I have an old small gear that somehow got flattened to an oval shape. The axle hole is too narrow to slide a bar through, although it is not completely closed shut.
How can I repair this gear?
The material deformation you are seeing is called ‘plasticity’. It happens when a material under stress is deformed and doesn’t return to its original shape (this would be called elasticity). In many cases (and it is the case with LEGO ABS plastic), this deformation weakens the material (as mentioned by Kramii).
To fix this part you have to deform it again thus breaking it even more. Here are two things you could do:
I have a bunch of those small gears, I wish I could just give you one.
One of these?
I regularly crushed these as a child.
You could try inserting something small into the hole and gently easing it open. I've had modest success with a small screwdriver.
Unfortunately, the gear will be weakened following a flattening. If it is badly squashed it is very likely break - either as you try to repair it or in use.
Your best bet, therefore, is to source a replacement. They're available from BrickLink for a few pence.
The other thing to mention is that in some cases, I've been able to use these gears even though they have broken in half. I just pushed both halves onto the axle and they stuck on well enough for my purposes. Even so, I'd still buy replacements where possible.
I don't think it's really possible to salvage it, as others said, you'll only weaken it more. Even if you can place it on an axle, there's a good chance it will break under stress.
But replacement should be very easy to find, and don't forget you could also contact the LEGO customer service for that. It sounds a bit overkill, but they will usually happily replace the part. It could also be that newer ones will be stronger, but I don't know that for sure. On the other hand, they'll very likely send you a dark grey one, since it's now the standard colour.
Here's what I did:
First I tried a screwdriver and awl, but couldn't get it to work.
What did work was pinching it with pliers to open the hole, and then sliding in a rod.
The piece is intact, but very flexible now. It is very deformed, but becomes uniform when attached to an axle. I'm going to leave it attached for a while to see if it evens it out more.