Driveshaft lengths compatibility
 

Hello to all of the road racing gurus. I recently suffered gearbox/driveshaft failure on the way to the track. The tailshaft housing broke apart at high speed possibly due to a drive shaft imbalance or u-joint failure. I will have to replace the early 5 speed gearbox in my SA22C race car with a later one from an FB. The car has what I assume is an FB rear axle as it has disc brakes but 4 X 110 mm bolt pattern. I am aware that I will have to cut the tunnel to accommodate the FB shift turret location. I will be ordering a new drive shaft from https://www.wholesaleimportparts.com...shaft_1979.php however, I am unsure if I will need to be specific with regards to the year of the gearbox and the year of the rear axle. Any advice would be appreciated. Thank you.
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Per the old Mazdatrix site: Driveshafts the same drive shaft worked for 79-82 RX7s so this would include the SA trans that you had.

The real difference is the mounting flange on the rear end. From 79-82 Mazda used a smaller pinion flange with a unique square bolt pattern. From 83-85 Mazda used a larger pinion flange. This means that the flange yoke on the earlier drive shaft is different than the flange yoke on the later drive shaft.

To determine what you have so you order a shaft that fits, look at the flange on your cars rear end. In general, if the holes are threaded then you have the small flange. If they are not threaded you have a large flange. The other thing to look at is how the bolt holes are spaced. On the small flange the bolt holes are not on circular pattern. The pattern is rectangular. On the large flange the bolts are in a circular pattern.

Also, based on the failure you had, you need to inspect the pinon flange on your rear end to make sure the pinion was not bent. It is easy to determine this - just put the car on stands and have someone rotate the tires while you watch the flange. It should not wobble visually and technically you need to put a dial indicator on it to make sure it is straight Next have the car alignment checked to see if there is excessive tow or camber on the rear end.

I have broken three drive shafts on my car as a result of various issues. I have bent the rear end pinion and bent the rear end housing as a result of this. So check it all out so you can avoid secondary issues when you put the car back together and take it racing next year.

As for the drive shaft itself. When I decided to replace the OE shaft when I was running in IT, I called Mazdatrix and asked about their reman shafts. They recommended that I buy a new one from Mazda. So I did. If you are registered with Mazda you will be able to buy a brand new shaft for a discounted price. It will likely be better than what you can buy anywhere else. Also, when you get your new FB transmission, make sure the tail shaft bushing is new. Then use new trans mounts and make sure your engine mounts are not torn or soft. All of these things can affect drive shaft performance in these cars.

Not to take this thread off topic but… is there a replacement transmission brace bushing make of poly or something stiffer than stock?

Yes. I believe there are competition trans mounts.

Carl

Stiffer trans mounts risk damaging the tailhousing unless you have stiffer engine mounts.

The engine and trans are mounted basically like a tripod. The drivetrain torque (what rocks the shifter side to side) is handled by the two engine mounts, drivetrain pitch (what pushes the shifter up and down) is "handled" by the trans mount. I quote "handled" because there are no forces to generate pitch, this isn't a Subaru or an Audi :)

Because of the way the trans mount attaches, if you have engine mounts that allow a lot of motion, and a trans mount that doesn't, the trans mount is going to torque horribly on the tailhousing. Given how utterly unimportant the trans mount is as long as it holds the end of the trans at the right height, I'd strongly suggest to leave it alone. There's zero benefit, and a lot to lose, with a stiffer mount.

Yeah like peejay says, mismatching mounts is not a good idea. When I ran in IT spec I ran stock rubber mounts and added a torque brace. The torque brace keeps the shifter from moving around when the engine twists under power.

Thanks. Good points.
In my case, I need to buy both engine and trans mounts. Probably going poly for the engine mounts unless I the Mazda Comps are similarly priced - FC subframe and engine. Race car, not street car.