This is intended to inform you of the tools most probably needed to remove and replace the rear axles (AKA, half-shafts), or their boots, on your Miata. It is also intended to be used with the shop manual. It is not intended to be a used as a standalone guide.
You will need to obtain new axle nuts prior to your installation; do not re-use the old axle nuts.
Optional and strongly recommended:
The axle nuts are torqued to approximately 180 lb-ft. It is tempting to use ½" drive tools, and they might work, but, during my axle replacement, I split a ½" drive socket and others have broken ½" drive breaker bars. Not only is this expensive and frustrating, it can be dangerous to you and by-standers! Dont try to save a few bucks - use the proper (i.e. larger) tools. The money spent on them gets you some nice tools and it is still cheaper than a garage.
Break the axle nuts loose while the car is on the ground. Use the hammer and round-tip chisel to bend the compressed portions of the old axle nut out of the groove in the axle. A pneumatic impact wrench will have the axle nuts loose in about 5 seconds each. A ¾" drive breaker bar will have them off in a little bit more time. A ½" drive breaker bar may leave you nursing skinned knuckles and driving around to parts stores looking for a new ½" drive breaker bar to replace the one you broke and to buy the ¾" drive breaker bar and socket you should have been using in the first place. After the axle nuts are loose, put the car up on jack stands. Remove the wheels and the axle nuts. Note: If I remember correctly, the brake calipers need to be dismounted only if the hub must be removed from the car because of a stuck axle.
On the early models, (90 to 3/95) unbolt the four bolts holding the axle to the stub shafts on the differential. On versions after 3/95, the shafts pull out of the differential itself. For the later models, you will have to remove either the top or bottom bolt that holds the hub to the upper or lower A-frame. This will allow enough room for you to lift the hub and pull the axle out of the differential.
If you are lucky, the axle shaft will pull out of the hub with gentle pressure or just a few taps of a soft-face hammer (brass, for example). If the axle is stuck, the best choice is to use a pneumatic air hammer to drive the axle out. I do NOT recommend banging on the end of the axle with a large hammer in an attempt to drive the axle out; it will mushroom the end of the axle and quite probably destroy the threads. You also have to worry about your safety while swinging the hammer and attempting to strike a very small target.
(Caution: Use hearing protection when using an air hammer because it is extremely loud. Use eye protection to prevent metal splinters from destroying your eyes.) Use a round-tip chisel in the air hammer and, applying constant pressure, you should be able to drive the axle out in 30 seconds or less. If you do not have an air hammer, remove the brake caliper and tie it out of the way. Remove both lower and upper bolts that fasten the hub to the A-frames. The hub with the stuck axle will come off in one piece. Take the assembly to a shop and have them place the hub in a vise for support and then use an air hammer to drive the axle out. Alternatively, a hydraulic press can be used but it is overkill.
At this time, on the early models, you can remount the hub to the A-frames. For models after 3/95, mount the hub to only one of the A-frames you will need to maneuver the hub for clearance when re-installing the axle.
On 90 to 3/95 versions, the axle outer end will slide into the hub and then fasten to the stub shafts on the differentials. On versions after 3/95, the axle will slide into the differential first and then the outer axle end will be fitted into the hub.
When re-installing the axle in the hub, use a thin coating of grease to prevent corrosion of the grooves. The fit is very tight so you may need to use a mallet to tap the hub onto the axle end. (Caution! Dont strike the hub bearing itself! Doing so can damage and destroy the bearing!) If it seems to be stuck, see if you can get several axle threads to extend beyond the hub enough so the new axle nut can get a good "bite" of 3 or 4 threads minimum. Be careful doing this - too few threads and you can strip them. Once you have the axle nut on, use a socket to tighten the axle nut. This will push the hub onto the axle shaft.
Now you can bolt the hub to the other A-frame (if required), re-mount the brake caliper (if required), and torque all fasteners except the axle nuts to their proper value as shown in the shop manual. Next, set the parking brakes and tighten the axle nuts until the hub begins to rotate you want to get the nut tight but the torque value is so high you must finish the job after the car is on the ground. Lower the car and torque the axle nuts to the shop-recommended value. Then use your trusty hammer and the chisel to bend the flat portion of the nut into the groove in the axle. This is to prevent the axle nut from coming loose and spinning off the end of the axle.
Double-check everything and you should be good-to-go!
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17 October, 1999