Trans Oil Cooler uses a 9mm x 1.5 o-ring. 9mm is the same as 3/8" or a -6AN (http://assist.daps.dla.mil/docimages/A/0000/0004/1956/000000077235_0000 00123996_LLLRQMYLYT.PDF ) See Page two of the PDF. For size conversions.