OK. Now we'll talk a little about differentials:
The Dana Powr-Lock clutch-pack differential is commonly just called the "clutch type". This diff is virtually indestructible, owing to its four-pinion (spider gear) design. The worst thing that happens to these are worn or (very rarely) broken clutches. In daily driving, this one acts more like a locker, with some noise evident. If the rearend lube is really beat, expect some herky-jerky rearend jitter in turns. There are actually three versions of this differential, the first being the early version with the oddball thrust buttons. Nothing wrong with this diff, assuming you have the drum puller required to get the axles out to pop the center section. Just replace the existing thrust buttons (look down the axle splines--it's at the bottom of the hole) with the commonly-available reproductions. If you don't, you'll never get your wheel bearings right, I promise. The second version is different only because it already has the right thrust buttons for the '65-up cars ('66-up Imperial, for some reason). The third version uses an oddball clutch-carrier arrangement and is actually the best of the three, but good luck finding one. Per Spicer, they only made it in calendar 1968 so it was used during the last 2/3 of '68 production and in very-early '69 models during the rolling change to the Anti-Spin. All Powr-Lok diffs use the same bearings and clutch packs; the clutch packs sold by Dana/Spicer are OE replacements while the MP version is more aggressive. It launches harder but you'll notice it more in corners/turns. Don't know what diff you have? Replace the thrust buttons anyhow. It's cheap insurance. For the record, the '68 revision is a dandy. If a seller knows what he's got, it's gonna be pricey.
The Borg Warner Anti Spin cone-clutch differential is commonly simply called the "cone type". It uses one-piece, spirally-grooved cones in place of the Dana's multiple-clutch packs. The reason Chrysler went to this design early in the '69 model year is that it's much smoother in daily operation--essentially seamless and silent. While it is plenty strong, it's not nearly the stone ax that the Dana unit is. Drawbacks include a case seam that runs through the pinion-shaft bore, only two pinions, and the fact that the clutches wear directly on inside of the case. These diffs are notorious for the pinion shaft galling to the pinion/spider gears, shearing the locating pin, and ovalling the pin's bore in the case. The seam running through the case's shaft bores doesn't help this situation, and its root causes are neglect combined with abuse. The lube gets tired, and jackasses like me keep pounding the snot out of 'em with big-blocks, etc. No, I never even considered the axle lube in my Charger because it never made noise and it always burned both rears. When we popped that axle apart with 187,000(!) very hard miles on it, it shouldn't have been a surprise to see that one shaft bore was a 2"-long oval. But it was a shock, because it was so quiet and still burned both! Anyhow, if maintained correctly this differential will live a long and happy life behind even a stout big-block/4-speed while turning slicks. Eventually, the cones will machine their way into the case halves, and that's when Chrysler, Borg Warner, and now Auburn (they bought the design and patent from Borg Warner years ago) says it's time to replace. Is it? Well, yes and no. Milling the back sides of the cones will return the diff to operation, but those who made it are right when they tell you your internal gear mesh is now out of specification. That will cause problems, make no mistake. But, remember how I told you to stay tuned? I found a workaround using factory pieces several years back... in an open differential, which guys are just throwing out, there's a neat pair of thick shims behind the side gears. Well, darned if they don't fit between the side gears and cones just perfectly on an Anti-Spin! They've even got a pair of nifty little oiling holes in 'em. Simply measure the thickness of the shims and mill a couple thousandths more off the back of the cones. Factory approved? Hell no. Does it work, and maintain gear clearance? Why yes, it does! How long does it last? I can't say. I did one for a friend's very-healthy 451-inch Road Runner 12 years ago and it's still fine, but that's not a daily driver either. Suffice to say if you do this hack, it would be a good idea to start saving your pennies for a replacement diff. It's a one-time-only deal. If she starts kicking one leg with that mod, there's no bringing it back a second time.
Swapping diffs: All 8.75" cases, 1957-'74, have the same outer diameter for the differential bearing races. The Powr-Lok and the Anti-Spin have different outer diameters for the bearing cones, though. It's pretty simple: If you're using a Powr-Lok, tell the guy you've got a '66 with a 742 case and Sure Grip. If you're using an Anti-Spin (or a new Auburn replacement) change your lie to a '72 with a 489 case and Sure Grip. This is true regardless of what case you're actually using and will get you the right bearings and races every time. I'll post the numbers here when I find my Big Blue Butt Book, where I actually have all this information written down.
Pinion splines: There are two variances here, 10 and 29. All 657, 985, and 742 cases were factory-equipped with 10-spline pinion yokes regardless of differential. 741 and 488/489 cases were by and large 29-spline, but both of the Powr-Lok-equipped 489 cases I pulled were 10-spline, so my belief is that clutch-type diffs used the coarse spline. 741 cases were 10-spline until the '69 model year, when they went to 29. I've not found a '69 Powr-Lok 741 to date, but I'd bet on a 10-spline if one was built. I've never seen a 10-spline open 488/489, period. Yokes are available in both splines, large and small U-joint, from several sources if you find a gotta-have gearset and it's the wrong spline for your existing yoke. Keep in mind, the 29-spline yoke from any other Chrysler rearend will work: 7.25", 8.25", 9.25" and passenger-car Dana 53 and 60. Dana 53?! Don't ask. Seriously.
