How do you change the fluid in a 8" rear diff?

Discussion in 'Classic Mustang Specific Tech' started by 65shelby, Jul 18, 2008.

  1. I've been looking for step by step instructions on how to change the gear oil in a 1965 Ford 8" rear end and have not been successful. I think the process must be different than for a 9". Can anyone give me the steps or direct me to a link where I can find them?
  2. 1st drain fluid.

    2nd safely dispose of said fluid.

    3rd open bottle of new fluid.

    4th slowly pour in untill barely leaking out

    :nice: :D

    Im not positive but i think there is a little rubber plug on the front side of the 3rd member for oil IIRC :dunno:
  3. The only way to completely get the old oil out is to drill a hole in the bottom of the housing, let it drain (I recycle all my old oil.........I use it to burn yard waste and treat Mange on the neighborhood dogs.:nice:) Thread the hole with a pipe tap and plug it before adding fresh oil. If worried about metal filings, pass a magnet up thru the hole to remove the loose stuff.
  4. The problem is I'm not sure where the drain plug is. I see the fill hole on the back, but where do you drain the fluid from? Is there another drain hole somewhere?
  5. There is no drain plug. You have a couple of options.

    Option 1:

    Remove the rear axles and remove the center section. The fluid will come out when you remove the center section from the housing. Replace the gasket and reassemble. Refill via the fill plug in the cover.

    Option 2:

    Remove the rear axle ubolts that hold the rear end to the springs. Remove the drive shaft. Remove the rear brake line. Remove the fill plug. Rotate the pinion straight up and drain out the fill plug.

    Option 3:

    Remove the fill plug. Connect to one of these:

    I wanted to post a picture of this but its huge.

  6. The Ford 9-inch rear end first came out in the Ford trucks and passenger cars in 1957. ALL '57 model 9-inch rears came with a 3/8"NPT drain plug in the bottom of the housing sump. MOST of the '58 & '59 model 9-inch rears came with drain plugs, but some didn't.

    By 1960, none of the 9-inch rears came with drain plugs --only fill plugs located on the driver's side of the 3rd member casting near the input driving pinion. In later years, the fill/sight plug was located on the back of the rear end housing.

    The 8-inch rear end came into production in 1962. None of the 8-inch rears were ever equipped with a drain plug from the factory. They only came with the fill/sight plugs on them and were located in the same places as you would find on the 9-inch rears.

    You can try sticking a siphon tube in through the fill plug to vacuum the old oil out. However, with this method, you will not get all the old oil out and if there are any metal shavings in the bottom of the housing, you most likely will not get all of it out.

    The best way to drain the oil, if you don't currently have a drain plug, is to pull the axles and remove the 3rd member from the housing. This will allow you to get all the old oil out, check for debris or metal shavings in the housing, and to thoroughly inspect the inside of the housing, sump area and the condition of the 3rd member, ring and pinion gears, etc.

    Once you have everything cleaned up inside, you could drill & tap the sump to accept a 3/8" NPT pipe plug and be assured of getting any and all shavings out before buttoning everything back up.

    An 8-inch without a factory Limited-Slip/Traction-Lok differential will require (2) bottles of gear oil to fill it. Pour it in through the fill/sight plug until it just starts to run out the hole.

    An 8-inch rear with a factory Limited-Slip/Traction-Lok will require 4 ounces of friction modifier as well as (2) bottles of gear oil. Pour the friction modifier in first, then follow with the gear oil.

    This link mostly for 9-inch rears, but there are parts number references for 8-inch as well:
    Replacement Parts
  7. I've been looking all around but can't find it. I have a 1999 mustang gt and i can't find the fill plug. it desperatly needs to be changed. can anyone put up a pictures of where the fill plug is?
  8. Look on the driver's side near the front of the rear end close to where the drive shaft attaches.
  9. my 8" in my 68 doesnt have a drain plug... you have to pull the 3rd member off to replace the gasket. Pull the axles... drive shaft.... all the bolts on the 3rd member and then make sure you have a jack for the rear end "its heavy". I can swap out my 3rd member in 45 mins now. The hardest part is putting the 3rd member back up. Have you ever pulled an axle?
  10. I use a cheap (under$10) oil pump that came from the local parts store. It can screw on to wide mouth oil bottles and has a manual pump that squirts about 1oz at a time. Attach a bit longer hose and suck the oil out of the housing thru the fill hole. Also works great for manual trans & transfer cases.:nice:
  11. The poster said he was looking for the fill plug on a '99 Mustang GT. The '99 (SN95) Mustang would have an integral 8.8" rear end. He would have to remove the rear cover to let the oil drain out, but upon re-filling it, he would have to remove the fill/sight plug located on the driver's side near to where the drive shaft attaches.

    --The only rear ends that came with factory drain plugs [3/8" NPT], were the '57-'59 Ford 9-inch rears.