How To: Swap a 20v by System

Courtesy of mr220v of International MR2 Owners Club | March 8, 2007 | Print

2. Transmission

All mk1 mr2 transmission will physically bolt up to the 4age 20v. There are considerations you must make though.

• A series motors used 2 different sizes of flywheel. Early mr2’s, up till around 1986, used a 200mm flywheel. From 86 on, a 212mm flywheel was used. (200mm and 212mm pertains to the griping area of the flywheel. The actual diameter of the flywheels was the same). The 20v came with the later size flywheel, so when ordering a replacement clutch and pressureplate, just ask for a clutch kit for a 1989 mr2. If using the smaller flywheel, ask for a set from an 85 mr2. Either clutch set will be sufficient for the stock 4age 20v.
• Early c50 transmissions only had one mounting position for the starter. With this transmission, you are limited to mounting the starter on the exhaust side of the motor. These transmissions are on the 85-87.5 mr2’s. Later mr2’s had the c52 transmission. This transmission had a secondary starter mounting position on the intake side of the motor, just like the stock 20v transmissions. If you plan on using your 16v exhaust manifold, which is perfectly fine with a silvertop, you can mount the starter on either side. If you plan on using a 20v header, especially the blacktop’s, the header will not clear the exhaust mounted starter. If you must use the exhaust mounted starter in this case, the header will have to be modified for clearance.
• The c52 and c56 that comes with a 20v can also be used with modification and transferring a few parts from the original mr2 transmission, namely your mr2 selector shaft, and the entire mr2 shifter assembly. On the silvertop c52, a hole will have to be drilled for the selector shaft and the oil seal. On the C56 however, there is no casting for the oil seal. One of 3 approaches has to be taken.
• The c52 and c56 that comes with a 20v can also be used with modification and transferring a few parts from the original mr2 transmission, namely your mr2 selector shaft, and the entire mr2 shifter assembly. On the silvertop c52, a hole will have to be drilled for the selector shaft and the oil seal. On the C56 however, there is no casting for the oil seal. One of 3 approaches has to be taken.
• The c52 and c56 that comes with a 20v can also be used with modification and transferring a few parts from the original mr2 transmission, namely your mr2 selector shaft, and the entire mr2 shifter assembly. On the silvertop c52, a hole will have to be drilled for the selector shaft and the oil seal. On the C56 however, there is no casting for the oil seal. One of 3 approaches has to be taken.
• Add the additional aluminum casting, and drill as you normally would
• Fab an adapter or a makeshift casting (this has been done with JB weld, not the ideal way, but seems to be holding up in this case)
• Reuse the center transmission casing from your original mr2 transmission. This requires taking both transmissions apart

Here are some pics of the problem area:

-6speed transmissions are available for the 20v, known as the c60. Some blacktops came with a 6speed stock, although it was not a real common option. The same considerations must be made for the oil seal casting that have to be made when using a blacktop c56 5 speed. Getting a blacktop with a 6speed is usually somewhat pricey.
One other option is to use a 00+ Celica 6speed. The bellhousings are different, but can be swapped somewhat easily. All that holds the bellhousing to the central casing are the external 12mm bolts, and two 12mm bolts on the inside of the bellhousing that are attached to a reverse lockout arm. Once the external bolts are removed, the internal ones can be removed and installed through the 1-2 inch resulting gap. It’s a little bit tricky, but is much easier than taking the transmission apart. Once apart, remove the old differential bearing races and shims from the 4age bellhousing and replace them with the ones from the original 6speed bellhousing. It is very important that the differential stay with all of its original races and shims. This is the only way to keep the tension on the differential somewhat within spec. Too much or too little tension, and you will burn your differential bearings.

The celica 6speed will also require the use of 1991+ mr2 NA axles. No drilling is required since the Celica’s selector shaft is already on the correct side of the motor. Just add the mr2 selector shaft and shift assembly.