For FIA historic rallying I can only use original homologated parts or, if those parts not available, something very similar scrutineers will accept (or not notice).

Mazda Sport gearboxes and gear kits are no longer available so I've chosen to use the Fb gearbox with Miata gears. In order to do that, the input shaft must be modified for RX7 clutch and flywheel.

Mazdatrix has a picture on their wesbsite with the dimensions and I machined mine to those but the input shaft did not fit. The splines with fouling on the clutch disc.

Below are the revised dimensions that did work.

 

What car are you rallying? Any pics or vids?

 

Currently engineering a Group 2 series-1 RX7, 1978 spec.

 

Interestingly, I have machined 3 to the Mazdatrix drawing and they have worked fine. Not sure why. In any case, an 1/8" shorter won't hurt a bit.

Carl

 

Hi Carl. I don't have enough technical experience with these cars to understand any differences in flywheels but the clutch in the car is a competition single plate and the fouling occurred on the front edge of the input shaft splines after it passed through the disc splines.

 

What were the symptoms of the problem? Could you not get the transmission bell housing to meet up to the back of the engine? If so, was it possible that there was too much grease in the pilot bearing preventing the input shaft from inserting completely into the back of the eccentric shaft? I have not experienced that problem personally, but have heard (and can imagine, which is sometimes the fatal flaw in problem analysis) it can happen.
Carl

 

That was the symptom but it wasn't a grease issue. The input splines were fouling on the flywheel.

GC

 

Interesting. Of the 2 cars I have used them in, at least mine has a lightweight aluminum flywheel, so maybe the interface with the input shaft is a little different. I will keep the shorter lengths in mind if I have to make another one. Thanks for the info.

Carl

 

When I did mine, I ran out of time to machine the input shaft and just sent it to Mazdatrix, it worked fine for me.



To build the ultimate first gen racing transmission with all Mazda parts requires 3 donor transmissions. As you can see in the picture the bell housing is from a NA FC transmission, it is stronger with some ribs, and has a vent to help keep the clutch cooler. The center section with the gears is from the Miata, but you do have to disassemble it completely to install the cut down input shaft. The tailshaft housing is from an FB transmission to fit the stock crossmember. The shifter tower is from the Miata transmission, it has a much better system spring and plunger system to center the shifter. You can see that I also have a Miata short shifter installed, you can use one of these with either the Miata or RX7 shifter tower.

 

I've never (personally) had issues with case flex that the bellhousing would make a difference for. The bearing spread is largely contained by the inner part of the bellhousing case that has an air gap around it, and the front input plate (whatever it's called).

The largest issue I have had with smoothcase transmissions is the bolts for the main bearing plate backing out. The plate absorbs ALL thrust loads under load! The bolts will back out enough that the mainshaft can rack back until 1st gear grinds against the midplate, filling the oil with metal and destroying the bearings, as well as making the synchros less effective. And then the bolts start to fall out, sometimes getting stuck in the gears.






I now replace the five Mazda "peanut butter bolts" with 25mm long Grade 8.8 bolts with serrated heads, Loctite, and torque to 40ft-lb. You can take them to 45ft-lb but sometimes the case starts to strip. There is room for 30mm long bolts but the threads are not cut that deep for some of the positions, so there's no point.