Radiator valve?

As Peccavi has said the heating system will empty. The value you speak of is not made anymore and the replacment value is totaly different. You would be better changing the value as folows.
1. Turn all rads off both sides check hot press for gate values on the heating flow or return close it. Turn value off at pump.
2 Air lock heating system by blocking vent and bung cold feed to heating system (small tank roof space)
3. Have small dish and bucket ready undo nut at rad collecting water in dish if you have air locked the system you will only get about half a bucket of water.
4. Change your value try and get a value with same connections to rad.
Job done very important to open vent and cold feed first then open pump value then cylinder then other rads then heat test.

N Ireland Plumber
April 2009

"How much water will spill out if I close the lockshield valve and take the thermostat off to fit a new one." (?)

All the water in your central heating system that lies above the open valve will pour out and the header tank (if you have one) will keep trying to fill the system up so there will be lots and lots of water - it will run for weeks - years maybe.

Just take the broken top to a plumbers' merchant and ask if you can just buy a replacement top - if the answer is NO buy a whole new valve (£10-£12 or so) and throw the bottom piece away and use the plastic control piece on the existing valve.

Alternatively engage the services of a plumber - £50 or so.

Peccavi
April 2009