Showing thermometer in protected housing, the thermometer screws into a 303 1" dia. holder that goes into the melt to protect the probe, I've made many such probe holders for babbit pots for lumber mills.
Looking down into the pot, the valve rod is 1/2" dia. 303, in fact all the valve parts are 303 or 304 except the bolt and roll pin. At the bottom of the pot is a 1" diameter valve body that screws into the base plate of the pot, there is ample guide area above the valve opening to ensure that the rod cannot come clear out of the valve body, which of course would be a catastrophe!
The top of the valve link, simple minds, simple designs.
You didn't ask, but here's a sideview of my snail trap. I have a 5 page set of drawings for anyone who PM's me, suitable for any plasma torch programmer to get burnouts made. 3/16" T-1 at maximum 30 degree angle to the strike, handles (Not in drawings) to lift and carry, and prong holders (not in drawings again) to hang targets on.