RX7Club.com - Mazda RX7 Forum - FB RX-7 Transmission Crossmember Bolts

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...:facepalm1: