Its been drained and refilled without any joy - what would a garage do? Do they have any simple (cheap) way of telling if the head/cylinder is cracked?

I would give a couple of garages/workshops a call. I have heard of a machine that can sniff the exhaust gases to indicate a blown headgasket.

Have you checked the water pump?

Sounds like the head gasket to me, some garages can pressure test the system to check if the gasket has blown. Other signs are water in the oil, which means the oil starts to change colour to cream or oil in the water.

Thanks for the ideas. Theres been no oil in the coolant. I'm going to drain the oil and see if theres any water droplets in there (though its only done a few hundred miles since it got put back together). The oil filler screw cap is nice and clean.
The water pump seems to be fine - with the rad cap off it shows to be circulating well.
Before I start phoning garages, someone has suggested draining the rad, removing it and giving it a good soak in soapy water (he said a sonic bath but I'll have to make do with a washing up bowl, some speakers and slayer on full blast). He thinks that as the water in the expansion tank is boiling, the likely culprit is the rad not cooling properly.

Fingers crossed cos the piggy bank is empty!

can't you put some rad flush in there

I'll use that as well but he was meaning giving the exterior a good clean - inbetween all the fins. I'll try anything at the moment.

it would have to have half a field in there to to stop it cooling to this extent.

I'd suspect the head or base gasket

Do you know where the best place to order the head and base gaskets is? Do all the 3tj models use the same size - including the imports? I've seen a complete set on wemoto for~£90 - this says its for the 3tj import models 1992-92, mines a '95.
Cheers

I think the 3TJ / 4DX model gaskets are the same

Hi reverend,
Been reading the thread.
That exactly do you mean when you said you have "put it back together"?
It maybe a dumb question but you are using coolant and not just straight water ?

The thermostat serves two purposes. 1) to bring the engine up to operating temperature quickly and
2) to regulate water flow passed the cylinders. Without the thermostat in water will flow passed the cylinders too quickly and not collect the heat to transfer to
the radiator. So don't leave it out after testing. The thermostat works on the bimetal spring which relaxes with heat. To test it, suspend it in a pot of boiling water on the stove. It will open after a while but only about 5mm.

Your fan should come on before it hits the red light saying that, when the bike is moving it should be getting no where near the line.

A simple way of pressure testing the cylinders is to;
Get a old spark plug and drill the electrode out. Thread or braise a air hose fitting to the spark plug top and pump in 45 psi of air with the cylinder
at "top dead center" of its compression stroke

More informative instructions link
http://www.startinglineproducts.com/ins ... Tester.pdf

It does sound like you have a cylinder head leak.
Before changing head gaskets try re-torque the cylinder head bolts.