Techniques - Removing seized/broken studs/bolts/nuts

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HarryMann: This is a general account for removing rusted/broken nuts/studs/bolts, 
 and not specifically written for cylinder head bolts or exhaust studs.

Penetrating oil then stillsons/molegrips

For penetrating oil use plus gas or rustbuster. WD40 doesn't usually work as well.

Drilling-out is often a last resort and although with practice and the right tools can be done succesfully pretty well 100% of the time, it is always preferable and frequently possible, to first try to remove a broken bolt or stud using any small part remaining proud (as little as 1/4" is often enough). If it's an exhaust manifold stud/bolt of course remove the manifold first to give a larger protruding length.

But not before acknowledging that you're in a spot and need to make your job easier first (wire brush any part protruding behind a flange for instance, penetrating oil, heat if poss.) Apart from custom stud removing tools, the two I usually rely on are a range of Stillsons, and a quality Mole Grip (e.g. not Chinese made!). By adjusting the Stillsons a bit loose and working back and forth, a small flat soon develops which can be grabbed by tightening them up - always watch carefully for the first signs of the stud moving, immediately wire brush and pen oil again and take it back gently to its original position, moving it more and more each time until like magic, you're getting half a turn either way.

Recently, I broke off all 3 studs on an exhaust silencer flange (not uncommon!). Two I removed with Stillson's/mole grips reasonably swiftly, technique as above; the third, broken completely flush and in the most inaccessible spot of all, I just managed to get a drill onto and although it took an hour or so, ended up with a good clean thread ready for a new tailpipe to be bolted on. These were all M8's. M5's would have been a different proposition in that area, but wouldn't be found on an exhaust! I also managed to drill out an M4 recently in a distributor without removing it from the car - fiddly, but can be done (started with a 2.2mm drill, then a 3 got it). The limit for a powerful battery drill would be say M12, depending on how deep the thread goes, bolt hardness and how much time you have I suppose. Do not use a large or heavy rechargeable drill for very small drill-bits, if you have a good lightweight drill/winder with a quality chuck, use that for drills below 1/8".

Stud extractors

Screw Stud extractors I'm not a great fan of left-hand-threaded tapered stud extractors (easy-outs). If they break you could be in serious trouble (they're hardened) and once you've drilled the hole, might as well open it out and get the thread out anyway. They also cannot be used on very small screws, studs, bolts. Where they might be useful is very long studs, but be warned... see other comments and See Also's below though

Also see £4.29 screw extractors

Also see cam type stud extractors for when then stud does not snap off flush used without drilling the stud

I'm still not impressed with either above two, alright for simple stuff, but when the going gets tough, they can both let you down IMO :-)

Flame Heat or MIG nut welding

These days, small MIG and arc welders have made it possible to quickly weld a nut to a broken stud or bolt

but this doesn't guarantee it won't then break further down, requiring drilling out - a good technique though, esp. as the short burst of heat can often loosen up the thread for you. Send the links in the Other Links section for more detail

Oxy-acetylene equipment isn't something you usually find in the home mechanics garage, especially now with both expense of it and the H&S police! However, small cannister propane burners are available, some giving quite a controlled and fine flame. So if you do have a source of high heat available, or can take the part to one, getting the seized nut or bolt almost cherry red (safely), then undoing when cooling is mostly succesful and being the quickest method, used routinely by professionals in garages and engineers workshops. Exhaust centres make regular use of this method.

Hacksawbob I have used a small gas soldering iron successfully on a number of occasions these kits cost around £30-£40 pounds and are worth every penny in my eyes. propane blow torches are available for plumbing work but I have found them unwieldy and difficult to get exactly where you need it when working on brakes and engine parts without risking damage to rubber pipes etc. These pen sized blow torches get the flame right to the nut, bolt or nipple and I am using it as matter of course and any bolt after applying plus gas and leaving for an hour at least. It may seem like it takes longer but, if you plan in advance and put the gas on the day before you will improve your chances considerably that the nut will remove with about 1-2 minutes of heat and gentle but sustained pressure from a decent six sided ring or socket spanner.

