I am in the middle of a mini-saga on the DIL's Seat Altea 1.6 petrol. (fwd transverse) It failed the MOT on "blue smoke under harsh acceleration" and "emissions". It had been using oil for a bit. Anyway, I checked the compression and all pots were top notch so decided to remove the head and do the seals. I know they can be done in situ but the previous owner/s had never looked after it and the gunge in the head was hellish. The engine sits slanted towards the screen and the bolts holding the head on (10 in total torx headed) on the exhaust side were a pain just getting the torx into the head. The gunge had hardened and had to be dug out with a screw driver. Even when loose I could not lift the bolts out without levering them and then had to use a hammer to get the torx bit out of the bolt head! Now I could not get the exhaust manifold off in situ as two of the lower nuts were very tight and I left the lambda sensor in to remove on the bench. As it was a smoker I wanted to check it for blockage although I reckoned I would have to change it. It made the head heavy and lop sided but got it out and finished off removing the manifold. Whipped the manifold in the vice and got my set of Lambda sockets out. Size 22mm, three different choices and all a waste of time. I then modified a 7/8 in ring spanner to fit by cutting a slot out to allow the spanner to fit and still no joy. I tried heating it with my inducer jobbie. No joy. Then up to the top workshop and got the gas on it . No joy. All I did was manage to round the head of the hexagon. At least my ring spanner got some grip. So I then decided that I will be getting a new one and cut the cable to allow a socket on it. I have a set of those sockets designed to grip rounded nuts and they generally work. But this time I decided to freeze the blighter with an aerosol I got on-line. And at last bingo off it came. It was tight and had to be screwed all the way. The thread on the sensor was damaged at the end but the manifold boss is quite good. What I would want to do is give the thread a run through with a tap. And here is the question to the heading. What tap do I need for these sensors? With a metric thread gauge it seems to be 15 which I assume is a pitch of 1.5 as the gauge has 10 threads and the length of the teeth are 15mm on the gauge, Now the internal bore of the sensor mounting measures 16.2 mm. Now with a BSW gauge its a 16 3/8in that fits. Both gauges put atop each other are exactly the same!