Mopar tapered axle rear brake conversion

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by: Centerline, Cobalt327, Crosley, Jon
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Contents

[edit] Mopar 8.75" tapered rear axle brake conversion

Click on photos for larger view.

[edit] Background information

Installed unit ready for the brake drum.

[edit] This article only applies to early Mopar (pre-1965) 8.75" differentials with tapered axles.

The prevailing opinion when anyone asks on just about any Mopar specific forum, is that if you have a 1964 or earlier Mopar, the first thing you should do is swap out the tapered axle rear end for a 1965 or later unit. Why this is the prevailing opinion is easy to understand when you consider that doing a brake job on one of these axles can be a real pain in the butt. It requires a special puller to remove the drum/hub assembly, and then you have to have a machine shop separate the hub from the drum if it needs to be replaced. The hub and drum are held together because the lug bolts are swedged (a procedure that is similar to riveting), which locks the two pieces together into one unit. This was a common practice back in the day and was used by Ford for decades. It's unclear why Mopar kept using this system well into the 1960s, but it's just something Mopar enthusiasts have to deal with.

[edit] Axle strength

First, let's dispel some of the false information that's been spread around about tapered axle 8.75" rear ends. The naysayers will claim these units are weak. This is NOT true. If these units held up behind the early '60's Max Wedge cars they sure as blazes will hold up just fine behind a stout street/strip 440 or 451 stroker. Now, if your purpose is primarily to go racing you should probably consider a Dana 60, but for general street/strip use these units are plenty strong.

That said, with any 8.75" rear end, its strength depends on which "pumpkin" (center section or third member) is used. There are three different castings: 741, 742, and 489. Each casting uses a different sized pinion. The 742 and 489 castings both use a larger pinion and are generally considered to be stronger than the 741 casting. However, all three castings are interchangeable and can be installed in any 8.75" housing, so the strength of any 8.75" rear end depends on which pumpkin casting is used and really nothing else.

The early 741 casting was primarily used prior to 1965 and is considered the weakest of the three, but that's like asking which is fastest: a Ferrari, Lamborghini, or a Corvette ZR-1. All three castings are extremely strong and for street/strip duty the 741 casting will do just fine. This unit held up behind the 413 and 426 dual quad Max Wedge cars all day long, but even if you need more strength there is no reason to change out a perfectly good differential when all you really need to do is swap out the pumpkin for a different casting. OK, so much for weakness being a reason to replace the tapered axle rear end.

[edit] Why the conversion is done

The real reason most people recommend trashing the tapered axle unit is because servicing (mainly brake jobs) can be a real pain. Well, its time someone shows how easy it is to covert the rear brakes from swedged drum/hub units to the later- and much easier to service- slip-on drums. Once this is done, the tapered axle rear end is almost as easy to service as any later model unit. "Almost", because it will still require a puller if you need to remove the axle from the housing. Other than this tool, there will be really no difference in servicing the tapered axle unit than any other 8.75" differential.

[edit] How the conversion is done

[edit] ID

First you need to identify your housing. Mopar used the tapered axle housing used from 1957-1964. If your vehicle is newer than '64, then chances are extremely slim you have a tapered axle housing. One way to tell for sure is if your axle has a big nut on the end. If it does you indeed DO have a tapered axle 8.75" differential.

[edit] Removing the drum/hub assembly

So the first question is, "How do I get the drum off this thing?" This is where that "tool" comes in. Below is a picture of the special puller required to remove the drum/hub assembly from the axle. These pullers are still available through Miller Special Tools (Universal Wheel Puller # C-844), but many times they can be found on eBay as well.

The picture below explains pretty well how the puller is used. You'll need a good sized sledge hammer as well (the first time you remove them anyway). When the hub is reinstalled, a little anti-seize will make this process much easier if it ever has to be repeated. Sometimes it takes patience, fortitude, a big hammer, some penetrating oil and a little heat to get one of these things off, especially if it hasn't been removed in 20 or 30 years. However, if you keep at it, you will eventually be successful. The trick sometimes is not only to smack the wings of the tool with the hammer (which tightens the puller) but also to smack the end of the puller to "jolt" the hub loose. One side may come off quicker than the other. Patience is the key.

