Mazda Diff and Rear End Swaps
There are two types of Mazda Differentials, the smaller M type and the larger P type. The M type differential is fitted on many small low power cars and trucks such as the miata and the B200 where the P type is fitting on higher power more heavy duty vehicles in general. There are some exceptions though.
|77-84 Ring & Pinion Ratios|
|Model||Gear Ratio|| Pinion|
M / P
|84-85 RX-7 GSL||4.076||…||…||…||Unknown|
|86-93 Ring & Pinion Ratios|
|Model||Gear Ratio|| Pinion|
M / P
|B2200 MT / AT||3.909||…||…||No||M|
|86-88 RX-7 5-spd||4.10||…||…||…||M|
|86-88 RX-7 GXL||4.10||…||…||Yes||Unknown|
|87-91 RX-7 T2||4.10||…||…||Yes||P|
|88 RX-7 GTU||…||…||…||Yes||Unknown|
|89-90 RX-7 GTUs||4.30||…||…||Yes||Unknown|
|86-88 RX-7 Auto||3.909||…||…||…||M|
|B2600i MT / AT||3.727||…||…||…||P|
|B2600i 4×4 MT||4.300||…||…||…||P|
Ok, if you have found a differential that will bolt in, you're swap will be fairly easy. You will start by getting the truck in the air and letting the fluid drain. while that is draining you will need to remove the drive shaft, both rear wheels, drums, and brake assemblies. Once you do this you will need to remove both drive axles to allow the differential to come out. This is done by removing 4 nuts on the flange for each axle.
Once the axles are free you can now unbolt the differential housing. This is also sometimes called the 3rd member, pig snout, or pumpkin. There are a lot of bolts here so it might take you some time. The differential will be heavy so it might start to work it's way out as you loosen the final nuts. Because of this I usually leave three evenly spaced nuts on making sure one of them is on top. When you're ready to let the diff out loosen the bottom two and carefully loosen the top one while supporting the differential. Once it's free it should pull right out and you will have an empty axle housing.
From here if you are using a complete differential assembly that will bolt on things will get a little interesting. Most interchange differentials will have a different pinion length, especially with any RX-7 or Miata differential. This will require the drive shaft to be shortened before it can be reinstalled. In the picture below the differential on the left is from a pickup, the one on the right is from an RX-7 Turbo II.
Going this route gives you a couple of benefits first being the shortened driveshaft. Since the driveshaft will be shorter you should be safe running a one piece instead of a two piece this will allow things to be a bit lighter and removing the extra u-joint will help free up a little power. The differential will be a bit heavier but most of this weight is dead weight from the casting and the longer pinion is much smaller in diameter than the differential so the power consumption there shouldn't be much different. Any of the differentials from one of the sports cars that have independent rear suspension will also have a location for a mount on the differential housing. Although you don't have to mount anything there you can use the stock differential mount intended for the donor car to make a mount to the frame and prevent you're axle from “wrapping up” when you accelerate hard from a stop. If you're driving with a stock engine this probably won't make any difference but if you've dropped in something else or installed a turbo and are making more power it will surely help keep that power going to the wheels instead of getting eaten up by drivetrain loss. The cons of going this route are that there will be the extra expense of shortening you're driveshaft and you are going to have to reset you're pinion angle. Once you have the differential back in and bolted up everything else is just a matter of reassembling everything you had to take apart.
Sources and Credit
Mazdatrucking.com - Website is no longer up
Thanks to Limequat for much of the information on this page! Salvaged by @reyxil