18mm. Just had to replace the 02 sensors on mine. Had to buy a box of 12 nuts! And a 18mm bolt to plug the old location.
Should be between 15mm or 18mm
18mm
m18x1.5 is the threading for Honda civic/del sols, and probably most other o2 sensor bungs
The sensor will be round and about the size of a silver dollar. The sensor can be found beneath the passenger side dashboard.
18mm
The engine size is needed to answer your question.
i think minimum size is 18mm
Use a tap and die set to do a rethread to the next size down.
1/1 is the o2 sensor on the engine size of the catalytic converter.
18MM
Before you mess things up really bad get a tap for the size threads on your O2 sensor and clean out the old threads with it, being careful not to really cut new ones just clean out the old ones.
Which sensor, please ask a new question with year, make, model, engine size and a description of the problem.