Swapping a THM-200C to a THM-700r4 Transmission

Determining what speedometer drive & driven gears you need

In order to determine which speedometer gears you need, you will need to know 2 things: what gear ratio your rear end has and your tire diameter. If you are unsure of your gear ratio you will have to pull the differential cover and find out (plus its an excuse to change your differential fluid (don't forget the GM Limited Slip additive if you have Limited Slip/Posi - Part# 1052358). There should be a ratio somewhere on the ring gear. You may have to put the car in neutral and turn a tire to find the numbers. Mine was 13:42 (42/13 = 3.23). Once you have this information, visit the 700r4 Speedometer Gear calculator. With the numbers that the calculator gave you, use the tables below to get the GM part numbers for your gears. Order the gears from any GM Dealership or using GmPartsDirect or you can get a complete kit from Bowtie Overdrives (includes speedometer sleeve housing, retaining clips, o-ring and gears).

Installing the speedometer gears and tailshaft housing

Once you have the necessary gears, clip, tailshaft housing and speedometer sleeve housing, you can install them onto the transmission.

First insert the clip into the cut out space on the drive gear and slide it onto the tailshaft. The tab on the clip should go into the hole closest to the transmission body on the tailshaft.

Before you put on the tailshaft housing, make sure you have a new o-ring for the gear sleeve housing, a square-cut seal for the tailshaft housing (GM# 1358899), the metal bushing (GM# 6260048) and a new end seal (GM# 8673526). Check out this picture if you need more clarification.

Install the bushing and end seal using a piece of a 2"x4" and a hammer. The bushing should fit flush with the inner lip as shown in the above picture (#1).

Once all the new seals are in the tailshaft housing (except for the speedometer sleeve o-ring), slip the square cut o-ring over the housing and slide the housing over the tailshaft with the opening for the speedometer sleeve on the drivers side bottom. You may need to use a 2"x4" and hammer to get the housing all the way in - at least I did.

Next loosely install the 4 bolts to secure the housing.

Now put the driven gear on the speedometer sleeve shaft (NOTE: I had to epoxy the gear to the end of the shaft because it was too loose). Insert the speedometer sleeve housing into the transmission and fasten down the retainer with the bolt. If you want to be sure that the gears are making contact, use a pair of pliers to shift the transmission into neutral (2 clicks, holding pliers from underneath, from all the way towards the front of the tranny) and turn the tailshaft using a pair of pliers with electrical tape on them (to prevent damage to the tailshaft). You should see the inside of the speedometer sleeve turning. If you don't, remove the speedometer sleeve bolt and try turning the inside while installing it to try to get the gears to mesh together.

Once this is all set, tighten the 4 housing bolts and you're done.

Transmission Pan Drain Plug

While I had the transmission sitting in my basement, I figured it would be a good time to install a drain plug. I got mine from summit from B&M for under $10.

To install it, first remove all of the pan bolts.

Next find a suitable location to put it. I ended up putting it on the curved area right underneath where the dipstick tube enters the transmission. This was the only spot where I felt like there was enough clearance from anything inside the transmission. Drill a 1/2" diameter hole where you want to install it. Make sure to clean up any metal particles from inside the pan. Also, if your pan doesn't already have a magnet in it, you should find a flat one about 2"x3"x.125" and put it at the bottom of the pan to collect any metal particles.

Next install the the drain plug with the plastic seal on the outside of the pan, right up against the pan. Tighten the outside bolt while holding the inside bolt steady with a wrench.

Install the plug nut while using a wrench on the large bolt head to ensure that the whole assembly doesn't rotate.

Finally, reinstall the pan (using a new gasket, if necessary) and torque all the bolts down to about 13 ft/lbs.

Removing the 200c

First you want to get the car up in the air. I had the luxury of using a lift, but it can be done with jack stands. Just make sure the car is high enough off the ground to get the transmission out from under the car once it's down. Also support the car by its subframes - make sure the rear suspension is not loaded.