Drifting/Punching/Shocking/Impact drivers

Shock horror - this can work! Starting with the use of multiple shock torques from a compressed air (or electric impact driver) as used in garages/tyre centres, this is an obvious thing to try, although as with a straight spanner torque the bolt/stud may still shear off - so as stressed above, always start by cleaning up the area with a stiff wire brush and soaking with penetrating oil or Rust Buster (see below).

Electric/12V battery impact drivers can now be had too - JCB do one that I've used, portable with a small motorcycle battery, works well on large high torque stretch bolts e.g. the Diesel pulley bolts, flywheel bolts. Especially where you are having trouble stopping a rotating assembly rotatating without damage from jamming it up e.g. crankshaft/camshaft/hub nuts etc.

A technique used often by old-hands on medium to large nuts/bolts is to attack the periphery of the fastener with a hammer and chisel in a controlled manner. The combination of a shaped drift, a club hammer and the correct approach can be very succesful e.g. on threaded fasteners such as drain plugs, countersunk Phillips/Posidrive screws in brake hubs, which are difficult to address in a conventional manner once seized due to their shape..

Here is a Wiki link to such a shaped-drift and it's correct use - get out of trouble easily, if you've got a bench grinder and a few old punches around, make one...

Bolt/Nut punch (drift)

On smaller fasteners such as allen bolts/driveshaft bolts a centre punch used in a similar way is always worth trying before the big push comes using the allen key i.e. pre-shock/loosen

N.B. It's the combination of shock/torque and an inward force all at once that combines to loosen. Get that combination right in a few repetitive blows and success can come surprisingly quickly. If you end up giving up, always then try the spanner, molegrips or stillsons - like as not you'll have loosened it anyway!

Drilling out

This is necessary when:

a) a stud or bolt has broken off in a thread hole, or with insufficient protrusion to allow even moles or stillson's to gain (enough) purchase;

b) the thread is rusted to the point of fusion inside the hole, so will shear off anyway (cut it off nicely rather then wait till it shears horribly at the flush surface)

c) you like practising your drill centreing and drilling out techniques even when you might get a stud or bolt out an easier way!

First clean the whole area up so that:-

a) you can see the extent of the stud and estimate it's true centre
b) there is reasonably flat face at it's centre (file, rotary burr)

With wickedly sharp pointed centre punch... Centre punch stud face spot bollock centre - VERY LIGHTLY If looks dead centre, go a bit harder in same place or move the centre pop to true centre by angling punch and then finally righting again for last pop. Inspect and be happy it's as good as you can do...

Very sharp (newish) quality 1/8" (3mm) drill unless stud/bolt is smaller (those new boron ones are great, even the cheap Silverline ones).... Cobalt drills are also v.good, but good sharp HSS drills have been used for years, just requires a bit more push to get it cutting in harder steel, and alwasy be aware of cutting speed, too fast or too slow is not as good as when it sits ther chipping nicely, so var speed a bit to find when it starts chipping nicely.)

Drill in as far as bottom of stud (estimate) - dead straight - keep stopping and checking drill angle in both planes - don't rush - spit on drill to keep it cutting and cool. Paraffin is a good cutting agent, nearly as good as spit!

Then open out in small stages until you are about out to the thread core diameter (see Tapping drill sizes) - at which point, you should be able to pick the thread out, collapse it or get behind it (scriber) or jamb a screwdriver in there and actually unscrew it! [whatever method appeals]*

With the one you've gone a bit off centre on - spend time trying to correct it with angled drill (and flanks of drill), then as you go up in size each time, try to re-centre the hole as best you can... as soon as you see the threads first appearing at one side of hole, stop, and see if you can either unscrew it (jamming sharp screwdriver in there) or collapse thread inwards with a fine small chisel and pick it out. A scriber can come in handy too, but depending on what scrim thickness is left in there, it can usually be unwound out, or picked out in bits - I now usually don't go in to hit the core diameter (good accuracy required) but try collapsing it or unwinding it out when there's still a fair bit of meat left in there (but the stress has nearly always come off the threads, maybe the heat of drilling helps)

Trick is to have a good range of quality HSS drillbits and when you get near thread core size, open out in very small stages (unless you've done it dozens of times before, when a bit of derring do gets the better of you!)

