Hi Rob and Jelly,
I know the blue ones you can pass the holder over an open flame, It doesn't take much, then insert the stem immediately and slide it in an out.
Keep doing it till it slides tight. The ones with metal clips can be opened on the inside top.
HTH
Rob,
I really think the most important part of a thermometer is easy accurate callibration. We bought a couple of ones that looked like yours.
The settings kept changing. I don't know if yours is self callibrating but our were never accurate. The ones with the nut under the dial
were the best for us to callibrate.
panini