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The rear transmission mount on FB's (and probably also SA's) is attached to a simple crossmember which connects from one side of the transmission tunnel to the other. Searching google for "FB Transmission crossmember" will yield pictures of what I'm talking about, but it's likely you already know of this piece if you've ever been beneath your 1st gen RX-7. The rubber transmission mount bolts to this crossmember, and the cross member is bolted to the body through rubber bushings on either side. In this thread, I am talking about the bolts which hold the crossmember to the body, not the ones that hold the trans mount to the crossmember. On a stock OEM car, "Bolt" is a misnomer as the original construction was a post with threads on both ends, and a nut. This likely aided installation at the factory by ensuring the crossmember was properly aligned before they tightened it down.
A couple of weeks ago, I was driving along and everything seemed fine with my car.. then WHAM! clunk-clunk-clunk-clunk-clunk. I pulled over immediately was was able to quickly determine that it seemed like my driveshaft was hitting my exhaust, since the sound was speed-dependent, obviously at a higher frequency than my wheels, and would happen when the car was moving, even with the clutch in or in neutral. I supposed that the driveshaft may have broken at one of the U-joints, but the car moved under it's own power, so I wasn't sure. Once I got it home & up in the air, I tugged at the driveshaft and noticed no play, but I could see that the driveshaft was resting on the exhaust. Moving forward I soon found the problem. The driver's side transmission crossmember bolt had simply fallen out. The transmission was hanging about 2 inches lower than it's supposed to be, so the transmission end of the driveshaft would hit the exhaust as it turns. "Great news!" I thought, "All I need is a replacement bolt".
Sounds great, except every parts supplier or car being parted-out that I could find didn't seem to have the very simple part, and my research kept leading to people who simply bought a bolt at the hardware store (and a nice big washer) to replace the missing OEM pieces. My initial plan was to remove the other bolt and support the trans with a jack while I went to the store to find a compatible bolt, but my smog-legal California RX-7 has so many catalytic converters, there is no way to get at that bolt without removing the exhaust system, and I wasn't ready to wait for my threadtester to come in the mail, so I measured the bolt hole diameter to be an M10 bolt and decided to use the guess-and-check method by buying various lengths and thread pitches. I am sharing what I've learned here.
If you are looking for a bolt to replace a missing transmission crossmember post and nut, here's what you want:
M10x1.25 Hex-Head bolt, 50mm long.
M10 refers to the bolt diameter, 1.25 refers to the thread pitch.
You also want a nice big washer to distribute the bolt force onto the bushing, since the washer that came with the bushing is likely missing if the bolt or post/nut is missing.
When I jacked up the transmission to install the bolt, I had a problem- either from the fall or the imbalanced force on the crossmember, it had flexed, and the hole in the bushing did not line up quite right with the hole in the body. In this case, the hole in the bushing was too far forward in the car. I tried pushing on the jack supporting the transmission, or pushing on the crossmember by hand, to no avail. What did work is fairly simple- a bottle jack. I placed the bottle jack so the base was pressing against the starter motor and the top was against the transmission crossmember. Since the transmission mount is rubber, there is give and this greatly helped me in aligning the bushing to the bolt hole.
Now my FB is all fixed up like there wasn't a problem... though I am thinking about taking the bolt back out and putting some threadlocker on it...
The manual trans FB uses bolts, the auto trans uses the post. The SA, both manual and auto, use the post style. I have an SA with manual trans and FB that was a auto and now a manual trans.
The manual trans FB uses bolts, the auto trans uses the post. The SA, both manual and auto, use the post style.
Thank you for the clarification! It seems the documentation I had found was for the SA. Either way, it seems like the same type of bolt will do the trick. I just hope that someone else in my situation can reference this page rather than need to guess & check, which can feel like
Does anyone know if these bolts coming out is a common problem or just a fluke? Actually on 2nd thought I see that diagram showing a lock washer, something my car didn't have. Looks like I'm going to need to take that bolt out again. Hopefully I won't need a bottle jack to get the holes to line up this time.
Last edited by ThirdPedalNirvana; Nov 14, 2017 at 12:47 PM.
I just had the exact same thing happen to me. Approaching a stop light in the city, going ~40kmh, I downshifted to 2nd gear and.... bang...ding..,. ding... I could see in my rear view mirror one of the transmission crossmember "Studs" bouncing to a stop.
I managed to retrieve the stud (complete with lower washer, lockwasher, and nut) The upper washer and bushings aka insulators, I did not find and didn't want to wander around the street looking for them. I was able to limp home going very slowly.
And I've since realized what happened. On my '82, these crossmember bushings are about the only chassis/suspension part I have not overhauled as I couldn't find new bushings from Mazda. When I completed my engine/trans swap (along with new engine/trans mounts), I went to remove one side (to flex cross member down for clearance) and instead of the nut loosening, the nut/washer/stud assembly began to thread out of the chassis. That seemed fine, and I torqued it back up after, but in hindsight, torquing the assembly meant that I was fighting a lot of friction from the big washers and the rubber bushings. Obviously I didn't achieve the right preload, and its been slowly working itself loose over the years.
Anyway, now I'm trying to decide what to do. Removing the cross member is a bit of a pain. The passenger side stud is behind the heat shield and my RB centersection exhaust.
Once I get it out, I was thinking to try to fabricate my own replacement bushings, by modifying some various Energy Suspension type bushings. I'll have my remaining two passenger side bushings to model them after. I'm very surprised that these aren't available anywhere. At some point I think Blackdragon had them until they went out of business.
Or, what might be even better and more robust, is to make up steel spacers to replicate the thickness of the upper bushing+washer, and simply rigid mount the cross member to the chassis. I'm already running Mazda Factory Competition engine/trans mounts, so there is already very little driveline movement, and I can live with a slight increase in vibration. The FB seems unique in its arrangement where both the trans and cross member are 'bushed'. Stud plus lower washer loosened as an assembly.
Mazda's part number 9978-01-060 is an M10x1.25x60mm long bolt.
way back in my youth i stripped one of these holes, so a stud seems like a good idea to me
My chassis is an 82 and this stud looks to match the parts catalogue.
J9, do you have any leads on replacement "insulators". How are these $10 bushings not available anywhere!?
I've got my car up on jack stands and starting to gain access to the passenger side. I can see it under the heat shield but cannot access it without dropping the resonator and heat shield.
Yes, the actual trans mount is still available, even new from Mazda, plus I think even the Factory Competition mount is still available (stiffer rubber).
What I cannot find anywhere is the set of rubber donuts aka insulators that go between the crossmember and the chassis.
Mazda went with a 'double insulated' setup on the FB, where both the crossmember and the trans mount are bushed.
FB RX-7 Transmission Crossmember Bolts
I pulled over immediately was was able to quickly determine that it seemed like my driveshaft was hitting my exhaust, since the sound was speed-dependent, obviously at a higher frequency than my wheels, and would happen when the car was moving, even with the clutch in or in neutral. I supposed that the driveshaft may have broken at one of the U-joints, but the car moved under it's own power, so I wasn't sure. Once I got it home & up in the air, I tugged at the driveshaft and noticed no play, but I could see that the driveshaft was resting on the exhaust. Moving forward I soon found the problem. The driver's side transmission crossmember bolt had simply fallen out. The transmission was hanging about 2 inches lower than it's supposed to be, so the transmission end of the driveshaft would hit the exhaust as it turns. "Great news!" I thought, "All I need is a replacement bolt".
It seems the documentation I had found was for the SA. Either way, it seems like the same type of bolt will do the trick. I just hope that someone else in my situation can reference this page rather than need to guess & check, which can feel like 
