FD Differential Remove and Replace - Writeup

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After deciding to replace the stock 4.10 FD gears with a set of 4.77 gears, I also purchased a Carbonetics Carbon Limited Slip Differential at the same time. ( free plug - Great deal, thanks Rishie! )

I would be remiss if I didn't also mention the smokin' deal I got on the gearset from Dial8 - thanks again. It included a Greddy diff cover (which is what made it such a great deal), which about doubles the volume of the differential so that it can hold 2 liters of diff oil, as well as all the seals and bearings needed to replace the differential.

I haven't got around to installing the Diff cover yet, waiting until I decide how to setup an oil cooler for it.

If you are simply taking your diff appart to replace the gears or the diff with another set of the stock variety, this job is probably a lot simpler. I know someone that replaced his Miata diff with another Mazda diff - it all simply bolted right up.

Using a non-Mazda gearset and/or an after market differential will create more work. It isn't too likely that the spacers will be correct for the new setup.

Changing the gears also requires removing the pinion, possibly replacing the pinion spacer, definitely replacing the crush collar and then setting the pinion bearing preload.

If you are reading this for entertainment, curiousity or are just bored at work, forge on head and read all of this.

If you are planning to do this job yourself, I cannot overstress the need to read the Differential section in the repair manual. Read it and study until you can almost quote it. Some of you may be old hands at this and will find nothing new here. I am not an old hand at it, and had I better studied the manual, could have saved some time and work.

The first time I re-installed the diff, I ran the car through the gears on the jackstands (very solid jackstands I might add) and didn't like the sound of the diff. It sounded rough. Had I paid more attention to the pinion bearing preload setup I could have avoided that.

Taking it all apart the 2nd time and re-doing all the setup was a bit easier than the 1st, as I already had pretty good idea of how to do it.

Next: The Documentation

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FD Service Manual: http://www.rx7club.com/showthread.php?t=614824

In addition to the FD service manual, the following links were found to be useful at one time or another.
YMMV.

I downloaded them so that local copies would be available should the web site disappear.

http://www.pirate4x4.com/articles/te...14b_Gear_Setup

http://www.dippy.org/svcman/sm03.html

http://www.differentials.com/install.html

http://www.rx7club.com/showthread.php?t=711712

http://www.rx7club.com/showthread.php?t=535290

http://www.gearinstalls.com/97fzj80rear.htm

Next: The Tools

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There are a few specialized tools you will need if you are planning major surgery on a differential.

If you don't already have the following, you may want to add to your tool collection:

Transmission Jack:
Really, I would not attempt this without a tranny jack or something like it.
Even with a buddy helping, putting a diff in place without a solid platform to hold it in place would be difficult.
What are your fingers worth? Mine are worth more than the price of a transmission jack.
This is the one I used: http://www.harborfreight.com/cpi/cta...temnumber=3185
I was able to remove and replace the diff by myself using this jack.

1/2" torque wrench that can go to 200+ lbs:
You will need to torque the PPF back to ~120 lbs.
For the diff you may need to go to 200+ lbs to the the pinion bearing preload correct.

Inch lb. torque wrench:
The pinion bearing preload is measured in inch lbs.
The spec for the FD is 12-15 inch lbs (look it up, I could be wrong)
You could use an expensive dial torque wrench, but this one worked fine:
http://www.amazon.com/gp/product/B00004SQ3B/ref=oss_product

Sockets:
For the diff you will need a large socket set.
IIRC the pinion nut is 26mm. As the plugs on the diff cover are 23mm and 24mm, it's a good idea to just buy a set of 20mm+ sockets.

Seal puller:
Pulls seals so much easier than with a screwdriver.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=35556

Bearing/Seal drivers
Indispensible for driving bearing races.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=95853

Old bearing races:
Sometimes nothing fits the new bearing quite like an old bearing race.
The carrier (side) bearings on the diff must be pressed all the way down, which is difficult to do unless you have a driver of the correct diameter. The bearing drivers won't do it. Tearing apart one of the removed bearings and using the old race worked perfectly.
Once of the attached pics shows a bearing race used to press the diff bushing.

