It is always a good idea to start with the smaller bit and carefully expand the hole if you need to.

First thing is yes the ITC-308 probe will fit in the SS Thermowell. Second the mm is how long the thermowell is. I bought the 100 and it's 4 inches into my pots. Third there may be many opinions of where to install the thermowell but truth is measuring in the same spot constantly will allow you to make adjustments to your process. Hence having a thermowell installed. I have 16 gallon pots and my thermowells are at the 6" spot from the bottom. Buy it install it and you won't complain.

The probe appears to be a standard 1/4” probe and should fit with no problem. I have the 10 cm thermowell which protrudes far enough to get a good measurement but does not interfere with stirring. I don’t use the racking tube so removed it and used that hole for the thermowell. If you want both I suggest putting the new hole at the same height but 1/4 of the way around from the racking tube again to avoid interference with stirring.

Yes the probe from the Inkbird Wifi ITC-308 fits perfectly inside this thermowell. The 100 mm length is suggested for the SS Brewtech 7 gallon Brew Bucket which places the tip of the thermowell nearly exactly at the center of the volume. You can drill and install your thermowell in any location you want but if you want to replicate the BME drill the center of the thermowell port 4.5" up from the point where the cone meets the body aligned on center with the ball valve. I found that if you want to brew smaller batches (say 3.5 gallons or less) then locate the thermowell port slightly lower based on the smallest volume of liquid you plan to ferment at one time. Measure twice cut once and Bob's your Uncle!

The 100mm is about 4 inches long (Depth of the well) and the 300mm would be almost 12" deep. I mounted mine just below the 1/2 way mark on the side of the fermenter (I have a Blichmann fermenter so it is split in the middle making it impossible to mount right at the mid point)

Hello Ann we carry the replacement o-rings. They are 16mm x 1.8 mm N90 O-rings.