It is also possible to buy Left Hand Thread drill bits, which can start to loosen the stud/bolt as its actually being drilled out available from:

Uni-Thread Ltd. Marldon House, Love Lane, Marldon, Sth. Devon, TQ3 1SP (01803 559595) Suppliers of helical wire isnerts, taps and drills

or.... LH drillbits

Had to drill four out on this manifold, none of which I could really see well to centrepop due heat and rust. Luck was with me, mostly, especially this one, straight down the middle and the drill seemed to spin it up and almost out. At this stage drove down the edge to collapse it inwards and 'oicked it out using a scriber. All 4 needed re-tapping after running out to a tap-drill size, very carefully, as this JX manifold is Cast Nickel Steel. Used 3-in-1 oil and a lot of backing the tap out, cleaning and moving on'

DrillBoltOut 01.jpg DrillBoltOut 02.jpg

  • can use centre pop to tap it around anti-clock to see if it will loosen. It's a matter of breaking it's seal, so as it starts to collapse it should start turning - usually have to pick out some bits left in there - a scriber is good for this, failing that a long oboe nail held in mole grip jaws. Obviously, a taper tap of the right size is perfect for finishing off the job, failing that and if the thread is a bit damaged, get a good hard bolt, file or saw some grooves along the thread length (triangular file is good for this) clean it up by running it up and down a nut, deburr it well, and that will be great for cleaning and tidying up the internal thread.

Oh, and luck, confidence and perserverance!

Don't rush, keep inspecting - change technique if going wrong - rushing is deadly!

More... Don't break a drill off in the job - if you have to angle to re-align, gently does it, modern drills just don't seem to be able to be bent like good old Presto HSS. Don't push the drill hard unless you know the pilot hole is dead true, use the speed control if the drill has it, slower sometimes cuts better! Spit on the drill tip if its gets hot, or won't cut. Try another drill, re-sharpen, anything to get it cutting.

If you do break a drill, it may still be possible to loosen and wiggle it out - but don't count on it!

NB. HSCO (cobalt) jobber drills make drilling out old, heat hardened or rusty studs and bolts much easier. A worthwhile investment say, a 3 or 3.2 and a 4.4 and 6.8 for an M8

Approx. thread core size is tapping drill size, don't exceed, see linked item:- Tapping drill sizes (ISO Metric)

See also - Another article (illustrated) for drilling out studs and bolts

Good luck...

Tex Ritter When you drill and tap an ordinary hole you have three taps available:

The first taper, second taper and finally a plug tap. In your case you will not be able to use the first taper, or possibly the second (unless you grind off part of the taper length) on the damaged threads, therefore you'll have to rely on being able to use the plug tap to clean up the existing hole.

If done carefully, you can drill and tap to a larger size without 'going through' the sump. As it's aluminium you should get away with using the plug tap to cut the new thread.

An 'Easy out' is a broken stud extractor device used to insert into the hole you have drilled into the remains of the stud, it has a left hand twist to it, so in theory the harder you twist the tighter it should grip on the shoulder of the drilled hole.

In my experience, the parralel extractors are the best for this job, but if you only have the screw type, and the hole is large enough, then they should suffice.

Spark or Wire Erosion

windysurfer: There is always away to get a snapped stud or bolt out of a hole.

I worked in engineering for years and used to use a spark eroder to remove broken bolts stud and even taps. Bud Would it not be cheaper to get a local engineering firm to erode your cylinder head stud out? Shouldn't take more than a couple of hours to do.

There a basic explanation at this link spark erosion

I did this for a living for a year or two along with wire eroding which works in the same way. I removed just about every type of bolt, stud, tap and dowel from anything and everything.

Helicoils and Timeserts (to be written)

Other Links, Rust Buster, thread inserts, EZ-Grip


I've had some good results with Rust Buster Eastwoods Item Code N150

HM:Maybe a couple of good results and lots of times not helped at all... think it depends on being very corroded rather than very tight (seized), the former may work, latter not.

Two other good links...

also see

Also see (annealing/hradening heat rustbusting methods)

Thread inserts


Left hand threaded custom stud removal drills (Frosts)

Impact stud remover (Frosts)

Frosts Tech article on several fastener removal tools

General EZGrip