sevenracer
sevenracer Reader
6/28/21 11:44 p.m.

This is post rebuild documentation for a first gen RX7 transmission.

A little history. I've been racing my rx7 for years, and I have used up a few transmissions. Usually these were junkyard boxes with well over 100k miles, and I would get a couple of seasons before the synchro's would wear beyond a usable point. I did actually have 3 transmissions built by 2 different race shops. One of those never finished a race, another lasted about one season before failing catastrophically, and the third actually lasted several years - the longest lasting box I've had. So, 1 out of 3 success rate on pro built boxes.

This time around, no luck finding any junkyard boxes and both shops I had used in the past have closed up now, so it was time to dive in and do a rebuild.

I learned a lot on during this effort, mostly how not to do it like:

Don't watch your buddy disassemble the trans while shooting the E36 M3 with him, taking no notes and maybe 3-4 vague pictures, then put everything into a couple of boxes  for 4-5 months. And don't pull everything out and try to get it organized, then box it up again to free up space for other projects. And buy the replacement parts you need a couple months later, then try organizing again and set aside, etc.

Now to be fair, that same buddy who helped me disassemble and diagnose, very kindly sold me one of his spares, so I thought there was no huge rush - I was happy to kick this can down the road. Which brings me to another how not to do it: Don't install a new transmission without testing it in all gears first. Yep, the trans I bought for some reason didn't have neutral, which I discovered after putting it in. Out it came and the old haggard box got put back in for one last race weekend.

 

When I finally got down to doing the rebuild, I was trying to organize all the parts. I laid them out on some paper on a table with notes and sequence drawn on the paper. But this really didn't do it for me. It was really hard for me to quickly figure out what went where (note I was fortunate to have a few examples, so I have a partially disassembled unit as reference sitting on the table with all the parts).

As a way to manage the parts better and keep them in order, I came up with this from scraps I had lying around - my transmission abacus! Really helped me visualize the assembly and keep things sequenced and facing the right way. There are 4 "gear stacks" in the transmission, so I have four posts on the left and corresponding ones on the right. If I need to get at any parts in one of the stacks for cleaning/inspection or to replace with new, I can just slide  parts over to the right side until I get to the one I need at the moment.

 

I also needed a reaalllly deep well socket for removing a nut off the main shaft. This took a whole lot longer to fab than I expected because my chop saw died before a got nearly all the way through the socket. I had to use a combination of much less suitable tools to gnaw it the rest of the way off:

infernosg
infernosg Reader
6/29/21 7:36 a.m.

This is something I've done twice now and I'll probably be doing again in the future. Let me know if you have any questions or need pictures of something for reference. The first time I did this I took a pretty comprehensive set of pictures. I like your method of organization better than mine:

Note this was before everything was cleaned but my general method has always been to tape parts to a table or piece of cardboard. The numbers are the assembly/disassembly order as shown in the FSM.

j_tso
j_tso Reader
6/29/21 8:57 a.m.

Good stuff. 

One of my back burner projects is a first gen transmission I picked up for free.

What have you all used for pullers? I found a Mazda dealer puller set on ebay.

infernosg
infernosg Reader
6/29/21 9:28 a.m.

In reply to j_tso :

Just a standard two-jaw puller with "extended" arms made out of some scrap iron flat stock:

Works well for bearings but the 5th/reverse clutch hub is still a bot of a pain because there's not a lot of room to squeeze the jaws of the puller between the hub and reverse synchro.

Brett_Murphy (Agent of Chaos)
Brett_Murphy (Agent of Chaos) MegaDork
6/29/21 2:14 p.m.

The transmission abacus is amazing. 

sevenracer
sevenracer Reader
6/29/21 10:23 p.m.

In reply to infernosg :

I used a set of 3 jaw pullers with extended arms, and I think a 2 jaw setup would probably work better. keeping the 3 jaw in place to get it tensioned is easier with 3 hands. I also ground down the jaws to get clearance.and a couple of harbor freight hammers helped perusade things as well.

sevenracer
sevenracer Reader
6/29/21 10:40 p.m.

