Building an RX7 LSD diff

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Hi guys, I asked Ian to create this section so I could have somewhere to put some things I was working on.

I’ve heard a lot of talk about putting MX5 LSD diffs into RX7s, but I’ve not seen a build thread anywhere or seen anyone who has done it.
So, I did quite a bit of looking into at and here’s what I came up with……

The Mazda diffs basics.

All FB RX7 diffs are 7” diffs, known as medium size diffs. The early piston diffs are small diffs and the 13B powered cars (RX4 and RX5) are large diffs. The diff size is based on the size of the crown wheel.

The S1 and S2 RX7 diffs are 24 spline. The S3 RX7 diffs are 26 spline. Early rotaries are all 24 spline, with the exception of the 13B large diffs which are supposed to be 28 spline, but I’ve not seen one so only speculating on what I’ve read.

The FB RX7 diffs are all 3.9:1 ratio, with an exception for the GSL-SE cars which I’ve read are 4.1:1 ratio. The JDM 12AT diff I had was also a 3.9:1 ratio. RX2, RX3 etc. are 3.9:1 ratio from the ones I’ve got.

Mazda produced open diffs and LSD diffs for the S2 and S3 RX7s. Only the higher spec cars got them, such as the GSL-SE, JDM 12AT etc. Apparently LSDs weren’t available for the Mazda small and large diffs, only the medium size diffs.

Now, in theory, you should be able to swap the side gears between the 24 and 26 spline diffs to enable the diffs to be used in your application. However, you cannot use open diff side gears in an LSD diff.

Here’s a picture of the open and LSD side gears side by side to show you the difference:



The 24 spline LSD diff side gears were discontinued by Mazda in 2006 (quote: Multirotor) and are now extremely hard to find. Momus on Ausrotary has had some made, but is selling them for AU$300 (£165) a set which makes them unaffordable imho.

The factory Mazda RX7 LSD diffs are clutch type LSDs. Mazda still has stock of the friction parts in the LSDs so if you find one, you can rebuild it. So, at least there’s some good news.

[Johnnyboy]

Good stuff... Thanks.

(and Merry Christmas!)

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The Mazda MX5 diff.

The Mazda MX5 diff is also a 7" diff and 26 splines, same as a S3 RX7. The MX5 diffs I have here have both been 4.1:1 ratio.

The MX5 LSD diff came as either open, viscous LSD or Torsen LSD (type 1 and type 2).

The MX5 diff will fit an RX7 axle and is a bolt in swap. However, you would need to grind off quite a bit of cast iron on the MX5 diff to get the clearance between the diff head and the Watts link.

This picture shows how close the Watts link runs to the diff head on an FB:





I intend to take an MX5 type 2 Torsen LSD diff and fit it to a S3 RX7 diff carrier. I have chosen to do this rather than fit the MX5 diff directly because, apart from the issues with the Watts link clearance, other issues include the difference in ratio between an RX7 diff and an MX5 diff, the difference in length between the RX7 diff and MX5 diff and the size difference between the prop shaft flanges between RX7 diff and MX5 diff.

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Comparing the RX7 diffs and the MX5 diff.

Here is a series of photos to show you the similarities and differences between the diffs.

This is your standard FB RX7 open diff (S2):







This is a S3 RX7 diff:



Here you can see the difference between a S2 RX7 diff and a S3 RX7 diff. Besides the difference in the number of splines, you can see the difference in the number of teeth on the crown wheel (and on the matching pinion). The S3 RX7 diff has more teeth (59:15) and the S2 RX7 diff has much less (43:11) but the S2 RX7 has bigger and deeper teeth. The S3 RX7 has more teeth to make it smoother and quieter apparently.

S2 RX7 diff in front:



S3 RX7 diff in front (so you know it’s not an optical illusion):



Here’s the crown wheels side by side. You can see how much bigger the S2 RX7 teeth are:





From left to right we have a S2 RX7 diff, a S3 RX7 diff and an MX5 diff. You can see the crown wheels are all the same size. 7” diffs:





In this photo, you can see the MX5 diff (in front) is a little longer than the RX7 diffs:



Here is a S2 RX7 and a S3 RX7 diff prop shaft flange side by side (S3 is on the left and is a little larger than the S2):



Using an MX5 prop shaft flange I have tried to demonstrate the difference between the 3 diff flanges.

