Just did this yesterday, after the oRing came in at Advanced Auto. Got a deep 27mm socket at Lowes, didn't want to have to make a return trip if a short socket wasn't deep enough!
I was able to get the cooler unit off and back on without removing the pipe, but it wasn't overly easy. ViceGrips were AWOL from my toolbox...
Advance Auto had 2 different parts for Oil Cooler Gasket/Oring. One looked like the seal from an oil filter (square edges along ring), the other was an Oring with two little tabs on the inside of the ring. I went with the Oring since it looked more like the old ring except for the tabs. (surely I won't regret this, he tries to reassure himself)
Now, on to the NEXT minor outpatient surgery for my NB
