Looks so obvious but I cannot find anywhere how to change fluid in this differential. No fill or drain holes or dif cover. The only way seems to be through the pinion cover; just wondering once taking it apart what about pinion angle, will it have to be again precisely set after fluid change or not?
Assuming an 8" rear end, should have a fill hole on the back of the housing or possibly in the cast center section. does not have a drain hole so the only way to drain the oil is to use a syphone gun to get the oil out.
Its not easy to get the syphon hose to the bottom of the diff, so I stepped it down to a smaller line to barely squeeze it past the internals. Its a PITA.
Only the early '57-'59 Ford 9-inch rear ends had drain plugs in the bottom of the sump in the center of the rear end housing. You will either have to vacuum the gear oil out, or pull the axles and then remove the entire 3rd member from the housing to get the oil out and to clean out the housing's sump. If you go the siphoning route, you still won't know if there's any debris/trash or small metal particles lying on the bottom of the housing. If you pull the 3rd member out of the housing, then you can not only see what's in the bottom of the housing, but you can also check out the inside of the 3rd member and see what kind of condition the ring, driving pinion gears, etc. are in. Taking the 3rd member out will not change any of the backlash settings of the gears. If there is no fill plug on the back of the housing, look for one on the driver's side of the 3rd member --near where the driving pinion goes in. I don't know if you have a Limited-Slip/Traction-Lok differential (?), but if you do, pour in 4 ounces of friction modifier (after the 3rd member and axles have been reinstalled). After that, you'll need two (32oz) bottles of gear oil. Pour all of the first bottle in and then pour in the 2nd bottle until it starts running out the fill plug. If you do not have a Limited-Slip/Traction-Lok differential, then ommit the friction modifier.
