Setting backlash on a RX-7 Differential

Last update: 12/24/08 Return to home page links

As per the purpose of this site, I'm not going to go into all the details of how you get the carrier out of the axle housing and other  mundane details of setting up a rear end. I am going to present pictures/details/hints of what I did that might help others when adjusting their backlash etc.

I WILL assume that you have read (elsewhere) the details and understand how to adjust a rear end. A service manual on your donor car would be a good source of information. And I am assuming that you have inspected the gears, bearings etc and your parts are in good condition and are ready to be set up. Read this article several times until you understand all the steps necessary to do the job. It's not hard to do, but it does take time and patience to do it correctly.

Here is a site that has details of adjusting a Miata rear axle.  

Ignore the fact that he seems to be setting up a gear set with broken teeth! Otherwise he does give the proper procedure to set up the rear end. The Miata rear end he is working with has a 7" ring gear that is said to fit into a 1st generation RX-7 carrier. Generally the Miata gear ratios are in the 4:1 range.

I pretty much follow the sequence that the above website does since that is the proper way to do a set up. But again, I am mostly presenting the tools and sequence that I use to set up a rear end.

Keep in mind that I am a perfectionist, and I don't mind spending hours doing something correctly even if it could be done "good enough" in a shorter time. That's the advantage of doing the job yourself. So I may repeat some steps that could be eliminated. But I only want to do this job once so I take my time.

The rear end is NOT a place to take short cuts. ALL the torque and power of the engine is going through those gears and bearings and they better be set correctly if you want the rear end to run quietly and to last. I did not do the pinion height or pinion bearing preload adjustments because they were OK in my rear axle.

My rear axle was making strange "grunching" noises (bad wheel bearing) and generally had a loose/noisy feel to it. After replacing the bearings and adjusting the back lash the axle is quiet and ready to go another 25 years. I hope I am too.

A quick review of how I do a backlash and spool bearing preload adjustment

1. Before you loosen the bearing cap bolts. Read and record the CW and CCW "pilot section" diagonal measurements. If they are within tolerance, you have the proper bearing preload. (How to do this is given later.)

2. Read and record a base set of backlash measurements at 90 points on the ring gear.
If the backlash is also within tolerance, you are done and didn't need to take your rear end apart.

3. If you need to do a backlash setting and/or bearing preload, loosen the 4 bearing cap bolts and re-torque them to ~1/3 the rated torque.

4. Set the backlash within the tolerance allowed.

When setting backlash, rotate the threaded adjuster rings in the same direction (when looking across the carrier). This causes the ring gear to move closer or further from the pinion gear. If you rotate both adjuster rings the same amount you should be able to maintain the same bearing preload. Keep in mind that both adjusters are right hand thread.

5. Tighten the 4 bearing cap bolts to ~2/3 the rated torque and remeasure the backlash. If the backlash is out of tolerance, repeat steps 3 and 4.

6. If the backlash is OK, proceed to the next step (7).

7. Read the CW and CCW "pilot section" diagonal measurements. If they are within tolerance, you have the proper bearing preload. If the measurement is less than the required amount, the adjusters have to be rotated inward.

To adjust the bearing preload, rotate the adjuster rings opposite of each other an equal amount, so the bearings are basically squeezed together or not squeezed as much.

8. Tighten the bearing cap bolts to full torque and go wash your hands. You're done.

And now, the gory pictures of a RX-7 carrier being adjusted.

To see an enlarged view of these pictures, click on a picture or right click and select "View Image".
U joint hanger

The first thing I did was loosen the wheel lug nuts and then placed a floor jack under the center of the axle.

I then jacked the car up until I could put two CBS building blocks; one on either side of the tunnel at the rear of the chassis. I then put two short pieces of 2 x 4s on top of the blocks and under the rear frame rails to support the rear of the chassis.

Place some rags on the frame under the axle so it doesn't contact the frame. Lower and remove the jack. The axle should now be resting on the lower frame tubes.

