oil cooler

I'd try liquid wrench

 

You must use a 1/2 inch air driven impact on the cap bolt. What you are trying to remove is not a banjo bolt holding that banjo styled fitting in place. It is merely a cap with a brass washer which must be removed to tighten a 13mm allen head bolt which lies underneath. The air impacts quick torque will jar the cap loose without breaking the soldered fitting. Although it may appear awkward this can be done without removing the oil cooler.

Note: You must use an allen hex key of adequate length afixed to a socket preferrably with a roll pin attached to a ratchet handle to accomodate working in such a close quarters area. It is sometimes difficult to find a 13mm hex key socket. You may have to buy a 14mm and surface grind each side to get to 13mm.