S2 RX7 diff (note that the flange has threaded holes):





S3 RX7 diff (note there are no threaded holes in the flange:





The diameter of the centre of the prop shaft is the same on al 3 diffs, so in theory you could assemble the rear half of the universal from the MX5 prop shaft onto the RX7 prop shaft, but you still have the issue of the extra length of the MX5 diff to contend with. Note, swapping the flanges from diff to diff requires removal of the pinion nut and in theory, resetting of the pinion backlash etc.

Here are some pictures of a S3 (26 spline) RX7 axle inserted into each of the diffs.

The S3 RX7 diff (small teeth on the crown wheel):



The S2 RX7 diff (large teeth on crown wheel):
The axle may look like it is inserted but it is sitting on top of the side gear and will not engage)



The MX5 diff (note the extra mounting pints which interfere with the Watts link):

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The MX5 LSD diff.

Here are a couple of shots of the MX5 Torsen LSD:







The MX5 open diff.

Here are a couple of shots of an MX5 open diff:





Here is the pinion removed from the carrier:







Note the collapsing collar on the shaft which is a single use part designed to maintain the preload on the pinion.

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Stripping down the S3 RX7 diff.

The first step is always to mark the bearing caps and bearing retainers so you can be sure to return them to their original location:









Take not of these 2 points as we will need them later on during the assembly:





Remove the bearing retainer locking clips which prevent them coming undone:



Then you need to remove the bearing caps. I am using one of the S3 RX7 axles to brace the diff while I undo the bolts (it’s only 30 foot pound and they come off very easily):



You will end up with this (this is an MX5 one but you get the idea). Make sure you keep the bearing races together with the correct bearing if you want to reuse them:



Remove the crown wheel, ending up with this:





Do the same with the MX5 LSD diff:





One other difference I noticed was that the S3 RX7 has a cast bearing retainer whereas the MX5 diff had machined steel bearing retainers. Here they are side by side:

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Assembling the MX5 LSD centre into the S3 RX7 diff.

Fit the S3 RX7 crown wheel to the MX5 LSD diff centre:









Having cleaned up the bearings and inspected them, they are fine to reuse. I have made sure I kept the bearings and races together so they stay as a working unit:



Here are the part numbers if you want to order them from somewhere other than Mazda, but they are about US$50 each apparently from the likes of Mazdatrix.





The S3 RX7 diff carrier:



Note that I have not removed the pinion from the diff carrier and therefore the preload setting and backlash are all correct. Make sure to clean the diff carrier, especially the mounting points for the bearing caps, bearing races and the threaded parts for the bearing retainers.
Check for any high spots created during the marking of the bearing caps and bearing retainers. You can use a fine file to flatten them down.



Insert the MX5 LSD centre with the bearing races into the S3 RX7 diff carrier:





Refit the bearing caps loosely and refit the bearing retainers so that all the marks line up as they were before you disassembled the diff:











Note that this is a starting point for the backlash only. It still needs to be checked as correct.

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Checking the contact pattern.

You will need some engineering ink/paint to check the contact pattern. Apply the paint to the crown wheel, allow to dry and then rotate the diff forwards and reverse and check the contact pattern. It should be in the middle of the teeth in forwards and reverse:













Setting the preload on the bearings.
Here you can see I have tightened up the bearing retainer beyond where it was originally with the open diff centre. The tighter setting ON ONE SIDE gave the above contact pattern.





I still have 2 setting I need to check; backlash on the crown wheel, which I expect to be correct as I haven’t removed the pinion and I am using the exact same crown wheel and pinion, and the distance between the 2 lugs on the bearing caps which I pointed out earlier.
I don’t have these tools at the house so I will check these when I go to my garage.

Currently, the diff turns nicely, there is slight backlash between the crown wheel and pinion and the bearing caps are back in the same location so perhaps only very minor adjustment required if any.

This is the finished article:

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And just for good measure, these photos show a S2 RX7 diff inserted into the MX5 diff mount:









Enjoy!

[KiwiDave]

Massively interesting build thread dude. Have just torn up my Santa wishlist and started again