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Plugging and Tapping

Cold Repair Technique for a seized or stripped thread

OK, well here is one of those awkward little problems often encountered working on an old vehicle. A sheared stud, screw or stripped thread..

This is the subject of this little example, it is the left hand exhaust manifold from a Range Rover V8. Made of cast iron, these things are pretty robust, but the problem with this one is the flange studs for the down pipe union. As you can see in the picture, one of them has sheared during removal, the other two are in a sorry state.

I think that some-one has probably forced metric studs into imperial holes here, but either way, they have got to be removed, and they don't want to play ball.

So what do you do?

Well the first step is to try and unscrew them in the normal manner. Two nuts on the stud, tightened against each other then try undoing the lower one. It SHOULD unwind the stud.

Unfortunately, in this case, that resulted in the lower stud shearing, and on the other two, the threads were so weak that they just stripped.

So, step two, attempt to remove with an eccentric stud extractor, or more usually a lot of force on a pair of mole grips or pliers!

Again, in this instance such attempts failed.

So what else can we do?

Well the next way to tackle the problem would be to try using an EZ-out, which is a sort of tapered anti-clockwise tap. Idea is you drill a hole in the sheared stud shank, then screw the EZ out back wards into it. Then when the taper in the EZ out is putting more force into the stud than the stud is putting onto the threads of the casting, its stops screwing itself in and starts screwing the stud out.

These work reasonably well on mild steel studs where they have sheared by being over tightened, but when the stud is severely rusted like this one, I know from bitter experience I'm as likely to end up with a sheared EZ out in a hole in the casting as I am actually removing things!

So, alternative methods have to be found. This is one such method and is known as drill and plug.

STEP 1

The first thing to do is to cut the studs down flat to the casting, so that you have a flat area to work to.

In this case, the casting is iron and the face not that critical, its not a sealing face or any thing, so I've decided, for speed, to  just lop them off and flat them down with an angle grinder.

If the casting was aluminium, or more critical, then I would take it nice and carefully and cut them as close to flush as possible with a hack saw, and then flat them down with a hand file.

Step 2

Now, before we start getting destructive, we need to know where on the casting the studs need to be, so right now is a good time to measure up the locations of the holes and note them down. Better still, if you can make a template for where the studs are, because we are going to drill them completely away and fill the holes with solid metal again. I simply use a bit of paper and a wax crayon, and do like a brass rubbing, or even a bit of cornflake box cardboard and a hammer (I RUB it over the surface, I don't hammer cardboard!), to get an impression.

Any way, next job is to start drilling the studs out.

First mark and centre punch the middle of the stud. Don't worry if its not absolutely central, by the time you have walloped in a big plug that little bit wont make too much odds.

Then start by drilling a pilot hole.

In this case I drilled the pilot hole in all three studs straight away.

Because I am drilling a pattern of holes though, I wont drill them all the way out to the size I want just yet, because I'm going to use the remnants of the old studs as datum's for where I want to put my hole for the new stud.

STEP 3

Old indian trick, if you want to drill a hole, start with one of your smallest drills, then having cut a small hole, gradually enlarge it using the bigger drills until you get the size you want.

Reason is that drills cut best at the corner of the 'bit' furthest from the centre.

If you use a big bit straight away, you will be trying to cut across the whole width of the bit, and you will be there for an eternity.

Do it in stages, say 1mm or a couple of drill sizes at a time you will get a much neater, more accurate hole, more quickly.

And on the same note, if you are drilling to a 'tap' size, then try and drill your last hole before the size you need as close to the needed size as possible.

I actually have a set of tap size drill bits that I keep ONLY for drilling tap holes. Reason is that they are more accurately cut than standard drills as well as often being weird sizes.

Standard drills, even if they are the size you need to drill a tap hole, will often drill slightly over size, so I always stop short, and then finish with the tap drill.

In this example, I wanted to drill the hole out to 10.5mm, which is the tap size for an M12 thread. So I started with a 3.5mm drill and worked up, 4.5mm, 5.5mm 6.6mm, 8mm, 9mm 10mm, using standard drills, then 10.5mm with my tap drill.

STEP 4

Time to tap the hole so that you can screw a bolt into it.

Right, if you don't know, a tap is like a cross between a twist drill and a bolt. Basically is a very clever cutting tool, and you screw it into a plain hole to cut a thread.

In this instance I am using a 'pilot tap', which is the one you use to cut a shallow thread to begin with.

As I want a tight hole that I'm not that bothered about getting my bolt back out of, I shan't be using a 'finishing tap' for this hole.

Any way, carefully load tap in to tap stock, grease, then align in hole, and turn until you feel the blades bite into the metal. Then 1/2 turn forwards, 1/4 turn back to break and remove swarf. Take your time, don't try pushing the thing through the hole or winding it like you are trying to start an aeroplane, and pretty soon you will have a nice neat and serviceable threaded hole.

STEP 5

So we have a nice threaded hole that we can screw a bolt into. Only thing is we don't want a bolt, we want a 'plug'. And we don't want it unscrewing.

Fact that we only used the pilot tap should mean that the bolt will be a bit of a tight fit in the hole, but even so, we want to be sure, so I have Araldited the bolt into the newly tapped hole.

Actually, I've used 'Metal Set' which is like Araldite, only it has stuff added to make it hard like metal and give it a lot of properties like metal.

But, Araldite would do, as would a good 'Loctite' type stud lock - but I'm more comfortable with a big gooey visible resin, so that's what I use.

So, mix up a small amount, and apply to both the treads of the bolt and the threads in the hole.

