CaptainPlanet

Will the rocking cause any damage or break the fork? This is for a cheap commuter bike. Weighing between leaving it alone or repair it, depends on urgency and ease of repair. How to troubleshoot and which replacement parts to buy for this issue?

cpach

If it's something actually worth servicing its better to replace the bushings relatively early, but for a cheap bike run it. You'll almost certainly be too annoyed by the play before it's unsafe.

CaptainPlanet

If I am to switch to a rigid fork, what information do I look for in the fork? It's a threaded fork, and I am wondering how long the steering tube needs to be to fit the frame.

dabac

There must be dozens of threads about replacing sus forks with rigid forks.
You have:
Steerer tube diameter
Steerer tube length
Axle-to-crown on old vs new fork
Type of dropouts
Fork rake or offset
Type of brake mount

That’s not hugely important. Switching from a threaded to a threadless headset isn’t a big deal.
.You have the fork and the frame at hand, you measure.
If you can’t manage that, it would seem unwise for you to attempt a fork switch.

CaptainPlanet

I don't know what to do with threaded fork if it's too long. Will I need to cut the steer tube afterward? How do I fix the threading? What if I couldn't find a threaded fork that's as long as my current one? What's the minimum length I need to secure it to the headtube?
With threadless forks, I just need to make sure it's equal length or longer and then I can get away with using spacers to adjust height and not having to cut the steer tube, can't do that with threaded forks.

dabac

Not entirely sure what you mean by ”afterwards”. Steerer tubes generally don’t protrude through the locknut, so it has to be cut to size during installation.

Preferably you find one where the threaded length you need fall inside the range of the fork. Otherwise, you need to find a shop with the die to extend the threads.
I’d screw on a worn top race to the cut line, do the cut OUTSIDE the race. File smooth using the race as a guide, then unscrew the race. A few more strokes of the file to take the edge off and call it done.1) search on
2) find a frame builder willing to replace the steerer
3) find a general metal fabricator willing to lengthen the steerer. It isn’t entirely reckless if you fab an inner strengthening sleeve to support the joint.
Look at what you have and figure it out from there.
​​​​​​​So plan for replacing the HS too, and use threadless. No big deal.
Although if you get an uncut steerer, you’d probably want to trim that too.
​​​​​​​

​​​​​​​

JohnDThompson 

Fork blades must be appropriately long for whatever wheel you intend to use. Brake mount(s) must allow whatever brake you intend to use function properly. Rake (offset) should be close to that of the original fork to maintain handling characteristics. Steer tube should be long enough to accommodate the stack height of whatever headset you intend to use. Steer tube can be longer; use spacers to take up any extra length. If excessively long, trim the steer tube, but it's best to err on the side of longer; it's easy to remove length, but hard to put it back. A longer steer tube allows for a wider variety of headset options if/when it is necessary to replace the headset.

JohnDThompson 

Screw a steel threaded cup down below where you intend to trim the steer tube. Un-threading the cup after cutting the tube will chase the threads. Don't use an aluminum threaded cup for this; it's too soft to re-form the threads. De-burr the inside of the steer tube as well.

Aftermarket threaded forks typically come with long steer tubes for just that reason. The minimum steer tube length you need is the length of the frame head tube (not including the cups) plus the stack height of whatever headset you intend to use. Measurement "A" plus measurement "B" gives stack height: