As mentioned in previous posts, the thermocouple came with an O ring. I take mine off every year when I close and lube the O ring with plumbers silicone. I have never had a leak.

Another method I have used is to carefully drill a 7/16 hole then tap with a 1/4 pipe tap, this allows you to use conventional pipe fittings & teflon tape to make connections. I used this method to install the injection port for my bleach injection system. Again ---- NO LEAKS.