Techniques - Removing rusty screws
HarryMann: Threaded or self-tapping screws through sheet steel or bulkheads can be difficult to remove due rusted threads.
e.g. the four screws holding the heater box to the bulkhead protruding through to behind radiator area.
Whenever screw threads protrude and rust, first wire brush when dry (long single row wire-brushes are as handy as the big unwieldy 4-row'uns), then pen-oil. If they're of the No.3 size or larger and well supported (heavy metal behind), then might try an impact screwdriver to initially loosen. Screw back in a bit, repeat wire-brush/penoil, then work them out slowly. Once they're a bit loose can spray threads from front as well.
NB. Only if can get small enough or correct bit for Impact Driver, some only have PZ3 and up
As we all know, half heartedly attacking with wrong screwdriver or not enough bodyweight behind it to start with, can be regretted in less time than it takes to find the right screwdriver or bit.
Another trick is to have someone tap the end of the screwdriver as you provide the torque. No result? Then grip better and get helper to tap harder (simulates an impact screwdriver).
Another take on it:
Scouse: Can anybody advise the best way to remove the self tapper screws which have seized, and just round off, and do you think this is my only option.
Screwdrivers vary, not just in size but quality and fit of the head, which can usually be 'felt' straight away, either it feels like it will or it feels like it won't, in which case pickup another one of the right size and try that.
Sounds like you're past that stage, so just drill the heads off with sharp drill slightly larger than your estinmate of the thread size, then when all is off and accessible, with a small sharp drill, drill the screw out of the hole. Or, wind the thread out backwards with a pair of molegrips or good quality pliers.
The problem is often that the first thing to do is not to try to unscrew the screw. It is to get access to behind it, wire brush thread thoroughly and apply Plus-Gas or a 'good' pen. oil and leave for a while. Then, try to move the screw in either direction, try slightly tightening as well as slightly loosening, alternate then brush and pen. oil again when it first moves and unscrew in stages.
It's also sometimes possible to get a 'very good quality' pair of moles or mini-moles onto the self-tapper's head, work them to create opposing flats. Again, once it moves, alternate tightening and loosening in stages.
Drill the head off
Drilling out - self tappers can be 'really' hard steel, a cobalt (Co8) drill should get the head off though, go for a size fractionally larger than you estinate the top of the thread shank. All you really need to do is get the head off first though, then can address the screw itself, might even then knock or wind out. Get to back of it and unwind with pliers, moles, wobble it etc.
NB. Cautions: can blunt your good drills this way, maybe start smaller than shank-size, get it 'cutting first' at lower speed, don't let it overheat, watch what's behind if blind!
Mini-moles can often start pan-head self-tappers moving, obviosuly not countersunks, although try if you can get at it from behind, even torque the threaded portion off, less to unscrew!
Drive head around using the technique described in General Tools Screw/nut/bolt punch (drift), maybe using a proper screw/bolt punch or deliberately blunted chisel edge. It works, specially when acces to the head with a screwdriver is poor or blocked off.
Of course, left-hand drills are availble too, ideal to help unscrew as you're drilling it out, whichever happens first... but not something you can buy around the corner.
Bombhead: If you can get it try this stuff... EZ-Grip