Next I disconnected the rear universal joint from the rear axle. The piece of wood and the string keeps the U-joint from falling on the floor.

Note: My tunnel top cover is held on with #8-32 pan head screws. This way I can easily remove it to get at the universal joint or do maintenance work.

Rear axle pulled back

Luckily the coil-over shock springs were loose when the axle was lowered down to the frame rails so I didn't have to disturb the spring adjustments. I removed the lower mounting bolts and rotated the shocks out of the way.

I then removed the tires, drained the oil and started taking everything else loose that held the rear axle in place. I didn't have to remove the emergency brake cables.

In this picture I have removed the fuel tank and it's mounting brackets, disconnected and capped the brake line and moved the rear axle towards the rear until the pinion flange cleared the rear bulkhead.

Once the flange was clear of the bulkhead I rotated the axle to the position in the picture.

The 2 x 4 is temporarily holding the pinion upright.

The shinny hoses are the FUEL lines. You don't want to starve the engine do you?

Rear end hanging left side

After being convicted and hung ..... Oh wait, that's another website.

My method of keeping the rear end pinion upright is to use rope through the link brackets on each end of the axle. The axle is resting on the lower frame rail and wanted to rotate CCW in this picture.

To be able to remove the carrier from the axle housing you have to move both axles a few inches outward to disengage them from the differential spider gears.

The RX-7 axle doesn't use "C" clips on the axle ends. The axle bearing retainers hold  the axles in place.
Right side of axle, brake removed

This is the right end of the axle after I removed the brake pads, disk rotor and moved the caliper back into it's normal position.

Next remove the three bolts holding each axle bearing retainer in place. They are located at the outboard ends of the axle housing. Mine were rusted so badly I had to split the nuts to remove them.

The axle flange will clear the caliper bracket while being removed.

You can see one of the CBS blocks and 2 x 4 at the rear of the chassis by the suspension links in this picture.

The next few picture are going to be somewhat difficult to explain but here's what's going on. If you are doing this job the Locost way you probably won't have the special Mazda tools like a slide axle puller.

To remove the axles you basically have to hammer them off. The axle bearings on the axle shafts are pressed  into the machined end of the axle housing and they may be stuck in place if they've never been removed before. Naturally, mine were stuck. (REALLY stuck!)

A slide axle puller hammers to PULL the axle/bearing out of the housing. It also produces evenly distributed pulling power to the flange and bearing.

You can also hammer on the inside surface of the axle flange to PUSH the axle/bearing out of the housing. The push is applied offset to the center of the flange but it does get the job done.

Since most Locosters aren't going to have an axle puller, guess which method we are going to use?

Special removal tool

The Locost approved axle removal kit.  One 3lb hand sledge and some wood.

I first tried to use a ~18" length of 1" x 2" hard wood and my special "convincing tool".

To remove the axle, you place one end of the wood on the inside surface of the axle flange and hammer on the other end of the wood. This did not work at first.

Machinists will tell you that you should not force anything.  Getting a bigger hammer is OK though!

Wood brace

What actually was happening was that every time I smacked the wood, the axle would move slightly and absorb the force of the hit.

Once I realized what was going on, I had to come up with some method of holding the axle from moving. My solution is shown in the above picture.

The piece of wood on the left is resting against the raised flange of the carrier. The piece of wood in the center is used to fill the space between the left piece of wood and the piece of wood on the right.

Another way is to use a longer piece of wood!  ;-)

Wood holding fixture

The angled piece of wood is resting against inside surface of the frame rails in the area

What all the wood does is hold the axle from moving when I hit the piece of wood applied to the back of the axle flange.. When you get one axle out, transfer all the wood to the other side of the carrier and hammer away.

The hammer wood is threaded through the ropes so one end is resting against the inside surface of the axle flange. You pound on the other end of the wood from inside the boot. It's going to take 4 or 5 really hard hits to remove a stuck bearing.

As soon as you see the bearing starting to move out of the axle housing, use lighter hits so the axle doesn't fly out. Hold the axle flange up with your third hand as the bearing comes out of the housing so the axle doesn't lay on the oil seal.