There wasn't too much wrong in there. Nothing broken, just worn out. Mainly the brass synchro rings were worn (no pics of those) and the clutch forks were amazingly worn -  instead of .010" gap in the clutch sleeve, had more like .125". The pads on the fingers were basically completeley worn off. Not really sure how much that was affecting shifting, but it was waay out of spec. The dogs on the clutch hub were also really chewed up. The bearings were just slightly loose feeling but really not bad at all.

 

Parts replaced:

bearings

clutch forks and clutch sleeves

syncrhro's

clutch hub keys and springs

snap rings (most of these deformed on disassembly)

lock nut

infernosg
infernosg Reader
6/30/21 1:04 p.m.

It's my understand the clutch forks are very much a wear item. I didn't replace them on my first rebuild but did on the second. Mine were out but not as much as yours. The Spec Miata guys go through them all the time and I've even seen some interesting ways they're tried to improve durability. The most common appears to be to try to harden the wear surface by heating and quenching. I also replaced all the bearings during my last rebuild but I did not replace any synchros or clutch parts. I confirmed everything was still in spec but am still kind of kicking myself for not replacing them anyway.

It's interesting to see how Mazda has revised these transmissions over the years. They made these from '78 in the first gen RX7s all the way through the 5-speed NC Miatas in 2013 and made constant changes. The biggest being a transition to a double synchronized second gear somewhere in the mid-90s. I've done the Miata gear swap so I just order parts for a 2013 Miata now.

Toyman01 + Sized and
Toyman01 + Sized and MegaDork
6/30/21 1:48 p.m.
sevenracer said:

This may be the most brilliant thing I've seen on here ever. Outstanding. 

 

dannyp84
dannyp84 HalfDork
7/11/24 12:41 p.m.
infernosg said:

It's my understand the clutch forks are very much a wear item. I didn't replace them on my first rebuild but did on the second. Mine were out but not as much as yours. The Spec Miata guys go through them all the time and I've even seen some interesting ways they're tried to improve durability. The most common appears to be to try to harden the wear surface by heating and quenching. I also replaced all the bearings during my last rebuild but I did not replace any synchros or clutch parts. I confirmed everything was still in spec but am still kind of kicking myself for not replacing them anyway.

It's interesting to see how Mazda has revised these transmissions over the years. They made these from '78 in the first gen RX7s all the way through the 5-speed NC Miatas in 2013 and made constant changes. The biggest being a transition to a double synchronized second gear somewhere in the mid-90s. I've done the Miata gear swap so I just order parts for a 2013 Miata now.

I'm reviving this because during my last race in my ITS RX-7, 5th gear caused the whole car to shudder, and I found a fair amount of brass in the fluid afterward. How difficult is the Miata gear swap if you've never build a transmission before? I pulled the transmission last night and I have two spares, but I'd like to rebuild this one a bit better than stock if possible.

sevenracer
sevenracer HalfDork
7/11/24 3:26 p.m.

In reply to dannyp84 :

You can run the entire miata gearbox if you swap the Bell housing, case, and tail housing. All the case pieces bolt onto the center plate. You do have to machine down the end of the input shaft - length and diameter. Doesn't actually meet ITS rules though.

I did this last year - I bought a new Miata box from Mazda Motorsports and had a rebuilder machine the input shaft and swap the case.

 

dannyp84
dannyp84 HalfDork
7/11/24 5:40 p.m.

In reply to sevenracer :

That's cool that new gearboxes are still available. For now though, I need to stay within ITS rules. Maybe in the next couple years I'll move the car up to EP, and have some fun with the modifications that allows.

RacetruckRon
RacetruckRon SuperDork
7/11/24 10:06 p.m.

I'm glad this got bumped, the custom tools and organization efforts are great ideas to save. I may need to replace a 3rd gear synchro on my CD009 in the near future. 

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