20b Cosmo diff setup
 

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.

https://cimg6.ibsrv.net/gimg/www.rx7...4194b5b3b7.jpg

https://cimg9.ibsrv.net/gimg/www.rx7...ab88e6c4e7.jpg

https://cimg0.ibsrv.net/gimg/www.rx7...97ca6ec1d5.jpg

https://cimg1.ibsrv.net/gimg/www.rx7...8231adc186.jpg

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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