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.
On my racing engine where we remove and replace the engine all during the racing season it is easy to have the copper washer slip off in the hurry to get ready for the next race. Of course it falls into an un-seeable and un-accessible place!
I have gotten into the practice of putting a small highly visible zip tie on the probe when it is removed to keep the washer in place. Easy to remove the zip tie when it is time to assemble.
See the three pictures for further explanation.