What temperature sensor do I use in new style head?

I installed a newer style 225 slant head (1975 vintage) on my old block (1967 or so) and everything worked out just fine except for the temperature sensor. The sensor hole is bigger on the newer head so I used an adapter but only the end of the sensor pokes into the water and now it barely works. I need the proper setup, whatever that may be. Can anybody out there help?