This was submitted by Lance Thomas of Alberta Canada (Abandon Metal Heaven). I think that is has much merit.
With a stud or bolt of which the head has sheared off due to corrosion or damaged (stretched) threads....
#1) However elementary, Please Take notice that a bolt/stud
almost always breaks off reasonably flush with the surrounding surface.
Example: In the attempt to remove the heads from say a 32 to 53(54) flat motor, some of the studs will back out and some of the nuts will loosen easily - but often we are left with studs that may have, over time - adhered to the surrounding cast metal of the block and under no circumstance, are they going to back out in one piece...
As a journeyman machinist with a welding ticket to back me up, I have run
across this situation countless times in the past 30 or so years, here is a tip that in practice will almost always back a broken fastener out of a cast iron block (or any cast machine part) without fail and will work (with minor adjustments) on cast aluminum as well-in less time than it takes to read this message.
Given- -in practice, I have found that the smaller (diameter) the bolt/stud is, the easier it is to twist off during disassembly-but by definition, the smaller the broken
stud is, the easier it will be to remove as well.
First - determine the diameter of the broken fastener: if the stud/bolt was say 7/16ths to begin with, then go to your bolt bin and find at least two 7/16 nuts for every broken stud. *Make sure that they are NOT cadmium or zinc plated (that the nut doesn't appear to look silver or gold).
Secondly - With a pair of vise grips, Hold ONE nut directly over the top off the broken stud as if to simulate where a bolt head would reside if
the broken stud had one. This will be as flush as possible with - and seated on the block as to simulate a bolt head. As mentioned earlier, most fasteners will shear off within +/- 1/16th of the blocks deck height and given these tolerances we will have plenty to work with.
Using a MIG welder, (you don't need to be a journeyman welder to do this, just handy with a mig) we will attempt to weld the inside diameter of the nut to the broken stump of the stud. This sounds far more difficult
than it actually is....
Using vise grips to position and hold the nut reasonably flush, carefully tack the inside of the nut to the broken stud, then stop and check for alignment. You can gently tap the nut into position remembering to keep it reasonably flush with the block's deck height.
Now weld up the inside of the nut - aiming your weld at the broken stump. Take care in concentrating the heat and weld AT the broken stud, NOT at the nut. This will assure that you transfer as
much of the puddles heat into the broken stud as possible (helps release the stud) and as you build your puddle up, it will automatically self weld to the nut. If your nut gets too hot, stop for a few seconds to let it cool as you do not want to melt or distort the hexagonal shape of the nut. This also allows more time for the heat to transfer further in. If you do by chance melt the nut beyond recognition, don't worry just start over with a new nut as the extra heat you transfer into the
block will not harm it, it will only assist in freeing the stud.
Do not worry about accidentally welding the nut to your priceless cast iron block as it ain't gonna happen, the reason being is that the process of welding the high carbon content of a cast (high grade steel) iron block (or any cast machine part) requires highly specialized techniques and materials that make it far immune to your standard Mig wire. Guaranteed.
Now, In practice you have just welded a new head to the
stud and should be left with what now looks like a standard bolt.
ALSO in practice, I have found that the smaller diameter a broken fastener is/was, the less heat it will require to release the stud from its surroundings, that is to say... When you now put a wrench or socket on your self fabricated "bolt" and attempt to unscrew it, chances are that a smaller (3/8 or less) stud will back out EASILY on the first attempt.
IF your newly formed 'bolt' head breaks off on
it's first attempt to back it out, don't worry. This just means that you either tried to back it out before it cooled (was still RED hot) or you didn't transfer enough heat into the stud. (Or possibly you may need to practice your 'nut' welding technique a few times on some scrap pieces). Most of all, don't get discouraged, as even a basic rod builder or back yard mechanic can master this in minutes with just a little practice (and in the mean time impress the heck out of your buddies).
Besides, In most cases it is very common for the nut to break free of the stud on the first attempt (this is why I had mentioned to find at least 2 nuts per broken stud), As generally the heat associated from the first attempt is seldom enough to heat and expand the broken stud thus freeing it from its surroundings.
I have used this technique on everything from 1/4 inch aluminum transmission pan bolts (ever try welding the inside of a 1/4 inch nut?) to 7/8 X 10 inch long D9 Cat
Headbolts and even in the case with a 7/8 X 10 inch long head bolt, it has worked every time without damage to the internal threads or surrounding metals.
*NOTE. Although physically possible, don't attempt to use a stick (rod) welder as you WILL almost certainly cause a stray arch/flash THAT WILL forever damage the cast iron and change/compromise the cast/grain structure of the surrounding block/metal. Don't attempt this with a torch either as you will be concentrating the heat far
more on the surrounding metal of the block and not on the stud. Use a Mig only.
*NOTE. One of the reasons we do not use a zinc or cadmium plated nut is that it is far harder to properly weld/adhere molten metal to coated nuts (read weld) until the zinc/cadmium plating has burned off thus releasing toxic (and in the case of cadmium, deadly) fumes. NEVER ATTEMPT TO WELD CADMIUM.