A bit late to the story.
To determine if you have a 7¼” or 8¼” – look at the axle tubes where they go into the center housing and look at the inspection cover.
The differential tube size (the tubes that goes into the center housing) is 2½” diameter for 7¼” differentials – which is the size of all 7¼” differentials (from ’60 to early ‘90’s). Now with that said, somewhere in the late ‘70’s, Chrysler changed the tube diameter on 7¼” differentials to 3” – but they have a distinct taper down just before the center housing. That way the ISO-clamp system can use either 7¼” or 8¼” differentials. The early F & M bodies with 7¼” use a different ISO-clamp because of the tube diameter has different spacing – but I doubt there are many of those 7¼” differentials left out there to worry about.
The tube measurements are outside measurements – even though it is hard to tell by my crude picture(s).
The 7¼” cover is shaped like a stop sign (octagon) and 8¼” is oval shaped.
All 8¼” covers have 10 bolts.
Some 7¼” covers have 9 bolts and some have 10 bolts (not sure when that change was made).
I have a few pictures of a 7¼” – but couldn’t find them, so I drew a rough sketch to demonstrate what to look for. The 7¼” is the ONLY differential Chrysler made with a step-down tube.
The only thing that will interchange between the two differentials is the yoke and brake backing plates (well, all brakes parts). Nothing else will interchange.
If you currently have a 7¼”, I would recommend finding an 8¼” (or 8¾”) and have it under the workbench until existing differential finally gives up. I have, what I feel, is the easiest procedure to change differentials in a garage (or parking lot) – if you don’t have access to a lift. Just perform a keyword search with my user name.
If replacing a 7¼” and 8¼” - the propeller shaft length will be a bit shorter (and shorter still with the 8¾”). Other than that, the two differentials will interchange without problems (. . . well, unless you have one of the very few early F/M bodies with the 2½” tube 7¼”, in which you will need ISO clamps, as well – or better yet, get rid of the ISO-clamp system altogether).
Anytime I’m working on a suspected differential noise concern, I would first get car jacked up (fairly high) and jack stands safely in place. Not necessarily in this order – but check all these items (with park brake off and transmission in any position other than park):
- At each wheel, grab tire at 3 and 9 o’clock, and push in/pull out on wheel. You should notice a very slight amount of play, only.
- Same hand position push one hand in/pull one hand out, then reverse (checking for wheel bearing play). You should notice a very slight amount of play (only).
- Move hands to 6 and 12 o’clock position and do same thing. Most wheel bearing problems will be noticed at this time.
- With a hand on tire always, rotate tire and listen to anything unusual (like bearing or scraping noise).
- Quickly start and stop tire rotation. You will feel more play, but it should only be a minimal amount.
- Repeat these steps on other side rear tire.
- Get under mid-position of car. Grab ahold of propeller shaft at front U-joint area and other hand on front yoke (if possible). Keep the hand on yoke still while using other hand to move the propeller shaft up/down and check for any play (non-should be present).
- Do same thing but this time trying to move propeller shaft side to side. Again, no play should be present. Note: there might be some play between front yoke and transmission – which I see a lot. There shouldn’t be any play there, but it happens. That is a bigger job to fix, than U-joints are.
- Spin propeller shaft (having a helper does wonders in this case) and check for any obvious kinks, dings, dents, bends, twists, etc. in the propeller shaft (none should be present).
- Go to rear U-joint and check for up/down and side/side as above, while keeping another hand on rear yoke (if possible). No play should be present in either U-joint or with yoke.
- Using both hands, move propeller shaft fore/aft – looking for any change with rear yoke and differential housing (non-should exist).
- Check the rear yoke nut is in place and appears to be tight. If that nut gets loose - it will cause a lot of clunks and other noises.
- Rotate the propeller shaft while feeling for/listening for any bearing noise (nothing should be detectible).
The pinion seal should not leak – but it is not uncommon for them to do so on higher mileage cars. Generally, when it does leak – it is more because of bearings are loose/not adjusted correctly than a seal failure. The seal just can’t compensate when bearing allows shaft to move all over the place.
If you have a leaking seal (leaking to a drip) – KEEP AN EYE ON OIL LEVEL!
Wait, did I shout that? No, well let me say it
much louder this time:
If you have a leaking pinion seal (leaking to a drip) – KEEP AN EYE ON OIL LEVEL!
Your leak will only get larger and there is not much lube in those differentials to begin with (about 2.2 pints of lube). Lose enough lube and you WILL BE stuck somewhere, digging out your tennis shoes out and getting ready to to start walking.
I’ve worked on a lot of flood vehicles, before. The important part is to keep travel to a very minimum and change fluids out as soon as possible. A trace bit of water left in a differential will not be a problem – for differentials get rather hot when in use, and any evaporated water will exit via the vent - in a short amount of time.
A couple tablespoons full of water (or more) will, I repeat, will wreak havoc on your differential bearings, in a short bit of time.