18mm. Spark plug anti-fouler makes cool O2 bung, but I usually end up cutting most of the threads off, then chasing the ones that are left after welding. Shannen I think my fire's out... Bruce Plecan wrote: > > GM O2 sensor, what size are the threads?. > 14mm is too small, 16 or 18?. > Bruce > Got 4 irons in the fire, two of which are about hot. >