Basically there are two different boxes that will fit, the K series and the T series.
On to the K series gearbox. This has a one piece casing with removable extension housing only. The bellhousing cannot be removed separately. It has a cable clutch, removable sump pan and came in a few variants. If you are planning a K series engine conversion then this is the gearbox for you. They can be found in almost all the KE series RWD Corollas. The main differences are the position of the shifter. There are 4 speed versions known as the K40 and 5 speed known as K50. The best gearbox for the Sierra would be a KE55 5 speed as it is the only 5 speed with the shifter in the forward position allowing the shifter to come through the original hole in the tunnel. Alas these are quite hard to come by these days. You can use the later model KE70 5 speed but it has the shifter further back. You can either cut a new hole in the tunnel or swap the extension housing for the KE55 type. Yes the extension housings can be simply unbolted from the back of the gearbox and swapped over, no mods required. Be careful as you still need the 5 speed extension housing, the 4 speed housing doesn’t have an extra part in it for the 5th gear.
Now if you plan on running an A series engine and are lucky enough to come across a K50 gearbox out of an AE70 corolla then this can be used. This is almost exactly identical to the KE series corolla gearboxes except it has the bellhousing to accept the A series engine. To clarify, you cannot bolt an A series engine to a K series bellhousing and vice versa. These are a very rare gearbox but what I have been running behind my 4AGE engine in my Sierra for quite some years now. It has held up surprisingly well for what it is as it was never designed for this sort of power. Now as this gearbox is the same you can still swap the extension housing for the KE55 item exactly as I have done.
Below is a picture showing a KE55 5 speed (left) and the K50 from my Sierra (right) side by side. As you can see they are almost identical except for the bellhousing. Note that as I have swapped the extension housing on my gearbox so the shifter location is the same between these. I no longer have the original housing but it does sit a good 100mm further back than this.
The best way to tell these gearboxes apart is by looking at the top of the bellhousing. The K series engine housing has a single central bolt mount location where as the A series has 2 top mount holes as shown below.
These gearboxes work well and having the cable clutch are more compatible with the Sierra. Although if you plan a lot of horsepower and/or torque then the T50 gearbox is what you want.
The T50 gearbox is the main box of choice behind the 4AGE engine. This was primarily the only factory RWD gearbox found behind this engine. It is a stronger but not much dimensionally larger box than the K50. This can be identified by its hydraulic clutch, removable bellhousing and split case body. Again as the K series gearboxes these are found in a variety of vehicles. They were also found behind T and S series engines. All these bellhousings are different so be careful where you put your money. The bellhousings can also not simply be swapped from one box to a different one e.g. you cannot fit an A series bellhousing to a T series engine T50 gearbox. Also as the K series box there are 4 speed (T40) and 5 speed (T50) variants. Another thing to note is that there are also 20 and 22 spline gearboxes. The 22 splines are reputed to be stronger but this is not confirmed. All Japanese spec AE86 boxes are 22 spline. The Jap spec boxes also have the clutch slave cylinder on the R/H side of the bellhousing where as the Aussie spec can be found on the L/H side. I really don’t know why this is. I can only assume there is something to do with the engines being 4AC's available in our local sprinters where the Jap's got primarily the 4AGE. The other quirk is that there is an AE85 out there that had the 4AC engine in it in Japan so this may be the same gearbox as our local AE86.
The picture below shows the T50 (left) beside the K50 (right). As you can see there is quite a bit of difference between these two.
Note the location of the slave cylinder on the T50. This is a Jap spec gearbox. These two gearboxes are pretty much identical in length so will situate the engine in the car the same way. Another thing to know is that I have also swapped the extension housing on the back of my T50 for a housing from the back of a T50 from a TA22 Celica. The Celica runs the T50 behind the T series engine but has a forward position shifter compared to the AE86. Again this moves the shifter about 100mm forward. The picture below shows the different positions. These are roughly the same positions with the K series gearboxes as previously stated.
There is a bit more involved with swapping the extension housings on the T50 as there is on the K50 gearboxes. If you plan to do this swap then you are also going to need the TA22 shifter and 3 internal gear selector rods. This means you will have to split the gearbox, remove the rods and replace them with the new ones. The new rods are shorter to allow for the repositioning of the shifter. After this the new extension housing can be fitted along with the new shifter as it is different all together and they do not interchange.
This pic below shows the 3 rods I replaced.
And this diagram gives an idea of what else is around them. The forks on each rod have to be removed and put onto the new rods.
As with each of these gearboxes make sure you purchase a tailshaft or at least a yoke for the output shaft so that a new jackshaft can be made using this and your original Sierra item.
Now if you want to go stupid and for some reason have some nuclear bomb (very powerful) powered Sierra you may want an even stronger gearbox. The only other gearbox I know of that has been successfully converted behind the 4A engine many times is the W series gearbox. Now there are a heap of different types of these out there but I am not going to go into them now. Basically they are stronger again and you may know them as a Supra gearbox. There are a few places making aftermarket bellhousings that allow you to bolt these gearboxes to the the A series engines. They are popular with high powered race cars. The best of the best can be found in New Zealand. I actually got one in just the other week for a customer building a 300hp 4AGE powered Clubman type kit car. He also bought the Supra W58 gearbox from me for it along with a RWD 4AGE and two 4AGZE engines. Nothing like having spares I suppose. Drew may chime in and give us more info about fitting this gearbox into your Sierra as I believe he has done it.
Well if you got this far without falling asleep then good for you. If there is anything I have missed or gotten horribly wrong in my current sleep deprived state then feel free to criticize. At very least I hope you learnt something after this.
