MHI (Mitsubishi Heavy Industries) turbos are used in mitsubishi and other cars. They are relatively easy to rebuild, and you dont need any special tools. The moving parts are individually balanced, and dont require balancing after assembly,
Assembly and disassembly are pretty straight forward linear processes, there are few if any things you can put in wrong, however you can damage or break parts, a little bit of caution is required.

Turbos have 3 main components, a compressor, a turbine, and the section that goes between. The turbine is the side that the exhaust gas blows by, the compressor is on the opposite end of the turbo, and its the part that compresses incoming air before it goes into the engine. The part in between is often called a center section or the CHRA.
MHI turbos have a descriptive part number, a TD05-12a turbo has a TD05 turbine, and a 12a compressor wheel. There are many size compressor wheels and housings, and a few sizes of turbine wheels and housings. There are a few ways to make different size turbos fit your application, however the easy way is to replace things with the same size parts. For instance, if you have a TD05-12a turbo that is failing, the quick and easy way to fix it is by buying an TD05-12a CHRA, and rebuild your turbo with the new CHRA. It may be your only option, but if your turbo is rebuildable, then you can save some money by rebuilding it yourself with rebuild kit (new bearings and such).
If your compressor or turbine wheel or shaft is damaged, then you will need to replace them too, so you should check that out before you get your kit. The part numbers are relative to the part sizes, a 12a is going to be a small compressor, a 16g is going to be bigger, and a 20g is even bigger. TD04 is smaller, TD05 bigger, TD05H more bigger, and TD06 is even bigger.

If you want to modify your turbo, then there is more work, and I will explain a little now, but more in other sections of this instructable. If you want a bigger compressor wheel, you can either replace your center section, or have it machined to fit the larger wheel, if you want smaller, you have to replace the center section to match the compressor wheel you need. You will also need to get a bigger compressor housing, or have yours machined to fit the bigger wheel.

If you want a smaller turbine, you need a new housing, and exhaust wheel/shaft (the turbine wheel and shaft are one part), If you want a larger turbine wheel, then you need a larger housing, or having your housing machined to fit the larger wheel. You will also have to completely rebuild the turbo center housing to change the shaft.

If you use a new turbine housing, it has to fit the right way on your cars exhaust housing, or the turbo will face the wrong direction, and you wont be able to hook it up.

If you use a different compressor housing, you may need to reclock the turbo, and make other modifications to fit the new housing to your car.

There are a few ways to do things, the best way depends on your needs, but sticking with the same size is always going to be the easiest, most folks want bigger, and thats always easier than smaller.
My car is fitted with a TD05-12a turbo, and that size is not very common anymore, but you can get a TD05H is pretty easy to find, and they come in many compressor sizes too. The first thing I did was take a TD05H-16G turbo, and rebuilt it with my TD05 turbine wheel. I used the 16G center section, compressor wheel and housing, but my TD05 turbine. The reason I did that was because it kept the turbine side all the same as factory and was the easier option. It means I have to take two CHRAs apart, and rebuild one with the right parts, but I dont have to mess with the turbine housing. I used the 16G housing that came with the turbo, and that needed to be modified to fit my car. it would be easier to fit the stock one, but I would have to have it machined from 12a to 16g before it would fit, and re-clocking and re-plumbing was easier for me to do. The good thing about fitting my car to the new style of housing make it easy swap in new turbos.

Step 1: CHRA Disassembly/Assembly

CHRA Disassembly/Assembly is the important part if you are rebuilding your turbo. If you are keeping the same wheels then its just a matter of taking it apart, replacing seals and bearings, and putting it back together.
In the first picture, you can see the turbo has been removed from the car, and put in a vice to remove the compressor housing. This is kind of difficult part, it requires removing a humongous snap ring. A large pair of needle nose pliers with a 45 degree bend works pretty good, but you can also use a screw driver, and pry it up, and around to get it out. Once that ring is out, you can take the compressor housing off, and mount the CHRA in the vice See photo.
With the CHRA in a vice (mount it on the flat side of the turbine wheel nut), then remove the nut on top of the compressor wheel (caution, sometimes they are reverse theaded), and remove the compressor wheel (may require a little prying to get it moving).
Look at your compressor wheels, they come in 2 basic flavors, flat back, and super back. If the back of your compressor wheel lays flat on the counter, you have a flatback, if its convex and curved not flat, its a superback. You need to know this before ordering your parts, because you need to order a superback or a flatback rebuild kit.
If you are changing the compressor wheel, you should get the kit that matches your new compressor wheel. You will also need to have your center section machined, or use one that is the right size for your new compressor wheel.
With the compressor wheel off, you can get to the smaller snap ring, a standard set of snap ring pliers will remove it with ease. the next several parts come off one at a time, I like to lay them out in sequence to make it easier to put them back in the right order.
At this point the shaft and bearings are still in the center section, but all the other parts are out. take the CHRA out of the vice, and gently pop the shaft out. Sometimes it helps to wiggle it while pushing, because it likes to hang on the oil ring. When you take the shaft out, the brass shaft bearings may still be inside, you need to pull them out too.