First making sure to clean all swarf and any crease left during cutting out with a thinners rag.

 The bolt can now be screwed into the hole.

Now depending on how tight the hole is, the bolt might be a bit tight from the start, or it might be quite loose at the top.

Either way, don't screw it in all the way so that the head is against the casting, leave some of the thread protruding.

Because it has to be cut of and faced flush, just like I did the studs to start with.

This may sound a bit silly, because we are back essentially to where we started.

But, what we have actually done is perform a standard 'plugging' operation.

Now if we drill and tap into the casting, we don't risk having an over lapping 'shadow' hole with just a sliver of old stud in it.

If we drill into the casting now, even if we drill on the joint between plug and casting, we have a big chunk of metal that wont wobble or move and allow the stud to pull out of our new hole.

STEP 6

So with the plug in place, it's now time to mark the location of the new stud hole, and centre punch it for drilling

Remember, I made a template of the casting face before I started drilling, and I used this to mark the location of the first new hole.

That is why I didn't plug all three holes at the same time, because other wise I would have lost the datum's for my template.

By leaving two of the old studs in place, I can locate the template over them to mark where I need to put the one I have just plugged.

Then, when that one's cut, it will serve as the datum to locate the template so that I can mark where the other two were after I have plugged them.

So this it the bit where you HAVE to be accurate. Which is why I took care and used my sharp little auto punch to mark the hole centre, rather than using a hammer.

STEP 7

Pretty much like steps 2 & 3 here.

Start by drilling a pilot hole on your mark, then opening that up in stages to the desired size.

Only main difference is that the plug was going to screw into an M12 hole, and our stud only needs an M8 hole, so instead of drilling out to 10.5mm, I only needed to drill out to 7mm.

Note that while 7mm is a standard drill size, I took the hole to 6.5 with standard drills, then did a final cut to 7mm with an M7 tap drill.

STEP 8

Pretty much as step 4, using a tap to cut a thread in the hole you have just drilled.

This time though the hole is smaller, so I'm using a smaller tap.

And, because I want to screw my stud into this hole, and stand some chance of getting it out again at a later date, I cut the thread with a 'pilot tap' to start with, which put a shallow thread into the hole.

Then I used a 'finish tap' which cut the thread that bit deeper and parallel all the way down, to give a nice fit with the stud.

And that's about it. On the finished artefact, you can see the location of the plug, and how it gives plenty of metal around the new hole.

You can also see the shadows of the other two studs waiting to be drilled out, which having done the first one completely, to give myself a datum to work from for the other two, I can do at the same time.

So, all I have to do is repeat the operation for those to studs, and that's that, Job done. One old manifold made as 'good as new'. Simple, hugh?

SUMMARY

A 'Drill and Plug' repair is a very useful technique, and doesn't need to be limited to old exhaust manifolds. It could be used almost any where a threaded fastener has seized or sheared in a casing or another component, or even where a lug or boss has been damaged, and you need to repair or replace material to an artefact.

In this instance I used a screw in plug, made from an M12 bolt. But, depending on the problem, a 'wound' or damaged area could be drilled out and filled with weld or braze or even glue, depending on the material and what strength was required of the finished component.

The tapped plug technique I used here though does have a number of advantages.

The first one is that it is strong. Provided that you have used a big enough plug, and locked it with a suitable adhesive, or even weld, or braze, then the plug will usually be as strong as the surrounding metal.

Next, it is a 'cold' process. The casing didn't need heating or welding or anything to make the repair. This can be handy if the component you want to repair is made from say thin walled aluminium that might be a real night mare to weld. Or if you don't have any welding equipment.

Which is the third major advantage. This technique does not need any particularly specialised equipment. The two main tools you need are a drill, and a tap.

Most people have a drill, and you can get a good tap and die set for about 20. They are easy to use, long lasting. Don't require any consumable supplies, and are incredibly versatile, so a worth while investment to any-ones tool kit.

As far as spanner ratings go, this is not beyond the scope of a novice amateur. The most demanding part of the operation is drilling accurately. But if you take your time and take care and drill your hole out in steps, that shouldn't be too much of a problem. But, use some discretion. My example was salvaging an old iron manifold, and there was not much risk of doing any major damage to the component, or much need for high precision accuracy. If it was a smaller plug, and on something more critical, it might be better to get some practice on some bits of scrap first - like everything, its common sense.

What's important here is the principle, as much as the practice. It's is fairly simple, basically you gut grind or file away the metal that's no good to you. You then drill a walloping great hole where you want whatever you want to be is going to go. Fill the hole with a plug of new metal to give you a working area, and then drill and tap that like the original thing was, and put a new fastener into it.

But it doesn't necessarily need to be a fastener; imagine that a control linkage lever has got a pivot hole in it that has worn over size over the years, you could drill it and plug it and put in a new pivot hole. Possibilities are pretty endless.

And, there is an important refinement of this principle known as 'heli-coiling' I ought to mention. It's not quite so DIYable, but of you are contemplating a P&T repair and thinking 'this looks tricky' it may be a salvage you could get a local machine shop to do for you.

Its generally used on on softer metals, or where there isn't very much room to put in a big plug, but the principle is the same, you drill and tap an over size hole, then screw in a wire insert called a heli-coil, that sleeves down the thread so that your smaller, original size screw or stud fits it. Its commonly used on aluminium cylinder heads where the spark plug threads have been stripped, or in aluminium cases where electrolytic corrosion has seized a stud in a boss, but the boss isn't really big enough to take a large plug, or plugging with aluminium wouldn't be strong enough, and plugging with steel would cause metal compatibility problems.

 

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