On the Mazda Comp website they list something about a pinion adapter for the fc rear end

http://www.mazdamotorsports.com/pdfs...Drivetrain.pdf

bottom of page 2

"NOT E : 1986-91 RX7 Non-Turbo Ring & Pinions: The 4.33 ratio gear set is the lowest gear set available for 1986-91 non-turbo
RX7s. To obtain an even lower gear ratio (4.44, 4.875, etc.), there are two options:
1. The least expensive modification is very complicated and should be done only by qualified machinists. It requires grafting
(splining and welding) of a short pinion shaft from an early (1979-85) RX7 to the end of an (1986-91) RX7 non-turbo
long pinion shaft.
2. Purchase an adapter kit that allows you to fit a short pinion gear set to the 1986-91 RX7. This is more expensive than the
modification explained above but does provide the ability to install a different pinion shaft later, if you need an alternate ratio."

Does anyone know anything about this? or have any pictures of one?

I did a quick search but didn't find much

 

I always just lengthened the pinion shaft as listed in #1. #2 sound like they sell you a new diff carrier to attach to the stock aluminum housing.

 

That's what i'm going to do . . . Any tips for lengthening the pinion shaft? I read this thread which seems to be not too complicated

https://www.rx7club.com/race-car-tec...-shaft-633805/

I have access to a lathe. I've machined external tapers before but never the internal ones. The only thing i'm really concerned about is maintaining a reasonable amount of runout

 

I remember seeing that thing on Mazdatrix website a loooonnnnnggggg time ago. It was super expensive. It basically made it so you could swap the pinion gears easier.

I'm not sure who fabricated it, Speedsource maybe?

 

We sent ours to Rebello to have the shaft lengthened.

I take on a lot of projects, but this is not one that i would do. I have access to lathes, tig welders, and most machining equipment. Just a risk/cost ratio for us. It wasn't worth it for me to potentially screw it up

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