150 mm, 1/4" male BSP thread. The upper fitting threads into the lower fitting, compressing the probe against the internal PTFE sleeve, creating the seal.
Use a permanent marker to mark the spot on your old probe where it came out of it's fitting. Make sure to compare the length of your old one to the new one, in an effort to keep the water level where it was prior to removing the probe. These can safely be cut with wire snips to any shorter length.
The auto-fill probes on Simonelli equipment are bent prior to installing into the side of the boiler, so be sure to keep your old one so you can match the shape of your new one to it.
If your existing auto-fill probe is installed in your sight glass remove it from there and exchange locations with your anti-siphon valve: if scale builds up in the lower pipe supplying water to the sight glass, it may block the pipe. If this happens, whatever water that is left in the glass will remain there, giving the probe a false reading as the actual water level decreases.
WARNING: This product can expose you to chemicals including lead, which are known to the State of California to cause cancer, birth defects, or other reproductive harm. For more information, go to www.p65warnings.ca.gov.