A little while back, my transmission was slipping and lost power (also made a nasty rattle noise) so I decided to rebuild it. It is the A4RA transmission that is found in the 1996 Automatic Honda Civic. My problem is that now it is leaking from the Transmission Housing Gasket. This transmission rebuild was my first time ever working on a transmission, but I think I did a good thorough job (No leftover bolts, springs, *****, or dowels). I followed the official Honda service book, and I also used a YouTube guide for reference as well as online forums. In this rebuild, I replace every gasket and every O-ring, cleaned all the residue from all the crevices (Including the valve body), replaced the filter, replaced the torque converter, and I replaced the clutch packs (everything new, nothing OEM). I did not touch the differential, although I did clean it. When reassembling the body, I also made sure EVERYTHING was to torque (even the 8.7ft.-lb. 10mm bolts, followed patterns, and even followed the two-step torque directions). The only thing I regret not checking was the clearances for the clutch packs (this is because, although the service guide informs you to check them and provides clearances, the YouTube guide didn't).

After everything was reinstalled, I filled it up with official Honda ATF transmission fluid (accidentally did about a quart more than recommended) and drove it (the same way it says to in the service manual). There were 0 issues and the car felt amazing, it was extremely responsive and it shifted through all the gears (including reverse). When I got back, was when I realized that it was leaking a lot. This was when I realized that it was overfilled, so I pumped some ATF out from the dipstick hole, hoping this would resolve the issue, but it didn't... I ignored it for the time being, but didn't drive it until I emptied the transmission and refilled it (again, following the service manual, and hoping that the issue would be resolved). The issue was not resolved despite making sure to fill it properly this time, so I thought "maybe it's the solenoid gasket since I've seen fluid there before" so I replaced it. That also did not fix it, and at this point I know it's coming from the gasket. I even let it sit for a while hoping it would stop leaking but it never did. It threw so much out, the dipstick didn't reach the ATF anymore.

So, my transmission housing gasket is leaking a lot from the top (Near the solenoid, towards the front end) and all the way back, probably all the way to the bottom. It's leaking considerably from under the starter specifically. The weird thing is that the mating surfaces were scrubbed with a soft non-abrasive and wiped cleaned with a degreaser prior to assembly and I used a new gasket (of course). Could this leak stem from a bigger issue like excess pressure which could be caused by user error in the rebuild process (like the clutch plate clearances), or could this be as simple as a failed gasket? I don't think it could be a failure in the reinstallation process, because there would also be an issue with shifting (Which it didn't have). The gasket was made of paper which I did find odd, but the other mating surface gaskets were also made of paper and it's not leaking (I think the original was also paper, but it had solidified over time). It's not overfilled and the breather valve is not blocked. I don't think it's any of the solenoids because, again, the car drove. Could it maybe be that the transmission overheated when it was slipping causing the mating surfaces to warp? What I want to do is check for proper pressure first, but before I do anything I'm looking for any advice at all. I'm new to the automotive industry so even the little reasonings I'm giving with my findings could be misinformed. Maybe a nudge in some direction or something obvious that I'm overlooking. The reason I'm asking and not just getting to work is that I've seen people with similar leaking issues who found a solution that didn't involve taking it apart.

It's worth mentioning that I don't have a check engine light but I did have a code indicating that the crankshaft position sensor was not connected properly, but I'm assuming it's fixed because there is no check engine light, and if it was broken, I don't think it would be the cause of the leak. It's also worth mentioning that I never found the cause of the nasty sound I heard when the transmission was slipping. If I missed anything, I apologize as I am also pretty new to the forum.

TLDR;
I rebuilt the A4RA transmission in my 1996 Honda Civic, carefully following the official Honda service book and additional guides. Despite that, I believe my transmission housing gasket is leaking. Initially, I overfilled the ATF fluid but later corrected it. Changing the solenoid gasket from the top didn't fix the issue (how I know it's the housing gasket). Now, I'm wondering if the leak is due to a larger problem like incorrect clutch plate clearances, or if it might be a simple failed gasket. Although I don't have a check engine light, I had a previous code concerning the crankshaft position sensor. I'm seeking advice before taking any more steps.

 

This is where I got the filter, gaskets (and seals), o-rings, and clutch packs:
https://www.ebay.com/itm/173873725536

Red arrows indicate about where it's leaking from that I know of (these images are from when my transmission was initially taken apart and was for reference in reassembly).


Mating side.

 

I'm not a fan of taking it out and changing the gasket, and since everything is new and clean, I think I'm going to JB Weld the entire mating area around the transmission and deal with it in 5-10 years 👍