Press:
You can do this job without owning a press.
A press will be necessary however. If you don't have a press, plan on spending a lot of time going back and forth to the machine shop.
Here's the press I used: http://www.harborfreight.com/cpi/cta...temnumber=1667
This worked for everything except pulling the larger bearing off of the pinion.
That thing had been on there for 18 years and wasn't coming off easily.
It took a trip to a Machine shop with a 40 ton press to remove it.
I would not have bothered if I hadn't needed that spacer that was under the bearing.
This spacer just happened to be the right size for the new pinion.

Workbench:
You need a solid surface to mount the diff to while working.
I've seen a number of arrangements while googling for ways to do this.
One was a very nice bracket made from angle iron, and it bolted to the workbench and allowed the diff to be mounted vertically.
Another was scraps of wood that suspended the diff on the rim of a 5 gallon bucket.
I went a different route however. Either of those arrangements would make it quite difficult to torque down the pinion nut when crushing the crush coller.
The bench can be seen in one of the attached pics.

My bench consisted of 2 wooden saw horses with shelves screwed down on them to create a bench.
Two holes were drilled in the bench top and the diff was bolted down.
An extra leg (a 2x4) was attached under the bench at the point of greatest stress when torquing down the pinion nut.
My diff required 16o+ lbs of torqe - I didn't want to break those shelves.

Brace:
You will need some kind of tool that will hold the companion flange in place while you torque down the pinion nut.
Google for it and you can see all kinds of home made tools.
I opted for a heavy piece of plate aluminum I had on hand. It worked pretty well for the job.
However you do it, set it up so that the brace will rest against something solid, like the top of the workbench.

The bench and the brace can both be seen in on of the attached pics.

Pipe:
A trip to home depot with some specs and a caliper yielded some pieces of galvanized pipe that worked well for pressing a couple of bearings. I am not providing exact specs, as you need to measure the stuff yourself. Galvanized pipe is not made to very high tolerances and pieces will vary. You will see them used in following pics.

GM Gear Marking Compound:
Googling revealed a variety of substances used for checking gear patterns. Toothpaste, lipstick, Prussian Blue (from NAPA), diaper pintment (really). I went with GM Gear Marking Compond. It is yellow, highly visible and doesn't dry out. Plus there is no need to remove it from the gears. You can buy it online or just get it from a local GM Service Dept.
Part # 1052351 - about $10.00 US.

A level:
Yup, it is useful for measuring the width of the diff assembly, as you will see. I tried a 2 foot Stanley level, but found a magnetic torpedo level attached to a steel straightedge easier to use.

Straight Edge:
Used for some measurements.

Dial indicator:
Essential. You cannot setup a differential without a dial indicator.
I used one from Harbor Freight along with a magnetic base.
http://www.harborfreight.com/cpi/cta...emnumber=33675
http://www.harborfreight.com/cpi/cta...temnumber=5645

Bearing Splitter:
You may need this to remove the rear pinion bearing.
http://www.harborfreight.com/cpi/cta...temnumber=3979

Home made gauge:
The gauge shown in the pic is something I made to measure the width of the inside of the diff case. A caliper just won't fit in there. There may be a tool already to do this, but I didn't have anything that would work. This is just a couple pieces of aluminum stock with a hole to slide a stainless steel rod through, and drilled and tapped for a set screw.

Next: The Parts

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In addition to the new diff and new gears, there are a lot of other required parts.

Seals, bearings, spacers, crush collars (plural intentional) and pinion nuts are required for this job.

Mazda differentials for the RX-7 (FD and at least some FC), RX-8 and MX-5 have a number of interchangeable parts. The entire assemblies are probably interchangeable between the cars, though I don't know that for a fact.

These differentials all one piece spacers to set the backlash changing the lateral location of the differential in the case. Chances are the spacers you removed when you disassembled your diff will not be correct for installation.

After fooling around a couple times and trying to order just a few of these spacers I broke down and bought the entire set. Yes, they are expensive. The entire set of ~20 spacers at 16.00 or so apiece is $300+.

