The little sealing copper washer is crucial to sealing the temperature probe fitting in the head.
If it is missing, no amount of teflon tape or other stuff will seal the fitting!
If you are having problems getting the probe to seal, check to make sure there is a copper sealing washer in good condition on the probe.
Read more at …… https://www.britcarfixes.com/shop/hints/temperature-probe-sealing-copper-washer/
MGB tips mods and maintenance 