Replaced the radiator in my 2002 F-150 (5.4L V-8) with an after market from Auto Zone. Everything tight and dry as far as coolant connections, but my oil cooler fittings began dripping almost immediately. After three days, pulled the radiator a second time to check fittings for damage/cross threading and found no problems. Replaced the o-rings in the radiator oil fittings, buttoned everything back up. Everything A-OK until exactly one week later, when oil leak reappeared.

Should I have used some sort of non hardening thread sealant or possibly Teflon tape on those oil cooler fittings? Any other suggestions?

Thanks!

 

Any threaded joint that has the potential to leak(from the threads)should have either tape or sealant.imho

 

If it is an o-ring seal or a flared fitting no goop or tape is needed infact it's detrimental. Tape or goop is only for NPT fittings.