After Diagnosing the Check engine light as the front sensor, I heated up the exhaust and tried carefully to remove the sensor by working the threads back and forward.

It still came out with the threads stripped.

I read somewhere that all Oxygen sensor threads are Aluminium. Is that correct?

The threads in the exhaust box don't seem too bad, so I need to know what the thread is to buy a thread chaser.

Tool shops only seem to be selling 18mm X1.5 or 12mm chasers.

Measuring an undamaged part of the sensor thread where the washer had been, gave about 17.4mm across the threads.

What is the correct size to buy please.

18mm spark plug chaser

Thanks for that information.

The legacies seem to have different sensors according to the model.

Is there a parts list somewhere to check the part number for a 2005 2.0i  EJ202 engine?

https://parts.subaru.com/

 

But that's for North America. You could try http://opposedforces.com/parts/ as they have European listings

Edited April 8, 20187 yr by Mike104

Thanks a lot Mike, The European link is the one for me. My Legacy is a 2.0i, which is not available in the States. 

Yours start at the 2.5i.

Paul