I like to clean things up, and lay them out in order for re-assembly, if you are using the same shaft, you need to start by removing the old oil ring. Its actually trickier than it looks, and easy to break because its very brittle. After its off, or if you are using a new shaft, its time to put the new oil ring on, and its important to be gentle with the new oil ring. Its very brittle and hard, so dont bend it too far, or it will snap. When you put it on the shaft gently slide it over and around the grooves. I havnt tried it, but I saw where someone wraps sewing thread in the grooves to make it easier to slide the oil ring on. This is probably the trickiest part of rebuilding, because you dont want to break the ring, or even bend it too far, because if the ring gets too compressed or spread, it wont fit quite right, and if it doesnt fit right, oil may leak. Be really really careful for a few minutes while you gently slight that ring into place.
lube things up with a little oil, fit one of the brass shaft bearings in the rear side of the center section, and heat shield. Put the lubed shaft in, as it goes through the center housing, slip the other shaft bearing on and in the center section. gently push the turbine wheel and shaft the rest of the way in, being careful to fit the oil ring, it should snap into place, and not come out easy.
Place the center section back in the vice, and start reinstalling the various new parts in the reverse order that you took them out. put a little oil on all the parts as you put them in, the oring in particular will fight you if its not lubed.
The last 2 parts are the compressor wheel and nut. After you slide the compressor wheel on, you should reposition the turbo on the vice so that the vice holds the compressor wheel in place while you tighten the nut.

I like to spin the turbo, just to feel that everything seem right. It should spin freely, and have no play forward and back or left/right/up/down, and should not be rubbing on any surfaces. Then its ready to put the compressor housing back on, and install.

Ive included a time lapse video, to show this step of the rebuilding process, you may want to use it as visual step by step by pausing at each step.

Step 2: Compressor Housings and Clocking

The last section, we removed the compressor housing to work on the CHRA, There are several styles of housings that fit on these turbos, The first picture explains the newer style, and the older styels. The next picture shows the newest style, then the oldest (non intercooled) style, and the last one shows them set with with different clocking. Clocking is how the output is pointed, and different housings are clocked differently for different cars, but they can be changed to fit your car.
The other thing to know about compressor housings is that they fit a specific compressor wheel, a 12a compressor housing and a 20g compressor housing will be radially different on the inside, but the outside may appear the same. You can also have any housing machined to fit a larger wheel,

I changed the charge piping on my car, so that I can hook up newer style turbos. This took a little bit of figuring things out as I went, but it was pretty straight forward. With the turbo off the car, you take the new housing, and orient the outlet facing the right way to hook up to your car. I had to hook up some adapters, to get things to hook up nicely, then disconnect the housing, and fit it back on the turbo with the oil input up, and drain down, like it will be when its back in the car. It wont fit on right, because there is no hole for the pin to fall into, but you can see where the pin needs a hole, and mark it. Now all you have to do is file or grind a bit of metal where the mark is, being careful not to remove too much material, but if the pin wont fit, remove a little more material. until it fits nice and flat. See pictures with comments in them.