I removed the axles completely so I wouldn't have to tie them up away from the seals. And I wanted to inspect the wheel bearings anyway.
Special carrier hold fixture

Next remove the  bolts holding the carrier to the axle housing and lift the carrier out of the housing. It weighs ~50 lbs so be careful.

This picture shows the RX-7 carrier resting in the Locouki special "carrier support structure". That's the four pieces of wood that keep the carrier from falling to the bottom of the "carrier holding fixture" (the white 5 gallon plastic bucket).

Hey, you want Lo-cost?  This is how you do it.

I use two pieces of hard wood under the flange to support the weight of the carrier.

Two thin pieces of plywood are used to hold the weight bearing  pieces of wood in place. Small finishing nails hold the whole thing together.

The tag in this picture is the backlash in 1/1000 of an inch after I had adjusted it (.00275"). The backlash was ~.007" when I started. (WAY too much!)

Pliers through bucket

So how do you hold the pinion from moving while checking backlash? Easy.

After you have the carrier sitting on top of the bucket look how far down into the bucket the pinion is. Next make a slot in the side of the bucket so a pair of grip pliers can be inserted through the slot to lightly clamp the pinion flange so it can't rotate when taking measurements.

The slot should just be wide enough so the pliers can fit through the slot but not move left or right. I made the slot by drilling a vertical  row of 1/2" holes with a wood drill. I then used the drill to remove the material left between the holes.

The spring is used to make sure the pliers don't move and also to hold the pliers in the slot when they aren't clamped on the pinion.

Now that your expensive pinion adjusting fixtures are ready ..............

There are three tools you can't skimp on and must have to work on an early RX-7 rear end;

1. A good dial indicator and the clamp to hold it in position.
2.  A torque wrench.
3. A 7" to 8" micrometer. (I made my own!)
Dial indicator set up

In this picture I am measuring what I call the CCW diagonal measurement. You should also do the CW diagonal measurement.

How to do the measurements are explained below the last picture on this page. Basically I wanted to set the rear end BACK to the factory setting when I was finished.

I made my 7" micrometer from some 1/2" thick aluminum stock I had. The three pieces are held together with flat head machine screws. Getting the pieces at right angles is very important to get accurate readings.

The micrometer head was part of a World War II "X" band microwave guide tuner. I've been planning on using it for something all these years. It's removable in case I need it for something else. My junk collection goes WAY back!

I keep the micrometer away from temperature changes while using it so the long aluminum bar doesn't change length between measurements.

That includes NOT touching the long bar of the micrometer between the measuring points.

I repeat;
Before you loosen the four bearing cap bolts, measure the distance between the "pilot sections" on the bearing caps with your micrometer. Write the data down, you can use it later to set the preload.
Dial indicator set up

The shiny spot near the right bearing cap bolt is one of the "pilot sections" (a Mazda term) that you use to measure the bearing preload.

The next set of pictures (below) illustrate how to do this measurement..

You can see the other pilot section on this bearing cap near the left side bearing cap. There are matching pilot sections on the other bearing cap.

Notice that the dial indicator is contacting the gear tooth at the required 90 angle. It really doesn't matter where around the ring gear you place the dial indicator as long as you use the same position for all four of the 90 degree readings around the ring (crown) gear.

LCD hood finished

This picture shows my Locost version of the special Mazda "differential wrench" p/n 49 0259 720. Locosters can use a small hammer and a 1/4" diameter flat ended punch.

Move the dial indicator OFF the gear tooth before hammering on the adjuster!

First loosen the four bearing saddle bolts and then set them to about 1/3 their rated torque.

Use the punch to rotate the threaded adjusters the same amount and in the same direction to move the ring gear back and forth. This sets the backlash.

You don't have to use a lot of force at this point, you barely want to move the adjusters.

.004" adjustment

I used a center punch  to mark both bearing adjusters and caps BEFORE I adjusted the backlash. The punch marks were next to each other before I changed the adjuster position.