Puller installed and ready to go to work


As you can see below there is literally no brake material left on these shoes. The difficulty of removing these units is why people tended NOT to service them as often as they should... and never in some cases. As you can see the hub is "keyed" in position on the axle. Although this is kind of an "old" way of doing things it still works very well. Don't lose this key...they are very hard to replace. The brakes are pretty much typical for any Mopar 8.75" rear end so there will be no surprises there.

When you do get the hub/drum off don't be surprised if you see something like this...

In this photo the brakes have been re-done and the axle is ready for the hub to be reinstalled. You can also see the key in this picture (red arrow).

[edit] The hubs

On to the hubs. When the hub/drum unit is removed it will look like the photo below. This unit is all one piece because the lug bolts are swedged onto the brake drum. This is a procedure where the hub and drum are assembled, then the lug bolts are expanded where they contact the drum. This, for all intents and purposes, rivets them together.

Drum and hub, as removed


Below is a picture showing what a swedged lug bolt looks like when compared to one that has not been swedged. The lug bolt on the left has been swedged and the one on the right has not. You can see how pressure has been applied and the lug bolt has been "riveted" to the drum. Certainly not the way to do things if you want to make servicing easy.

Unswedged lug bolt on right

[edit] Separating the hub

The next job is to separate the hub from the drum. On the surface it would seem you could just press the studs out and be done with it. This is NOT the case. If swedged studs are simply pressed out they will enlarge the holes in the hub and ruin it. No one is reproducing these, so care must be taken to do the job right. The right way is to use a swedge cutter on a drill press and remove the swedges. Then, the drum will simply slide off. An easy job if you have a swedge cutter, but most people don't. You can still find them occasionally for about a hundred bucks, but you don't really need one.

If you are not going to keep your old drums, here's an easy way to not only separate the drum from the hub but also remove the stud from the hub at the same time.

  • First, you need to cut the stud off flush with the outside of the drum. You can use a cut-off wheel for this or a hack saw; just make sure you have them flush with the drum surface.
  • Then, center punch each one and drill a 3/16" - 1/4" hole in the center of each stud about a quarter inch deep. This will relieve the stress and allow the swedged area to curl in on itself when the stud is pressed out of the hub. You need to do this for all five studs.
  • Then from this point, it is a simple procedure to press out the studs. With this done you now have the hubs and drums separated. Remember this procedure will basically ruin your drums so don't use it unless you plan to replace them.

Here's where the fun begins. You now need to press in new studs. The stock lug bolts on these old Mopars are 1/2-20 UNF thread with a 0.622" knurl. The knurl is the ridged part of the stud that fits into the hub. In order to have a good tight press fit you need to make sure whatever stud you use has a knurl size of at least 0.622", but it can't be too large either. If it's too large, it will be difficult to press in and you stand a chance of damaging the hub. NAPA has studs that have a 0.623" knurl (part number BK 641-1128) and these work very well. Below is a picture of the hubs with these non-swedged studs installed.

New non-swedged studs installed


When you reinstall the hubs on the axles, it will save you some work if you clean the axle and inside surface of the hub with a good solvent. This will get rid of any rust or other contaminants that might make it more difficult to remove the hub again if you ever need to remove it or the axle. You may also choose to coat the axle with a little anti-seize compound. The nut is torqued to 120 ft/lb and is held in position with a good sized cotter pin; this is not all that different than how front hubs are held on- and they're full of grease. So a little anti-seize can make disassembly a lot easier if the axle ever has to be removed to service the pumpkin. Below is the installed unit ready for the brake drum.

Installed unit ready for the...


Now for the best part. NAPA lists the rear brake drum for a tapered axle car (in this case a 1964 Dodge), part number ND 4401056 for $92 each. You no longer have to use these expensive drums. You can now use the 1965 and later drums, part # ND 4401129 for $37 each. All that's left to do at this point is to adjust the brakes and slide the new drums on. If you ever need to do brake service again, just slide them off... it's that easy.

NOTE: Parts pricing current as of early 2012.
...new brake drum

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