CJ T-18A Transmission Swap
After several trail runs with my 78 CJ-5 equipped with a T-150 3 speed trans it became obvious that a gear change was essential. My CJ has 3.54 rears, a Dana 20 transfer case (2:1 low range), and low a gear in the T-150 that is only 2.99:1. This equates to approximately 21:1, this is rather pathetic, I found I spent a lot of time having to slow down to prevent running into the Jeep in front of me and having to do the old bump-bounce-stop over rocks. Not exactly rock crawling if you ask me.
After some extensive research I decided on a T-18A swap. The T-18A was a factory option in the late 70′s CJs and is equipped with a 6.32:1 low gear. This combo will make the CJ’s crawl ratio about 48:1, more than twice what I had. The most appealing things about this swap is that it is essentially a bolt-in operation. The big problem with this swap is finding a trans to use. Amazingly a friend of mine knew of one sitting in the back of a Jeep pickup at a local junk yard. The T-18A also was a factory option in Jeep pickups. These transmissions require some modification to fit a CJ. First I must say that this trans is massive, next to my T-150 it is 3.75″ longer and about 3″ taller. It will be necessary to alter the drive shafts due to the increase in length. I am probably going to use a CV style rear drive shaft since it will only be about 14″ long. Also my dual exhaust system may have to go but I will worry about that when everything is in.
The Jeep truck models use a different input shaft which is about 15″ long. The bell housing is deeper and it also uses a 5″ spacer between the trans and bell. I learned that the input shafts are interchangeable and better yet I found a place Hick’s 4X4 (Evidently Hicks is no longer in business – link was removed) in California that sells a kit to do this. The kit came with a new input shaft, pilot bushing, and a new bearing retainer. I also ordered a rebuild kit which came with gaskets, bearings, synchronizers, and seals.
I completely disassembled the trans (not a job for the mechanically challenged) and cleaned it up. I found a couple of strange pieces in the bottom which turned out to be pieces from the shifter fork rods, at first I got worried but luckily there was no damaged components. I then reassembled the trans with the new and old parts. After a new paint job it was ready to go in.
I gave a quick call to Mike at OK 4wd looking for a new clutch, we decided on a Centerforce clutch and disc. The T-18A uses the same bell housing and clutch configuration as the T-150 so everything fit perfectly. I also sent my flywheel out to be resurfaced (this is essential for a clutch to properly break-in.
This transmission probably weighs 100 pounds, my dad and I wrestled it under the Jeep and managed to get it bolted to the bell housing with the help of a floor jack. A few times I almost pictured this anchor called a transmission coming crashing down on us. The skid plate on the Jeep needed to be moved back but there is existing holes in the frame and the plate bolts-in. I had to make some minor modifications to the trans mount since this trans came out of a Jeep pickup.
the trans was all bolted in I attached the transfer case, next I had to extend the rods attaching the 4WD selector to the transfer case. This was due to the fact the trans is 3.75in longer.
I did a little measuring and the sent off my old drive shafts to be redone to facilitate the length change of the new trans. I had the front lengthened and the rear shortened. I also had a CV style joint installed on the rear drive shaft to reduce vibration.
As soon as I picked up my new drive shafts they were installed and the project was nearly finished. Just an adjustment of the clutch and also a set of 4 degree shims in the rear axle.
I fired up the Jeep and out the door I went. High range driving using the 2-3-4 gears is all but identical to the old 3 speed. But slip it into 1st (or Low) and you creep along. Even in high range. Low range is really cool, Lo gear is incredibly low. I would say I can do 5mph at 3500 rpm, I found I can just let the Jeep idle and I can crawl up on most anything. Reverse is even lower which makes it even easier to backup on the trail.
The first off road trip with my new trans was the most fun I’d had. I now could let the Jeep idle and creep over anything in my path. It is a rare occasion that I had to use the clutch or the Jeep stalled. It took me a while to get used to which gear I should be using, sometimes I was going to fast in 2nd and sometimes too slow in 1st. One draw back with this trans is that 1st gear is not synchronized and if I am in 2nd I have to stop to go into 1st.
In summary this was the most dramatic and worth while change I have done to this Jeep yet. UPDATE: Be sure to see the article about adding clearance to the clutch arm to avoid excess wear. CJ T-18A Transmission Swap Revisted – A Tip