I thought I should make a post about changing the ratio in a 20b Cosmo diff. I struggled to find much info on Cosmo diffs, not many people seem to have messed with them. I've got a 26b pp Cosmo and the 3.9 factory ratio is just not ideal for a high revving, low torque engine. I have only dealt with the 20b diffs not the 13b diffs.

Here's some differences between the 13b and 20b:

-13b stub axles are 30 spline ( same as rx7 FC, FD and rx8 )

-20b stub axles are 31 spline ( larger OD, apparently the same as the light duty mazda utes and vans? )

-13b crown wheel bolts are M10

-20b crown wheel bolts are M12

Here's the similarities between the 13b and 20b:

- Bearings and seals are the same, also the same as rx8, FC and FD

- Pinion length is the same, including rx8 and FD

- Crown wheels PCD is the same, including rx8, FC and FD ( only difference being the M12 threads in the 20b crown wheel )

Knowing this info a bit of mixing and matching can be done to setup a different ratio in your Cosmo. I got hold of an Rx8 6 speed diff for $200nzd, depending on the year these will be either 4.44 or 4.77. The one I got is a 4.44 which has completely transformed the car, accelerates much better now.

 

Because the 20b stub axle splines are different and I don't have any 13b Cosmo axles I had to retain the factory 20b lsd center. These 20b lsds are viscous I believe, ( works all right so far )

Next issue I then had was the 12mm crown wheel holes in the Lsd center, ( the rx8 crown wheel has M10 threads ) so I machined up some spacers to go on the M10 bolts.

The pinion height was a fair way off, the rx8 pinion was sitting far to low. I made my own pinion spacer adding a total of 0.243". I set this up using the factory service manual for an FD rx7.

Since I had the diff apart I also machined a solid crush tube spacer. Somehow I fluked it and got it bang on, requiring no shims!

I reused the original bearings, had bugger all kms on them. I replaced the pinion and axle seals.

Other than these differences it's all just standard procedure for setting up an rx8 or FD diff.

 

Here's some photos, ( RX8 pinion is the one on the right, Cosmo on the left ). Solid crush tube in last photo.

 

Nice work, excellent info for Cosmo owners. Did you consider converting to an 8,8 at all.

 

Nice work and handy information

 

Thanks, at this stage I don't need to upgrade the diff. If I was going to go for a stronger diff I would use something like a Winters quick change.

Thanks

 

I am not very knowledgeable about rear diffe. Is the parts needed to do this swap able to be bought any where ? Like spacers and such. I have a spare rx 8 4.30 diff. Could you just swap ring and pinions and it work ?

 

The spacer for the pinion i made myself. A new factory crush tube could be used instead of a solid one. Your 4.30 ring and pinion will fit. It will need setting up the same as any diff, gear mesh etc. A diff shop should be able to do it for you.

 

You able to make another set of the needed spacers and tubes ?

 

you can get that made any where, even a small home garage lathe can do it. I can get the ID and OD dimensions as I have an OE spacer laying around, but he will have to provide or you measure the thickness needed.
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