8-1.25mm pitch threaded hole going right through about 25mm long. I want to buy a tap and carefully try to get this thread usable again, but have never done this before. I see that there are bottoming, plug and taper taps. Also there are spiral or straight types. Which of these would be best for my purposes or would I require more than one? I have a bottle of CT-90 cutting and tapping fluid which I presume would be suitable to use with it.

Any advice or suggestions welcome.

Hallo Peter. I have done this kind of repair myself but I would hesitate to jump in and give you advice on it as I know that there are some very knowledgeable and better qualified people on this forum who will soon give you the options open to you. Meanwhile, it may help to know what the item is and the sort of use and stresses to which it will be subjected. As I read your post the hole goes right the way through and is therefore not a blind hole?

Mick

Peter, you say it is an alloy, is it a copper alloy ie brass or bronze, an aluminium alloy, or is it stainless steel?

If you need to retap with the 8 mm thread, I'd drill and tap the hole for 10 mm, and make a plug of the same material, and fit it in the hole with some Loctite, then drill and tap the plug 8 mm.

Ian S C

Edited By Ian S C on 04/12/2014 08:19:49

It depends on the damage - if just a little damage at the hole entry (say from trying to insert a bolt cross threaded) then cleaning up with a tap may be possible. If there is more serious damage then you have to consider how strong the remaining material will be - in which case a heli-coil or maybe a threaded sleeve could be be more appropriate (both in a larger tapped hole).

Thread repair taps exist for this kind of work - an ordinary tap will probably try to cut a new thread rather than follow the existing one. I've had some success in the past using plug tap in the absence of a repair tap, but care is needed.

If you can access the rear of the hole, running tap through from un-damaged side should be easier.

High Peter,
If the thread is stripped then trying to re tap it will not solve the problem as a tap cannot put metal back. If it is not stripped then I would go for a plug or bottoming tap (I thought plug an bottoming were different names for the same thing and a second tap was between them and a taper.) so you do not start a new thread out of step with the original as you may do with a taper if the end you start from is very damaged. Start from the least damaged end so there is less chance of starting a new thread. The threaded part on an M8 tap will probably be less than 25 mm long so try to get a tap that is ground down to less than the core size at the end of the threaded part so you can go deeper into the hole. (If you can't get this type you could probably grind the tap down yourself. I am not familiar with CT-90 so I do not know if it is suitable for aluminium or zinc alloys. (I am making the assumption the you mean aluminium or zinc alloy rather than brass bronze or other alloys.) I normally use WD40 or paraffin (Kerosene.)

Les.

Making a lot of assumptions, I'm guessing that this is a aluminium casting and the thread has 'stripped' through being over-tightened or pulled out. retapping won't put the metal back and the resulting thread will be very weak.

The best repair is an M8 helicoil insert, drill oversize, tap for the helicoil ( a spring-like insert with a thread on both insert and outside), screw in the helicoil with the special tool (which inserts it easily, but leaves it tight in the hole), you are left with an M8 threaded hole that will the stronger than the original one.

Helicoil sets typically come with the drill, tap, insertion tool and several inserts so they are about £30, but as they can save you £££s...

Here's a supplier, make sure you get teh right thread pitch as well as diameter: Tracy Tools

Neil

Again I don't know what the job is, but for light damage you may be able to use a M8 bolt. Cut a groove or two along the bolt and go slowly, unscrew every turn to clear the threads. A hard allen screw is best.

In the first instance I would run a first taper tap down it and then inspect the threads and check it with a bolt. I don't much like helecoils but it is an option provided drilling the hole out larger to accommodate it doesn't weaken the part.

I went to a local engineering firm to pick up some steel this week and the guy happened to show me an alloy casting he repaired with weld and then taped again. Partially filling the hole with weld is another option then if you can get it done.

It's not really for a newbie to jump in here but as yet another alternative I recall seeing some aluminium repair rod that could be 'melted' with just a MAP gas burner and such holes fixed. the demo vid (can't remember the site, sadly) showed the guy fixing such a thread.. by dropping a bolt into the filled hole while still hot and liquid and just unthreadng said bolt when cooled - no need to tap. And the filler material was stronger than the original alloy. I think they had something similar for steel too.

To answer Peter's specific question, since the thread is pretty much already cut, a bottoming tap is your best bet. You might want to drill an 8mm hole in a block of something you can offer up to your subject to ensure the tap is entering the hole squarely which is half the battle when cutting threads.

.... and as you mentioned it was a through hole, if you can start from the undamaged end, that would help. Don't worry about spiral flutes. Just a regular bottom tap is fine although a good quality one is preferred.

If the M8 hole is only stripped half way through the 25mm section ( or is only tapped half way through) then re-tapping with a standard size M8 tap(s) and using an extra long bolt may be the answer . It's unusual to have a 25mm deep hole completely tapped through for an M8 thread . Normal practice is approx. 2D deep for say aluminium.

If you can shine a light through the hole this may help you to decide what to do.

I've had good experience with Helicoils but they do need to be put in properly. If they are good enough for aircraft parts they should be OK for most applications.

I've used High tensile steel inserts in VW engine cases without any problems ( ( 1/2" BSW cut down H.T. bolt with M8 tapped hole)

Edited By Dinosaur Engineer on 04/12/2014 21:02:26

Wow!! What a large and informative response. Thanks very much everyone.

The threaded hole is in a mount for a small telescope. Very much like a pan and tilt head for a camera. The hole is for the tilt arm handle, which is about 300mm long to screw into, but one end (the end to screw it into) is damaged/cross threaded along the first few turns, but the latter half of the hole has good threads. The metal is some sort of fairly soft lightweight (Chinese?) cast alloy. I can use the mount reasonably well by actually pulling the telescope around, but it would be nice to use the tilt handle instead. At a push, I could just drill out the cross threaded section, and if necessary, epoxy the tilt handle into the hole. I just thought that it might be best to try re-tapping it first. The tilt handle is "proper" steel and the thread is undamaged.

I have found the CT-90 cutting and tapping fluid works great when drilling holes in steel, so thought it might aid the tapping of this alloy.

So my problem is not too critical, and I have been managing without the tilt arm at all. Re-fitting the tilt arm would bring the mount back to its original condition and make it nicer to use.

Thanks again.

Depends how damaged it is but an easier solution might be to screw in a greased good quality long M8 screw from the undamaged side to just reform the thread.

Helicoils are the best engineering solution (IMHO) but a bit OTT for something like this.

Russell.

Edited By Russell Eberhardt on 05/12/2014 08:05:23

Sounds like you just need to run a tap through it from the good end if you can.

Hi Peter,
I imagine that the thread on the tilt arm is not 25 mm long so here are two more suggestions. 1 Extend the length of the thread on the tilt arm so that it screws in further and thus enters the undamaged part of the thread. 2 Just drill the damaged thread out to 8mm clearance (8.2 to 8.5mm) to allow the plain part of the tilt arm can enter this plain section ad the threaded part engage with the undamaged thread.

Les.

I imagine that the part is similar to my tripod for my Camcorder, about a foot long and the part it screws into is split, so that you can give the handle a twist to lock the camera head. On my one the thread only goes half way through, and could be extended.

Ian S C

Most permanent solution if possible would be to use a stud and nut to put the wear on the stud and not on the threaded hole.

Peter,I had success with exactly the same repair. I used lard (pig's fat) as a lubricant and fed all three the taps from a set of hand taps through from the bottom (back side) of the work piece . It worked like a charm.