Keel bolt #3 - Drilling, tapping

While I’ve decided to continue drilling I wanted to try yet another approach while the bolt is close to surface: to drill smaller hole in bolt’s core and try tapping it for M8 or M10 bolt. That would give me another chance to pull the keel bolt.

Piloting the hole with smallest drill I dared - 4mm - and enlarging it to 6,7mm for M8 tap took me an hour. Unfortunately - it is hopeless. The bolt did not rust evenly so I have a portion of good metal core on one side and rusty bit on the other side. No way a thread will bite into it and hold during pulling.

Trying to drill on healthy part is too late - tapping bit will not reach that far into the hole.

Anyhow - I have now 1cm deeper pilot hole which would make it easier for 14mm drill to bite. Then I will need to steady continue grinding through the bolt.

Still no electricity - and I cannot use oil lamps as I use compressed air to dust away metal bits. Why “compressed air” container is in fact propane-butane mixture is beyond me. Probably easier to liquify than air but no open flame allowed in vicinity.

Once we have electricity back in boatyard I can try yet another option: freezing down the bolt to -50 C and trying to crush it with impact drill. That method should be safe for surrounding oak - unlike electrolysis method or pouring phosphoric acid into the hole. First I will heat up the bolt with soldering iron and then apply freeze spray. That should make a thermal shock and make metal brittle enough to crush with impact drill.

Sources for freeze spray, in Sweden:

Maston TECmix Coldspray (-55 °C) :
http://carsystem.se/produkter.asp?refnr=1646&action=single&parentrefnr=1633

Kall spray E-COLL (-45 °C):
https://www.esska.se/esska_se_s/Kall-spray-400ml-lokal-kylning-till-45-C-E-COLL-907570602590-16590.html,shoptype_set=b2c