Greater Midwest Classics


By: Keith Wickenhauser

Getting your stuff:
• 3.21 comes from a 66 or 67 tornado or a 67 Eldorado and has an 8 bolt cover on it. Make sure you get the shaft and bracket as it’s a different size.
• For the final drive housing gasket I used Napa FPG RDS30094. You might want an extra in case you wreck 1.
• I cut my own final drive pan gasket using gasket paper.
• I ordered my bolts from McMaster-Carr part number 91271A644 (3/8”-24 Thread size, 1 1⁄4” long). You will need 12, although these come in a package of 25. You will also need 3/8” lock washers.
• Also have a good quality 3/8” drive socket 12 point 3/8” socket as this bolt requires a lot of torque.
• You will need a good quality 3/8 drive 12 point 7/16” socket for CV joint removal.
• You will need to pull your tranny dipstick, so you may want an O ring for replacement while it’s out.
• 7/16” x 5.25” bolt. Grade 5 or better.
• We put new seals in, mine did not have vent holes. The seal were SKF 27SS2420 and National 9568.
• 2 quarts of differential gear lube. I used Lucas 75W-90 synthetic. Make sure whatever you use it is for final drive, otherwise you can buy a differential additive.
• Blue thread lock
• Quality calibrated torque wrench.
• Time; this project took Adam and myself about 6 hours.

Nice to have:
Final drive jig to attach to a jack. I borrowed mine from Jonathan Roche.
• MAC S147 differential wrench. We used Adam Libby’s.
• Ground down 5/8”, 6 sided 3/8” drive socket with a wobble extension.
• Another person to help.

• Pull transmission dipstick out.
• Remove battery cable as you will be working near the starter.
• Jack up the front end and secure into a safe position. Do not raise higher than your jack can lift the final drive. Make sure to block the rear wheels as you will be disconnecting the final drive from the wheels.
• Remove the final drive cover to drain oil and give more clearance.
• Clean the surfaces where the differential and the drive shafts mate. I used a wire brush on a drill.
• After removing the shafts, using bungee cords hook the shafts (that are still attached to the wheels) forward and up so they are out of the way.
• Clean all the areas you will be working by not to contaminate you internal parts.
• Remove the vacuumed modulator from the tranny to drain transmission fluid. Then reinsert the vacuum modulator. (I skipped this part as I pulled the tranny pan for a filter change anyhow)
• Turn the steering wheel all the way to the left, or right.
• Have someone hold the breaks as you loosen the constant velocity shafts using a good quality 3/8 drive 12 point 7/16” socket.
• Remove the right output shaft.
• Remove brackets.
• Unbolt the mount between the engine and the final drive. (5.25” bolt)
• Remove the bolts that hold the final drive to the transmission. 7 bolts and 1 stud.
• Put the jack up to the final drive and have it ready to support the final drive as you remove it from the transmission.
• The top bolt is difficult to remove. It can be removed easier with the long curved wrench. It can be a nut or bolt depending on the coach. Removing it from above the engine with the special MAC wrench was not difficult.
• Note: there should be one stud that stays in the transmission. You will just remove the nut.
• Lower the final drive with the jack.

• This would be a good time to inspect the plastic governor gear that is the tranny. Replacement can be done at a later time if needed.

• Install the gasket on the transmission using gasket adhesive to let it set up.
• Take the cover off the 3.21 differential to allow clearance room, and also you may need to turn the gear to make sure the splines line up.
• Make sure all your mating surfaces are clean, there is no contamination or potential for contamination to the differential internals. Also make sure your new output seals are installed.
• We installed a second stud to help line things up. (See picture). Then we removed it after getting some bolts started.
• Lift the final drive into place with the floor jack. Be patient as you will need to finesse the component into place without cutting your gasket.
• Center the seal before tightening the bracket.
• Transmission is aluminum, finger tighten to start making sure you threads are good.
• After this reinstall as removed.

• We torqued the CV axles to the final drive flanges using 3/8” 12 point bolts to 65 ft/lb using blue thread lock. These are not OEM bolts and this is not the manufacturer’s spec. Original bolts may be sourced from GMC suppliers. OEM has 7/16” head. It is not recommended to reuse old bolts, although some people do.
• Remember to check your tranny level when done.
• Fill the final drive to 1” below the fill whole, otherwise it will leak out.

• Shown is the housing above the final drive. I was unable to get the 7/16” x 5.25” bolt in time so I used a 6” bolt with a spacer. Also I didn’t get the housing with 3.21 so I had to modify the one I had. Modification was done after the final drive was bolted tight to the transmission.