If you have the racer discount from MazdaMotorsports (I do) you can buy them for $10.15 each.

I was lucky with the pinion spacer - the one from my stock pinion happened to work.

Attached is what I think is a complete list of parts needed. The prices are all MazdaMotorsports racer discount prices, but you can probably get close to that from Malloy Mazda.

There are two pinion shaft seals included because they are about $10.00 each, and it's not too unlikely that you will mess one up and need another.

Buy lots of crush collars. I think I bought 3 or 4. There is one unused one left in my parts box. They are about $6.00 each. Buy extras. You may get the pinion preload right the first time, you may not. If not, the crush collar you used is now trash. Don't try to reuse a crush collar.

Next: Removal and Disassembly

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I'm not going to go too much detail here because it is well covered in the Service Manual.

You will need to remove the PPF, the drive shaft (prop shaft in the manual) and pop the axles out.

No doubt someone will dispute the need to remove the PPF and or the drive shaft. My advice is to just remove them. It isn't that hard to do, and removing the diff will be much simpler.

Don't forget to prop up the motor and tranny. I used a small screw jack on top of a stack of bricks under the transmission.

To remove the axles you can get by with removing the brakes (caliper and rotors) and disconnect the upright from the upper suspension arm. The half shafts can than be popped out of the diff and the whole assembly can be pushed back out of the way so the diff can be removed. To avoid having to deal with sealing the brake lines and then doing lots of bleeding, I just tied the calipers up to the coil spring.

In my case everything was already removed after replacing all the suspension bushings, so there was a little more room to work. That was the first time. The second time I removed the diff, I did it just as described in the previous paragraph.

Once you have the diff mounted securely, it's time to start taking it apart.

If you don't plan on salvaging the side seals, then pull them out now. They are just in the way otherwise.

The following instructions differ a bit from the manual. If this diff is mounted horizontally to a bench, this may save your fingers a bit of damage, and maybe the diff too.

Start by pulling off one of the bearing caps as shown in the pic. Then drive out the shim. I used a small chisel and a dead blow hammer.
The square edges of the chisel were ground down to better spread the load on the shim without deforming it. If the diff has been in the car a few years you may need to give it a few whacks to get started. Once the shim is out, hold on to the diff with one hand while removing the other bearing cap. Now you can ease the diff out of the case.

Had you removed both bearing caps, the diff would pop right out of the case after the first shim was removed, and then it just might roll over a couple of your fingers on its way to the floor where it will start looking for your toes.

Once the diff is out, you can remove the pinion. After setting up your brace to hold the companion flange in place, remove the pinion nut. It will be hard to remove. Get a two foot breaker bar to break this nut loose. Or if you have a capable air gun, you can probably use that. Lacking the air gun, I used a $12.00 harbor freight breaker bar.

After removing the nut, you can use a pulley or gear remover to pull the companion flange off. A 2 jaw puller worked well for me. You can also probably remove it by gently tapping the back of the flange on each side until you work it off. The puller is a little easier.

Now to remove the pinion. The FSM (Field Service Manual) says to use a brass hammer IIRC and "tap" it out by striking the front end of the pinion shaft. I'm not sure what is considered a "tap" by the engineers that wrote the instructions. If "tapping" includes whacking it quite hard with a dead blow hammer, then I guess I did it correctly. It takes some force to do this, as what you are doing is driving the pinion out of the front bearing.

Don't forget to put the pinion nut back on before you start hitting the pinion, or the threads will get pretty messed up.

Once the pinion is free, set it down and then remove the front pinion seal. Once the seal is out you can remove the front pinion bearing, crush collar and spacer.

The final bit to remove from the diff case are the races for the front and rear pinion bearings. Shown in the pics is the rear pinion bearing race being removed with a hammer and punch, and the front race just after being installed.

