If it’s not hitching or pinking then it’s probably happy combustion-wise, but might be getting rather scary exhaust gas temperatures increase the risk of exhaust valve problems and possibly HGF. Difficult to know without measuring the temperatures…… My Vitesse will run to mid 16s at low cruise (60ish) without hitching but I don’t run it beyond mid 15s. With GTs plasma plugs we could get it to 18:1 before it hitched. It didn’t drive nicely though.

Is it not the same as a standard drain plug? e.g. NPTF 3/8 I'm pretty sure it is on a 4-cylinder at least.

The four season Valve is available in the States from RockAuto for apprx £50+ Or from Amazon for a little less. The Taxi FX4 valve is good if you have no easy place to screw in the valve. But the screw in version is excellent on the TR range of engines.   Roger

Not offhand but I will find it….. It’s a taper thread iirc.

Yep. And the welds haven't even stuck as they're tiny little tack welds trying to stick to a vice 🙃 Helicoils are a good idea. First job will be to try and extract them to see what the threads are like. The main issue is that the heads appear to have mostly departed company with the shank, meaning there's nothing to turn them by. Not terribly difficult to sort, but it's somewhere in the bottom third of a long to do list at the moment...

No need whatsoever for a welder anywhere near the vice. 

It is also possible to sort the screws if the threads are completely gone, I fixed mine with helicoils. https://www.engineeringsupplies.co.uk/516-x-18-bsw-thread-repair-kit.html Phil

Thank you, Biturbo!    Certainly, a hand held die grinder can act homicidally, so that the minutiae of workshop practice are a lesser concern.    It seems to me that a more human reason explains the difference when it cuts nicely on one side of a hole, but climbs out, or cuts in the wrong direction on the other, and it's our (my) muscles.    The hand and wrist is a complex tool, designed for grasping a tree branch and adapted to hold tools in a wide range of positions, but not very rigidly.    The reaction to the cutting action of the grinder comes in various directions, some of which are not those where the hand/wrist's strength is greatest.     So when I find this happening, I change my grip- sometimes it works! John

Nick, do you know what thread side is in the head for the heater valve take off pipe?

I must admit on my spitfire (OK, it is "only" 25 years old! 16V so a more modern design) I run 15.5 up to 3500rpm which is 80mph or so in 5th. I have been working on TPS readings, about 15% is where I start getting a bit more serious with the fuelling, setting to 12.5 at 80-100%  When I connected an AFR to my Vitesse, it runs at high 15's at about 60-70mph, I need to have another go at the needles with a file. It doesn't hitch though, nor pink. So who knows if that is OK?