Step 3: Turbine Housing Aka Welcome to the Hotside

The turbine housing is the part that the exhaust pushes through to spin the turbine, and the housing is made of very hard cast iron. Its very durable, and hard, but its also brittle and can crack, or break. The turbine wheel fits snugly into the turbine housing, with a little bit of clearance all around. If you are rebuilding your turbo, there isnt anything you need to do to the housing or wheel, The housing bolts to the exhaust manifold on one side, the other side is where the turbo mounts to the car.
If you want to fit a bigger turbine wheel, you will need to rebuild your turbo with a bigger wheel, and have the housing machined to fit the bigger wheel. and put it all back together. its pretty simple and straight forward.
I have a spare housing that had a lot of damage, bad things happened to it, messing up the area where the turbine wheel fits. I used the damaged housing to make a TD05H housing (the next size up from TD05), this only requires grinding a small bit of material, but its very very hard material, and it takes a lot of work to move it.
I started by mounting the housing on a vice, and gathering materials. I started with a dremel, but it became clear that was going too slow. I got a corded drill, and some grinding bits, and basic safety gear. I marked the area that I wanted to remove material, and evenly ground down where I put marker. this only took off a tiny thin layer, and I had to repeat the process many many times, it took a whole day of marking and grinding. When I got close I could fit the turbo into the housing without making contact, with it just sitting, I could spin the compressor, and it would freely spin. I marked the area, and pushed the turbo into the housing until it made contact, and gently wiggled and turned the compressor wheel to mark the high spots, and I went back and ground them down real good until I could tighten the clamp real tight, then spin the wheel without it making any contact. To insure that there would be no problems after it heats, or bearings wear a little bit, I ground the housing another couple sets of marker ink to make a nice clearance. See next to last image.
I spent all day grinding on that housing, I went through several grinding bits and sanding drums and it did make quite the mess, but at the end of the day, I only had a small pile, and I think a most of that pile was the grinding bits, not the cast iron.
There are 2 good reasons to modify the old housing to a larger size. One is because its easier and cheaper to buy TD05H turbos, that TD05 turbos, often you have to pay more for them, if you can find them. The other good reason is because it works better than the TD05. An adventurous person could even push it to TD06 sizes.
When you are all done fitting the turbo, take the clamp off again, and separate the turbo and turbine housing. Put the housing back on the exhaust manifold and fasten those big nut real tight while its easy to get to them. Fit exhaust back on the turbine housing and then put the O2 sensor back in, and tighten it all up.

Step 4: Fitting Your New Turbo

Now that you have rebuilt your turbo, all thats left to do is put it back in. If you stuck to the stock sized and didnt modify the turbo, it should go back easy. Gently put the turbo into the turbine housing being careful to not bend or dent any blades on the housing. Turn the turbo so that the oil inlet is on top (there should be a mating mark to line up with. Put the clamp around the joint, and put the screw in and tighten it till its good and tight. With the turbo firmly mounted on the exhaust housing, you can now put your oil lines and your water lines back on the car, New copper washers will help prevent any leaks, but you can also reuse your old ones if you are careful. The inlet and outlet fittings should fit back on, but if you modified your compressor side, and have a new housing, it may take some fiddling to get things liked up right. I had to add some metal piping and a silicone sleeve to get mine all plumbed in nicely.
If you did a simple rebuild, all you have to do is hook up your wastegate, and you are done!
it should all work just like a new turbo.
If you modified or changed your compressor housing, your wastegate may not line up right, and you may have to fabricate a bracket, or a different way to mount your wastegate, so that it still works right.

Step 5: Mounting the Wastegate and Manual Boost Controller

If you changed your compressor housing, or clocking, then you may have a problem mounting your wastegate actuator (because the mounting points are no longer in the right place for your car). You can see the pictures I have modified my wastegate actuator so that it works on my car. Its a fairly simple mechanism that pushes a rod as boost pressure rises, that rod actuates the wastegate, allowing exhaust to bypass the turbo to prevent over boost.

Ive also modified my ECU so that it no longer restricts fuel at higher boost levels, this makes it so that I can run higher boost pressure than its set by the factory, but it also means that I need to a manual boost controller. There are better boost controllers, but you can build a boost controller with about $10 worth of stuff from a hardware store.

In the picture, you can see its made from some brass fittings, a spring, ball bearing, and a couple washers. Put it all together, and it makes a valve. The more you tighten it, the more pressure needed to open the valve. Adjust it by tightening until you get to the right boost pressure. The valve only works one way, and Ive put an arrow on it to show you the direction that the valve works.

The boost controller fits between the wastegate, and the vacuum line that operates the wastegate. What it does is it prevents boost pressure from getting to the wastegate actuator, until it gets to the pressure level that you set it at. It takes some fiddling with adjusting, and a mechanical boost gauge to set it. 15 PSI is a good setting form most of these turbos, but you can run them higher. I ran my factory TD05-12a turbo at 22psi.

If you are going to run higher boost then you should also consider a recirc or blow off valve. or you may prematurely wear your new turbo thrust bearing. I have one, but I havnt figured out a nice place to mount it where it wont be seen. I want this to appear stock, but the insides of the turbo have been modified to be a much bigger turbo, that makes the car go a lot faster than stock.