If all the bearings are being replaced, you will need a bearing splitter to remove the rear pinion bearing. If this is the same bearing that has been on the pinion since the car was new, you might want to just skip the bearing splitter and take it to a shop with an adequate press. The 12 ton press I tried to remove it with was starting to bend. The shop where I took has a 40 ton press and the owner said it took most of what that press could do to remove that bearing.

Next: KG Gears

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More Pics

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The gearset installed is the 4.77 KG (Garage Kagotani) gearset. These gears are not actually made specifically for the FD RX-7. That is apparent when you consider that the pinion requires a bushing to bring the diameter up to the correct size for the bearings, and the ring gear requires a spacer to get in the the correct backlash range.

Nonetheless the gears work fine. One potential problem with this gearset is that the ring gear spacer may not be thick enough. The setup on my diff required the 6.50mm shim on the left side of the diff to get the backlash set correctly. As 6.50mm is the largest shim Mazda makes, if the 6.50mm ring gear spacer had not worked a new spacer would need to be made.

I didn't measure the ring gear spacer thickness, but it should probably be another 0.2mm (0.008") thicker.

Were I to do this again I would probably just get the 4.77 RX-8 gear set.

Next: Pinion race install

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The new pinion bearing races must be pressed into differential case.

This is really quite simple using the bearing drivers and a press as shown in the pics.

Next: Pinion bearing install

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If you are assembling a new gearset, the pinion spacer may be the incorrect size. This spacer is used to set the depth of the pinion so that the gears mesh toward the middle of the ring gear, between the heel and the toe of the tooth.

The FSM shows how to use some special tools to measure the spacing and determine if it correct. If you have a way to do that without those tools, please post it. Lacking the tools (I didn't try to order them) I opted to use the spacer that was on the stock pinion.

Actually, what really happened was this: I could not remove the rear pinion bearing, and made an attempt to order the same size spacer. The attempt was rather lame, as after the diff was assembled, the pinion was too deep. That's when I made a trip to the machine shop to remove the rear bearing from the pinion, so I could retrieve the spacer. As it turned out the stock spacer was 3.20mm, while the one I had ordered was 3.35mm. Just enough to screw it up.

Once the rear bearing was removed at the shop, I found that the stock pinion spacer worked fine, so no need to order more. Of course without the special Mazda tools, you won't know if the spacer is correct until you have assembled everything, set the backlash and checked the gear pattern.

The rear pinion bushing was pressed on using a 6" piece of 1.25" galvanized pipe from Home Depot.

The rear bearing was then pressed on using a 6" piece of 1.5" pipe. You can see this in the attached pics.

Once the bearing is on it is time to install the pinion and set the preload.

Next: Pinion Preload

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Now comes some of the fun stuff, setting the pinion preload.

"What is preload and why is it needed?" you might ask.

There must be sufficient pressure on the pinion bearings for proper operation. If they are too loose, you may hear some bad noises from your diff, maybe a low howl at low speed, and whine at higher speeds. This is the sound of your pinion bearings self-destructing because they are not being held in place well enough. If they are too tight their lifespan will likewise be significantly shortened.

Getting the preload wrong is why I did this job twice - complete assembly into the car, immediately followed by complete removal and disassembly and starting over. Better to miss the first autocross of the season than make it and blow up the (expensive) diff and gears.

This is where the inch lb torque wrench will be used BTW.

Read the FSM instructions. Then read them again.

Keep reading them until you know them by heart and can recite them from memory. Seriously, this is one place you don't want to screw up.

The diff will need to be very securely mounted, as you will be putting a lot of torque on the pinion nut.

There isn't too much I can add in addition to the FSM instructions.

Here is the little I can add.

Once the collar begins to crush, you will know it, as the pinion nut gets very hard to turn. I don't think I can overemphasize this point: Tighten in small increments, remove the brace frequently, and check the pinion preload.

When you check the preload, be sure to spin the pinion a few times to relieve any binding. It will be quite bound up after you take the wrench off the nut and brace off the flange. I put a little lubricant on the bearing, then put a drill on the pinion for 2-3 seconds to made sure it was loosened up. If you don't relieve the binding the preload measurements will be way off.

