This is one of those things where, for repairing discs, particularly for the GameCube, there are few more economical options for a business than the machine you see above. If your problem is with the shattered housing for the resurfacing arm in the fourth picture, I'm afraid this is not the Instructable you're looking for. If your lower spurs cracked off or are trying to, though, you're in luck. We didn't have much luck getting this problem resolved in a timely manner, so we took matters into our own hands. Needless to say, this will void whatever warranty you might have.

All the pressure of the spring (which exerts the force required to resurface the disc) gets placed on the little tiny spurs you see pictured. There is... well, there's really nothing to reinforce these, and after about 200 discs the plastic just gives way. It's not nearly tough enough to last any longer. As you can see above, my self-taught (though generally effective) ugly plastic welding was insufficient to handle the load required. It survived better than the severed stump was doing pre-weld, but even with the more-elastic plastic I used, it would fall apart after 20 more uses at the most optimistic. I decided their over-extension blocker (which is what this is) simply wasn't good enough, so I installed my own. That's what we'll be doing here today.

Step 1: Enter: the Disc Drive

Xbox 360 disc drives are notoriously worthless once they die, but they, like all other modern disc drives, have two bars that the laser rides along to scan the disc. This is our new tension bar. It's small enough to not upset the mechanisms, but sturdy and long enough to hold the forces it needs without breaking down in a month. If you're having a hard time figuring out how to get ahold of one of these, a couple of screws, some other metal rod, anything will work. These are just everywhere if you have craigslist and 5 bucks.

The resurfacing arm, the thing we're repairing here, essentially has two positions.

Open position is when the lid is up and the spring is fully extended. Without the blockers that are breaking off, the spring will flip the arm too far out, potentially dislodging the spring, damaging the arm elsewise, hurting people or discs (by scraping as it opens or flipping out and mashing fingers) or just plain being inconvenient to open and close every time.
Closed position is when the lid is down and the spring is compressed. It's only relevant because our new blocking bar needs to not interfere with this position, or the discs can't get the pressure they need to be buffed back to shiny goodness.

Before I started drilling, I had to make sure that I installed the bar in a non-interfering location. Ideally it'd be as close to the spring as structurally feasible to equalize strain between that and the hinge. Unfortunately, their design means it has to be pretty far back or it would stop the lid from closing all the way (or apply too much pressure to the disc). I set the arm in each position and marked it with a Sharpie to see the max range. It ends up being just behind the rearmost vertical support on the Easypro model.

Step 2: Drill It Out

The rod itself is 1/8 inch, but I wanted a really snug fit to prevent it falling loose from vibration, so I settled on a 7/64 bit for the final hole size, then expanded it slowly by working the drill bit around until I could force the rod through. Before that, though, a pilot hole.

The 1/16 bit meant minimal damage if my aim was off. I expected it would be because of the cramped condition I was drilling in. I also started off with a centerpunch to hold my bit better, as is my preference. The pilot turned out to be prudent because the harder to access side of the machine (from inside) was a bit too tight for my drill, so I ended up nicking the arm. I compensated as best as I could to reduce the damage, and I don't think it'll amount to much harm in the long run.

I did consider melting my way through, but my experience in restoring all the other cracked plastic showed me that this plastic is particularly noxious as smoke, even in a well ventilated area, so I opted out of it. That was not a headache I wanted to repeat.

Step 3: Success!

Using your imagination, push the bar through the hole. I used the back of my centerpunch and some mild violence. A quick check showed that it interfered with arm motion only as I had intended, preventing it from overextending! Just as extra insurance that it wouldn't vibrate out in spite of the tight hold, I gave it some encompassing fixative on each side. Hot glue seemed easiest, and it's got a snug hold.

When I get around to repairing that other catastrophic housing damage (fourth image from the introduction) I'll let you know what to do about that, too. Right now I'm leaning toward refabrication via sandcasting and recycled aluminum, but that'll be some time from now.

As of hitting the publish button, this has been through about 40 successful GameCube repairs in this first day with no evidence of deterioration or reduced resurfacing performance. I'll call that a success!

Best of luck, fixers - let me know if this helped or if you have any other improvements to the machines!