Dear Blindboy,

Hope you're well. As to the grommet, I always put the thicker section inside the radiator's side-tank. The thicker rubber makes it less likely that over-pressure blows-out the sensor, which leads to coolant loss.

I presume you'll use brass wire to hold the sensor in place. Therefore, which side of the grommet goes inside - the thicker or the thinner side - matters little.

Hope this helps.

Yours faithfully,

Spook