The FSM instructs you to assemble everything except the front pinion seal, set the preload, and then record the ft lbs of torque required to get the preload correct ( 12-15 inch lbs). Following that you disassemble, install the oil seal and re-assemble, torquing the nut down to the recorded value. In addition, I torqued the nut down in increments, making sure the preload didn't exceed the specified value. The torque required for mine was 166 ft lbs, and it came out correct at reassembly.

When measuring the preload, measure it while the pinion is turning, as more torque is required to start the pinion turning than is required to sustain the motion.

Be sure not to forget the spacer on the crush collar, and the washer under the nut.

The attached pics show the inch lb torque wrench being used to check the preload, and a bearing race being used to seat the front pinion oil seal.

Next: Differential bearings

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If you are installing a new differential or replacing the bearings on the old diff, you will need to press on new bearings.

Removing the bearings is straightforward - just use an appropriately sized bearing puller. I used one from Harbor Freight to pull a bearing from an RX-8 diff so the race could be used as a drift to seat bearing on the new diff.

The bearings can easily be pressed on in a press using a bearing driver. This will not completely seat the bearing however. After the bearing is pressed on you can see a small gap between the bearing and the body of the diff. The bearing must be pressed on until it reaches the body of the diff.

Mazda makes a SST (Special Service Tool) for this. If you don't have the proper Mazda SST the next best thing is a bearing race. This is where the race I removed from the RX-8 diff came into play. As seen in the attached pic, the bearing race can be used in the press to finish pressing the bearing on the last few thousandths of an inch.

Next: Differential measurements

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The FSM specifies that you need to measure both the inside width of the diff case and the outside width of the differential. This is so that you can determine a starting point for shim sizes. If you are using the same diff and just replacing the gears this won't be necessary - just use the shims that came in the diff as the starting point.

When replacing the diff and the gears as I have, the measurements will give a good starting point for shim sizes. Measuring these sounds simple until you try it. A regular caliper will not fit inside the diff case to measure the inside width. If there is a tool for doing so, I don't know about it.

Building a tool to make this measurement was pretty simple. Two small pieces of round aluminum stock, and a 12 inch piece of 1/8" stainless rod were all that was needed. A hole was drilled in each piece of stock to slide the rod through, and a hole was drilled and tapped in the end of each piece so a set screw could be used to secure the rod.

As shown in the pics, the tool was adjusted out to the width of the case, then a caliper used to measure that.

Measuring the width of the diff with bearings and races install is somewhat tricky. I finally settled on using a 1x1x12 inch piece of steel as a straight edge with a torpedo level attached. After leveling the straight edge on top of the diff assembly I measured the height on each side and took an average, which worked well enough. Make sure you are working on level surface.

The math for determining the shim sizes is pretty simple and in the manual. One funny thing about it is that you are supposed to add .002-3" to the diff width to determine the shim sizes. It's funny because the shims only come in .5mm (~0.016") increments.

Next: Install Differential Into Case

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You may be tempted to slide the diff into the case with the bearing race and spacer installed on one side, and once the diff is in place install the spacer on the other side.

That won't work, at least in my experience.

I spent several minutes trying to do that. There's just no way to get the spacer in there without damaging the bearing, at least I couldn't do it.

Attempts to do so lead to the edge of 3 of the roller bearings digging a small gouge into the bearing race.

Yeah, ouch.

Fortunately I wasn't really trying to force it all that much - the bearings are pretty hard and it didn't take much to cause the gouges. The bearings themselves were fine. The little ridge on the edge of the gouge sanded out with some 400 grit wet/dry sandpaper.

Attempts to really force it (like whacking it hard with the hammer) would have destroyed the bearing I think.

To install that diff into the case, you need to put the outer races onto the bearings, and the spacers need to be held in place as you slide the diff into the case.

It's kind of tricky at first, but a couple of attempts and you will get the hang of it.

The diff of course will not actually 'slide' all the way in. It must be 'encouraged' with the dead blow hammer. After you get the front edge of the diff in place, you can hold on to it with one hand and tap it in a bit using the dead blow hammer.

