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I’ve just been and had another look at my barrel, (it’s on the bench). The centre of the hole on mine is just shy of 3/8” from the gasket face, the thickness of the casting between that face and the water space is about 1/8″ give or take a flake of rust or two so if your barrel has been opened out to 12mm you are running out of metal.

All is not however lost if you can do a bit of silver soldering. I am suggesting that you make a 1/8″ BSP bush with a diameter that will go into the ruined thread and silver solder onto this an oval flange, (oval, just ‘cos it’s pretty), previously curved to match the profile of the barrel plus a soft gasket and drilled for a pair of screws. You might think it advantageous to smooth the area of the cylinder casting around the hole but a thick soft gasket should do the trick. You will of course need to drill and tap a pair of holes into the block to match those on the flange.

A little tip here for silver soldering operations: If “Tippex” liquid paper if painted on those areas where it is desireable that the solder does not run it will create a most effective “dam” even when everything has got red hot. No, it don’t make no sense to me either but it works!

It would of course be possible to turn a “top hat” and file and bend the “brim” to shape but most commonly available brasses are “short” and will weaken or even break if treated thus. A scrap of 1/8″ (3mm) brass sheet however will not and it would be infinitely easier to get a fair curve on before soldering it to the bush.

That said if the ruined hole is 12mm then there is enough metal for a 60% thread if re-tapped 3/8 BSP, however the O.D. will then be 0.656” and 0.375” – 0.328” = 0.047” and that ‘aint a lot! It is always possible to move the centre of the hole “up” the block and even offset the new hole to retain its relative position but there is only 3/16” or so of wall thickness so you are getting into the area of needing special, (fine pitch), threads if you were to go much larger in thread diameter. On balance I’d avoid that route.