The dividers now are resting in the two punch mark holes (after the adjuster was rotated to correct the backlash) and shows how much I had to rotate the adjuster to move the backlash ~.004". The divider is used to make sure that each adjuster is moved the same amount.

As you can see, it doesn't take much rotation of the adjuster to move the gear a lot. (.004" is a LOT when you are setting backlash.) The adjusters are right hand thread.

The adjuster in this picture was rotated CCW. Therefore the other adjuster should ALSO be rotated CCW when viewed from this direction through the spool. That would move the all the parts including the ring gear, towards the camera and the pinion. CCW rotation results in LESS backlash.

Start with about 1/4" of rotation and recheck the backlash the first time around. This will give you an idea of how much things move.

I found that as I tightened the bearing cap bolts to 2/3 of the required torque that the backlash was reduced by a few tenths of a thousandths. If you are near the lower backlash tolerance, you have to take this into consideration.

This picture clearly shows the two flat spots on either side of the divider which are the "pilot sections". Those are the places where your micrometer touches while you measure diagonally from side to side while setting the preload on the spool bearings. The other bearing cap has the same pilot sections.
CCW preload measurement

Taking the CCW diagonal measurement while setting preload.

The next job is to set the bearing preload. This is done by very carefully and EQUALLY rotating the bearing adjusters inward or outwards. In affect the bearings are squeezed together so they are under pressure (or "preloaded"). If you don't rotate the adjusters equally, the backlash will change.

You will have to remeasure the backlash at 90 degree points around the ring gear to make sure it hasn't changed.

Are you wondering how you know when you have enough preload on the bearings? That's where the 7" micrometer comes into play.

On the 1st generation Mazda RX-7 and 1994 to 1997 Miatas you tighten the bearing adjusters until the diagonal pilot section measurements are between 7.3004" and 7.3033". Yep, +/- 0.00145" thousandth  total tolerance!

Since my micrometer is homemade I couldn't be sure of the absolute reading within a few thousandths of an inch. (I did a transfer calibration by using two 4" dial calipers.)

Measuring the diagonals BEFORE I loosened the bearing mount bolts was a method of checking the micrometer.

My initial measurement, before I loosened the bearing bolts was within the +/- tolerance. At that point I had more confidence in the micrometer readings.

I then set the diagonal measurements as close as I could to the original numbers.  (You did write them down before taking things apart didn't you?)

CW preload measurement

Taking the CW diagonal measurement.

Here is how I take a micrometer measurement.

First, wipe off any oil from the surfaces to be measured. You want to be able to feel the friction of the micrometer anvils sliding on the surfaces.

Notice that the left hand is holding the stationary micrometer anvil against the far side pilot section and the base of the micrometer is resting on my right hand to support it. My right fingers very gently turn the measuring head to take the measurement.

You do NOT want to -tighten- the micrometer. You want the movable anvil to just graze the pilot section. As you move the micrometer back and forth, you can feel (and hear) the movable anvil gently sliding across the pilot section when you have the setting correct.

Once the preload is correct, recheck the backlash and if correct, torque the bearing saddle bolts to the rated torque.

If you think all of this is a lot of work, just remember that I didn't do the pinion setting and pinion preload adjustments. My gear wear patterns are very good so I just left things as they were.

Now you know why the good shops want $100/hr to do this. I was recently quoted $500-$600 to do this job. Since I was able to do the adjustments in one day at a leisurely pace I figured I came out pretty good for a days work.

If you aren't sure YOU can do this, figure that after building a whole car are you going to let a few thousandths of an inch bother you? Besides how many people do you know that can set up a rear end?  Join the crowd!

As usual Mr. Phelps, if you or any of your helpers are apprehended while doing this job incorrectly, the Dept. will disallow any knowledge of your activities.

Cat, bad hair day
I don't smoke! But this is what can happen if YOU smoke!

While you have the rear suspension links loose, read my page titled, "Rear end service note".

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