After it is in far enough to stay put without any help from you, you can then use the dead blow hammer and some tool to tap the diff and the spacers. Once the spacers were in far enough I used a rounded off chisel to tap the spacers on the front through the axle openings, and a dead blow hammer directly on the diff. Don't hit it too hard though. I don't know if the diff can be damaged by hitting it with the dead blow hammer, but I really didn't want to find out.

Sorry, no pics of this as my hands were full - I should have got someone to take pics while I did this.

Next: Differential backlash

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Setting the backlash is what took the most time for me.

I took the measurements, determined what I thought was a good range of shim sizes to order, waited for the parts, set the backlash, and then found out I needed different shims. Lather, rinse and repeat.

Following the second round, I ordered every shim available just so I could finish this job.
(Nope, the shims are not for sale)

After a few rounds of this I got the gear pattern where I wanted it with correct backlash.

If you are assembling new parts here, be sure to torque the ring gear down on the diff. If you don't then the backlash will open up .001 - .002 after you do torque it down.

The links in an earlier installment of this include some good references for acceptable gear patterns.

One tip I do have to offer for this section is the use of the dial indicator.

When dealing in measurements that are specified to be between 0.0032 and 0.0045 you would like to make your measurements as accurately as possible. I found that it is very difficult to tell when the gear tooth touches the dial indicator without actually moving it, which will really screw up your measurements.

The solution I found was to set the dial needle to -1 or -2 thousandths. Then set the plunger next to the tooth, and turn the pinion until the ring gear tooth just touches the plunger. Turn until the needle is at zero. Holding the pinion in place then the ring gear can be moved back and forth to accurately measure the backlash.

The FSM says to measure in three places around the ring gear. I was rather anal about this, and measured and recorded every tooth. This may cause a panic until you re-read the manual where it does make some provision for variation. There were a few outliers, but nothing worth getting excited about.

In the attached pics the dial indicator is shown at 0.005 which is out of spec. Shims were adjusted to correct this. The gear pattern is shown where the pinion is too deep, along with two pics showing the final drive and coast patterns.

Now install the side seals, put some RTV on the diff sealing face, torque down the diff cover and let the RTV cure for awhile.

Next: Final assembly

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All that's left is to put it all back together. If you are using a tranny jack, mount the pumpkin on the jack. In addition to the chain that comes on the jack, I used a small ratcheting tie-down to secure the diff on the jack. This would be a good time to fill the jack with oil. If you're careful not to tip the jack too far to the side or forward, the oil won't leak out. It sure beats spending 15 minutes pumping oil into the diff while under the car.

Diff oil tip: When you empty the diff, drain the oil into a plastic milk jug or some other translucent container. Use a sharpie to mark the level, then drain the jug. When filling the diff, fill the jug to the mark, and you have just the right amount. This assumes of course that when the diff was drained it had the right amount of oil in it.

Get the diff under the car, jack it up to where you can install the bolts though the diff bushings and put those bolts in. This will require a bit of maneuvering - take your time. No need to tighten the bolts yet, they just need to keep the diff in place. Next install the drive line and the PPF. Now tighten everything to spec.

If you've followed the FSM you probably ordered new clips for the half shafts. If you did, then now you need to replace them. Replacing those clips is the single hardest piece of this entire job if you ask me. Now you need to re-install the half shafts. Make sure the open end of the clip is facing up, and gently slide the half shaft into the diff. You may need to rotate it a bit to get the splines to engage.

Once the splines are engaged you need to get those clips to snap into the groove inside the differential. You probably can't do this just by pushing on them. What worked for me was to pull the half shaft out about 1/2", then give it a quick hard shove. You will definitely hear and feel the clips snap into place. Re-assembling everything twice this method worked well for me.

That's pretty much it. If you feel you have the car securely in the air, you may want to try running it through the gears while on the stands just to see how it sounds. Make sure no one gets near the car while your doing this.

If all seems OK, put it all together and take it for a spin.

Whew! It took almost as long to write this up as is did to do it.

I hope someone finds this useful.