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Does anyone know what thread size the Stud - Tank Mounting is?

Why? did you break it?
Off the bat, no I don't know the size, but if you have the wreckage of the old one, take it around to an engineering hardware supplier such as:
Blackwoods, Coventry Fasteners, United Fasteners, Bolts & Industrial Supplies etc. Drop it on the counter and say: One of these in high tensile, thanks mate"

Not so much broken it, just need to change the gaskets and will put some of the aviation gasket you told me about to ensure no leaks. While doing that I will follow Hasbeen’s advice and change the bolts to studs to make the fitting easier. I think the thread on the oil pump to block is 5/16 and if these studs are the same I can just buy two of them, save shopping around.

I have a spare bolt though, so could go and see the guys you mentioned and get some from them.

I believe them to be metric M10. Mine are currently away for galvanizing so can't check them
You can check their size in the BL workshop manual.
There's a complete list of all bolt/thread sizes in there

Confused here, are we talking fuel tank to body mountings or fuel pump to block mountings?

The fuel pump to block bolts are 5/16" UNF.

The fuel tank to body fixings are M10 but they are special "studs" anyway.

Yep, the fuel tank "bolt" is a double ended stud, screwing into kept nuts in the body.

One suggestion before you try to undo the nuts holding the tank straps, locate the inboard ends of these 4 studs, 2 in the boot/trunk, & 2 in the cabin, behind the trim panel behind the seats. There is enough thread to fit lock nuts to these.

If you don't do this, the nuts securing the tank straps won't undo. The stud undoes from the chassis, with the strap still clamped, winding it into a useless pretzel like thing.

Ask me how I know.

Hasbeen

Originally I was talking about the studs to the fuel tank. However, with them M10's and the bolts to the pump being UNF, it will not work.

Hasbeen wrote:Yep, the fuel tank "bolt" is a double ended stud, screwing into kept nuts in the body.

One suggestion before you try to undo the nuts holding the tank straps, locate the inboard ends of these 4 studs, 2 in the boot/trunk, & 2 in the cabin, behind the trim panel behind the seats. There is enough thread to fit lock nuts to these.

If you don't do this, the nuts securing the tank straps won't undo. The stud undoes from the chassis, with the strap still clamped, winding it into a useless pretzel like thing.

Ask me how I know.

Hasbeen

Yes Hasbeen, I know just how you feel, this is the straightened out remains of what came off my car. Rusted straps was the original problem but there was no way the nuts would come off, I just had to let them twist up and keep going. However the studs unscrewed from the captive body nuts really nicely.

My gas tank studs were still bright metal but as I undid the nuts the straps still tended to twist (a function of friction between nut and strap). I used a long handled adjustable pliers (that I picked up at a discount store for $8) to grasp the edges of the strap by the nuts (which still allowed a socket on an extension to undo the nut) thereby preventing the strap from twisting until the nut was free of the strap. Not sure how well this would have worked on rusted nut/bolt but it's the only chance you've got to preserve both.

FI Spyder wrote:My gas tank studs were still bright metal but as I undid the nuts the straps still tended to twist (a function of friction between nut and strap). I used a long handled adjustable pliers (that I picked up at a discount store for $8) to grasp the edges of the strap by the nuts (which still allowed a socket on an extension to undo the nut) thereby preventing the strap from twisting until the nut was free of the strap. Not sure how well this would have worked on rusted nut/bolt but it's the only chance you've got to preserve both.

That idea was not an option for me as you can see from this picture, and YES, that is fuel leaking from the tank!

So the strap was acting as an alternative tank seal

dursleyman wrote:That idea was not an option for me as you can see from this picture, and YES, that is fuel leaking from the tank!

No worries about twisting the fuel tank straps there.