3.21 FD

It should also be noted that you should not fill the FD with oil up to the filler. If you do, it will push FD fluid out where ever it can. Those of us that run a 3.21 have found that fill to 1" BELOW the filler. Some of us have actually welded in another filler that is 1" below and to the right of the OEM filler. Then fill through the OEM until it starts to run out of the lower filler. Also while welding in another filler, I welded in a drain at the bottom of the FD cover to make fluid change possible without popping the cover off. Be careful of where you put it relative to the ring gear so nothing can hit on the inside.