Bill,
If chaimv has an original 1951 216 or 235, and that sender is the later 235/261 electrical sender, it is made for a larger hole in the head (1/2" x 14 NPT). If chaimv has the smaller hole, would there be an adapter available?
chaimv,
What year engine do you have in your car? If it is late early 55 or older, the hole will most likely be too small for that NAPA electrical temperature sender.
What is the eBay seller's username? It is hard to believe that the gauge set is NOS if it is for a 1951 car (unless the car engine used an electrical sender/gauge).
Tim