Now that the car is off the ground, and the rear suspension is unloaded, remove the two bolts holding the torque arm to the rear end. I had to use a 7/8" open end wrench on the top of the bolt and remove the nut with a 3/4" socket. Once you have taken the two bolts out, you can remove the torque arm. Be careful when taking it off just incase it is still under any load, although mine was not. I used a rubber mallet and tapped towards the drivers side of the car to get the torque arm off the rear end. Next just slide the torque arm out from the bracket on the transmission.

Next remove the driveshaft. There are 4 bolts holding the u-joint to the pinion yoke. You might need to manually shift the transmission into gear to allow the driveshaft to turn to get at all 4 bolts. Once you have removed the bolts and brackets, ease the rear of the driveshaft down and slide it out of the transmission tailshaft housing.

Now you can remove the 4 bolts holding the dust shield to the bottom of the tranny. Once you get the dust shield out of the way you can start removing the 3 bolts holding the torque converter to the flexplate. A flywheel turning tool would be a very handy tool to have right about now (although I just pulled on the belts and/or the flexplate). I turned each bolt to between the oil filter and the block because that is where I had the most room to remove the bolts.

Now that the converter is no longer bolted to the engine, I removed the attachments on the drivers side of the transmission. The electrical connector should just unplug. The speedometer cable should just unscrew from the housing.

Now we can start removing the 6 bolts holding the transmission to the engine. Use a jack with a good size piece of a 2"x12" to support the transmission by the pan. Remove the 2 bolts and 2 nuts holding the transmission crossmember to the frame. Be careful not to loose the bolts that are inside of the frame. I lost one and just used some wire threaded around the bolt to pull it back through the hole. Also you might need to get a 9/16" open end wrench inside of the frame rail to keep the bolt from spinning (this is where it is good to have a friend with small hands).

Once the crossmember is just hanging freely, ease the jack down as much as you can to allow you more room to get at the top 2 bolts. To remove the two bolts, I used a 24" extension, a 6" extension and a deep socket. It is good to have a friend holding a spotlight where the bolts are so you can see what you are doing. Remove the bolts and they should stay in the deep socket so you don't have to worry about loosing them.

Next remove the remaining 4 bolts (2 on each side). You might want to raise the jack a little to make it easier. I used the same extensions to get these bolts as well. The only one that gave me trouble was the top drivers side bolt. I could not get a socket on it. I had to use an open end wrench to loosen it a bit and fed my hand up from the front of the transmission to loosen it.

Once you have all the bolts out, you can now remove the remaining lines from the passenger side of the transmission. I will start with the T.V. Cable. First get out from under the car and pop the hood. Remove the T.V. Cable from the Carburetor/Throttle Body. Get back under the car and loosen the small bolt holding down the T.V. Cable to the transmission. I used a small open end wrench. Once it is out, just pull the cable up a little bit and you should just be able to unhook the cable from the transmission.

Next you can remove the transmission dipstick tube. This should just pull out. It is a good idea to have someone holding a bucket or something (I cut the side off an antifreeze bottle and used that) because some fluid is going to come out.

The last thing you have to remove before you can get the tranny out are the two cooling lines. I used a 3/8" flare wrench. The lines are kind of tough to get at so you might need to lower the transmission to get at them.

If you are working on jackstands, it is probably a good idea to now remove the transmission mount and crossmember from the transmission so you have more room to get the transmission out from under the car.

Now that there is nothing attached to the transmission you can gently lower it out of the way. Be careful that the torque converter does not slide off of the input shaft (it weighs around ~50 pounds when full and could hurt if it landed on someone). You will probably have to lift it off of the jack to get it out from under the car (unless the car is high enough off the ground where you can slide the jack and tranny out at the same time). Have a friend help you slide it out and you are now halfway done.

Take a break and enjoy a cold beverage ;)

Installing the 700r4