Nearly frozen bolts

I agree. A lot more heat quantity is required so the bolts actually gets hot. A soldering iron will only luke warm it no matter what the wattage of the iron. A small gas point flame may provide enough heat to soften and permit easier removal.

I could probably bring an impact driver to bear but I think the problem is now broken Loctite clogging the threads. Luckily at ground level.

Thanks guys.

Naked flame isn't an option because of surrounding paint and components.

Have noted the extractor and will return to the fray tomorrow.

Could you place a few pieces of fibro cement around the work site in order to protect the paint and components?

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Yes "gbc", but it is necessary to allow the bolt to then cool before attempting removal.

To extend the explanation of my scheme of removing valve seats, the method was to apply heat directly to the inserted seat which had the effect of burning the sealant and also expanding the seat which applied pressure to the sealant residue. Of course, the valve body surrounding the seat also became heated and would expand somewhat but not as much as the seat.
It was imperative then to allow the whole lot to cool to ambient such that the seat shrank back to its original size and could then be undone with very little effort.
The technique worked every time.
Initially we used an oxy torch but refined it to the welding current which better controlled the process and concentrated the heat to the seat.

Well here's the washup.

All out without stripping a Torx socket or shearing a bolt - but it was close. Probably because I'd gone out and got an extractor.

What I think helped:

Repeated to and fro.
Walking away when the leverage felt like it was getting excessive. I think there was some kind of settling going on.
Heat.
More leverage.

Maybe the heat expanded the bolt and helped crush the locker.

Extending the leverage from the key length of about 9" to double that led to one loud crack and shudder and I thought I'd torn it but no. The extra leverage was only needed on one bolt.

I did try a cordless hammer drill and the clutch slipped.

The amazing thing is that only about a cm of thread had been treated with the locker.

New hex head bolts have gone back in with breakable locker and a spring washer.

Thanks again for your advice gents.

Would it have helped with a clogged thread, not just a frozen one?

I've found that they will not strip the heads out of fixings but provide much better turning force than a standard cordless drill. definitely the tool of choice for your job.

Thanks.

I looked at my manual one but the chuck wouldn't take my Torx bit.