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Discussion Starter #1
The S54 found in the 94-99 Celica GT is the same as the one found in the 93+ MR2 except the shifter being in the "back" of the transmission.

The upgrades for this transmission over the 91-92 are the upgraded synchros, digital speed sensor, extra engine mount holes along the bottom, and it is more available than the MR2 one.

Below is how I modified it to fit my 91 MR2 with cable drive speed sensor, but the case modification steps should apply to all cars

First split the cases and get the middle case by itself. You'll notice that there is a small hole in the front of the case where the gear selector mechanism sits. This will have to be drilled out so the MR2 shift rod can go to the front. Thankfully its about the size of a US drill bit and there is a sleeve to help keep the bit centered. After drilling through to the other side, knock the sleeve out.
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Next go to the other side of the case and drill out the red shaded area. Your goal is to take out just enough material to get the shaft outer o-ring to sit in the case. Remember to test fit the rod with the oring in steps so that you don't remove too much material.
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Next up is the gear selector mechanism. Remove everything from the MR2 rod and install everything from the Celica one except for the C-clip on the front of the shifter. It won't go into all gears with that still attached and the shaft gets bigger right before it so it isn't needed.
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Reassemble the transmission, install the gear selector mechanism, and make sure all gears engages correctly.

According to Toyota the number of teeth on the differental speedo gear is different between the 91 MR2 and 94+ Celica. The solution is to either convert to a digital speedo and run the Celica sensor or modify the MR2 one to work with the Celica gear. This will be a problem as the Celica one is signifiganctly shorter than the MR2 one.
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To solve this you add threads (M22 X 1.5) to the MR2 one and then trim it down until the speed gear pin sticks past the gear housing enough to engage the Speedo cable. I confirmed I trimmed enough by installing the cable and gear/housing together and then spinning it while watching the speedometer.
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Was it worth it? Yes because there weren't any 93+ transmissions near me that weren't at least double what I could get a Celica transmission for. Some people would suggest to swap the outer case from the MR2 but I've heard that the bearings are different between the 2 cases for 91/92 Vs 93+. I forgot to confirm that this was true, but swapping cases could also require re-shimming the differential which I was not about to do.

FYI the 92-01 Camry speed gear (33403-39265) works with the Celica differential should you want to adjust how the speedo reads as the Camry runs larger wheels than the Celica.
 

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Discussion Starter #4
This worked well because there's already a marked spot to drill through the case. If that spot exists on the Solara then most of the steps should be similar.
 
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