Part 22

Transmission Woes

A busted transmission cut our first race short, taking us out just about an hour after the green flag on Sunday.  This was something that obviously had to be addressed/repaired before the next race. 

In the forums, when we described our race weekend and what had happened, someone suggested that we could improve the gearbox by getting ahold of a later-model Miata gearbox and modifying it to fit our RX-7 drive train.  The Miata gearbox is stronger and has a better set of ratios than our stock box, so it sounded like a good idea.  All we had to do was:

1. Find a Miata transmisson
2. Disassemble it and replace any worn parts
3. Disassemble a 2nd gen transmission (which we already had as a spare)
4. Send both input shafts out to have the Miata shaft machined down to the correct length
5. Reassemble the Miata gearbox
6. Re-clock the shift rods to work with the 2nd gen shift linkage
7. Bolt on the 2nd gen bell housing and tail shaft housing

What could go wrong?

We also needed to figure out what went wrong with the original transmission because we wanted to fix it so we could keep it as a spare.  Disassembly was fairly easy, even though the factory service manual is a POS.  But one thing became clear pretty quickly: getting the thing back together was going to be much more difficult than getting it apart.

Here's the actual "gearbox", with the bell housing and tail shaft housing removed.

 These are the shift rods.  Each rod controls two gears.

 This is a view of the same end, but looking into the box. 

 This pic shows the gear and synchro sets (most of them, anyway).  On the left are the shift rods and forks.

Gearbox with the housing removed.

 A lot of these pics I took just so I'd know what it looked like before I tore it apart.

 With everything torn down...

An important thing about rebuilding a transmission is the installation of the bearings.  You don't want to pound on them too much or in the wrong way or you'll damage them.  After that, it's just a matter of putting everything on the main shaft, input shaft, and lay shaft in the right order and in the right orientation.  If you can turn the input shaft and the output shaft turns in each gear, that's a pretty good sign.  All told, it probably took me 5-6 hours to rebuild my first transmission.

I'd like to say "and that's all there is to it", but that's not exactly the case.  I had just finished rebuilding the engine and I wanted to get it into the car and get it running.  You can't really install the engine by itself due to the way the engine and transmission are mounted in the car.  I ended up installing this transmission (our backup box) temporarily so we could run the engine.  For some reason, even though it was working perfectly before, it now would not shift into 3rd or 4th gears.  It was stuck pretty good. 

So out it came...again.  I'm getting pretty good at dropping the transmission out the bottom of the car...

I knew the general area the problem would be found in; it's the same shift rod for both gears so it had to be something with the rod, the detents (ball bearings that keep the shift rods in place when a gear is selected), or the synchros.  Carl was over that day and helping me out, so it was a lot easier getting it apart then doing it all yourself. 

We took the covers off and, of course, the problem disappeared.  I inspected the shift rod, the fork, the synchro; all looked good and were now working properly.  We removed the shift switches (used for emissions signaling, reverse lights, etc.) and I noticed that one had a thick aluminum washer/spacer and the other one didn't.  The washer definitely made a big difference on how far the switch housing protruded into the case, and the clearance between the shift fork and the switch.  I inspected the switch housing and found some wear marks; looks like a root cause to me...

We reassembled the transmission and everything seems fine; that could have ended a lot worse.

One transmission down, one to go...