It's a shame you're not closer to the fens to try my seat solution.  It's a Frankenstein of mx5 rails with DIY brackets to fix a lotus Elise/exige/vauxhall vx220 seat.  The seats are hard to find cheap, and aren't everyone's cup of tea as there's very little padding, but if you're the right shape they are excellent, light, and can be made very low.

when I changed the gearbox in the Herald for the untested 3 rail J type OD box, it worked fine apart from the OD sticking so as I was about to go on a trip took  the solenoid out of the original good box and fitted it to the box in the car and it worked perfectly. Move forward a year and I thought its time to clean and fit new O rings to the original solenoid, so between playing the host with our Christmas guests I had  a short time in the garage. Looking at the faulty  solenoid I realised its missing the circlip from the front! So was it never fitted or is in in the gearbox.... 

This has been getting a decent amount of use. Over 2k miles on the rebuilt engine now and seems good. He uses it when the roads are dry and non-salty so it’s been parked in the drive over Christmas. Sorted a non-functioning brake light this afternoon. Yes, the bulb had gone. However, swapping that revealed an earthing problem on the bulb holder (surprise!), so I soldered that and now it works again.  He fixed a random rattle from the rest by tightening the boot latch striker plate. He’s also been “auditioning” the modified MX5 ex-Frankenspit drivers seat. Not sure it’s a hit. He’s too tall for it.

Did you ever resolve this? There are various versions of the Subaru CVs, which do you need?

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.