I searched, and didn't find this one anywhere, so for those who are intrested in this..here ya go:
First of all decide if you ever plan to go turbo. This will decide which 60mm TB you need to get your hands on. If you're staying N/A you should get a TB from a 1G 4g63 NT Eclipse '91/'92 and up (you have to look closely at the part, some of the earliest 4g63s had the IAC cast directly onto the TB making them useless for this project. If you don't see a seam between the TB and the IAC don't buy it!) These, like the 4g64 TB have a lip cast onto them allowing you to retain your factory intake set-up. They're also a LOT easier to find. If you ever plan on going turbo, you need a TB from a turbo Eclipse which are harder to find, but have a removable elbow to make routing of the turbo pressure piping possible. Either a 1G or 2G will work. Keep in mind this tutorial is for the N/A version. Some details might vary for turbo applications.
Now if you want to have minimal down-time it'd be wise to get a donor TB from a 4g64, but that's entirely optional, but highly recommended.
With both TBs on your work bench, use an impact driver to remove the screws from the bottom of the throttle bodies. DO NOT attempt to use an ordinary screwdriver. Out of the 10 that I've taken appart only 2 screws have succumbed to a mere screwdriver, while only 3 resisted the impact driver and had to be drilled. While you're here, clean both the 60mm TB and 4g64 IAC thoroughly. Feel free to through the TB into a parts washer to soak, but clean the IAC by hand--you don't want to mess up the delicate parts within. Your really only need to worry about the IAC passages proper (they'll be the two dirtiest ones)
This is where the only "modification" comes into play. Part of the coolant passage on the 60mm tb will have to be filled in. I use RTV for this since It's cheap, easy to use, and resistant to antifreeze. (it's also removable if you "oops" unlike JB Weld) In the center passage, fill the half that's on the same side as the throttle cable attaches to. Use the 4g64 IAC as a reference--you need to fill the side that water DOSEN'T need to be in. You need to fill it to just over the level of the gasket surface, remember it has to be water-tight. I do 2-3 light applications with at least 12 hours in between. Just be sure to give it plenty of time to set-up. Also the center bolt hole on the 4g64 IAC needs to be filled with RTV as well.
I STRONGLY suggest replacing the TB/IAC gasket at this time It's Mitsu PN# MD611406 and they're $2.86 from MitsubishiParts.net Or approximately double that from your local dealer. It's still cheap insurance against coolant getting sucked into your engine or your idle being screwy from air slipping by.
If the original screws aren't too messed up, you can re-use them. I use new SS screws and lock washers but I'm just thorough like that. You need one 3/4" and two 2 1/4" if you're using new ones. Now with the center gasket in place, bolt up the IAC with the three holes that line up (yes, they're all on one side, but it seems like it's plenty...mine hasn't leaked in 40K miles so I'm fairly confident. Now, blow through one of the coolant bypass hoses while holding your finger over the other one. If you hear excaping, remove the IAC and lay on another thin coat of RTV to seal off the coolant passage.
The 4g64 TPS will have to be transfered to the 60mm TB as the Galant has no wiring for an external closed throttle switch, it's built into the TB. Be sure you use a wrench on these bolts, again a screwdriver strips out the soft metal easily.
At this point the TB is done! But it still has to be installed which leads to some new interesting problems.
The intake manifold now should be enlarged to match the new 60mm TB (you have the appropriate gasket...RIGHT?!?!) The exact process is detailed elsewhere, but it's pretty straightforeward. Also, the three vacuum nipples on the top of the TB now face the opposite direction. It's a fairly simple matter of re-routing them to the other side and remove that odd routing device from the Intake manifold since it's no longer needed. Now just bolt it up, make sure everything is reconnected and try to start the car. The TPS and BISS will have to be adjusted. You can try to do this yourself, but the dealer can do it much more easily. Slip a tech a $20 and it'll take him 10 minutes with the Mitsu scan tool, versus over an hour with a volt meter and screwdriver in your driveway.
Now enjoy improved throttle response, mid range punch and top end power. This is one mod that really is pretty much a free lunch in that there's really no downside...well except it makes off-idle response a bit jumpy. You'll get used to that in no time though and you'll recall how "sluggish" the car was off the line before.
