I have a rear fixed wheel, the axle on one side has the first couple lines of thread damaged so I cant screw in the axle nut all the way because it will just loosen again.

Do I have to replace the hub completely or can I just replace the threaded axle rod? It's a sealed Suzue hub

Probably u can replace it (the axle), OR... use the cone thing, the one that press the bearings (sealed cartridge?) to redo the thread. Just unthread, unscrew it and it will redo the missing threads. I dont know what it will happen with it after wards but it should work. I got the same problem years ago and i fix it like that.

Use the tread tapping tool. Take off your lock nut and figure out the threading. Then tap the threads and you should be fine. If that doesn't work you can replace the axle.

Is the hub sealed bearings or loose ball?

Axles are hardened steel. You do not try to thread hardened steel if you like your tools very much.

Sometimes you can shift both axle locknuts a couple of millimeters - essentially moving your axle to the left or to the right. That way your axle nut will be biteing on different (hopefully undamaged) threads.

It is highly unlikely they are so hard as to defy a die, hard metal tends
to be brittle, not a desirable feature in an axle, it may be relatively
hard but using a cone or other nut from the opposite side of the
removed axle is one way to try cleaning the threads, a very fine
file is another, absent the proper sized die. If the bunging is not
circumferential the affected part could be filed down.

The axles might be case hardened, much like the studs that hold the wheels on a car are. If they are, the only way to rethread them is a thread file. A typical die is not hard enough to cut threads on a case hardened fastener. The other option would be to use a carbide bit on a lathe but I doubt most people have access to that kind of stuff.

Attempting to use a cone to clean up the threads is a good way to ruin a cone, regardless of the hardness. Either use the proper die if possible or a thread file.

Thread file: https://www.thetoolwarehouse.net/shop/KD-2228.html

There is no doubt that axles are hardened but they are tempered back to a very tough spring temper for the good ones. A GOOD threading die will cut it but it'll certainly dull the cutting teeth quickly to the point where it may or may not finish this one job. I tried to trim an axle with my hacksaw years back and it took the tooth set right off the blade. And I don't use cheap bulk pack blades either.

Axle threads are very often a non standard thread pitch falling under specialty items. You won't find the die for them at the normal hardware stores even if those stores handle metric tools. You'll need to hit up a specialty machine tool outlet such as KBC or Emco. Not to mention that the good ones cost as much or more than what a new axle would cost.

I've been lucky over the years and only had one axle with damaged threads on one end. I worked around it by just removing the stuff off the other end.

That's where the $10 thread file comes in really handy (available at most automotive parts store BTW)

You're right, I meant to second your mention of the file. Now mind you on the better axles it'll wear the cutting edges on the thread file fast as well. But it's hopefully not something we need to do often so if it only lasts for a couple or three thread repairs on hardened axles its paid for itself even if it is ruined in the process.

Use a washer the thickness of the 2 damaged threads between the dropout and nut. This will take those compromised threads out of the equation.

I think running a nut or one of the cones over the affected area from the other side will do it.

The object is to straighten the threads, not recut them.

OK I tried this and it fixed the problem. Thanks. That black nut that presses the bearings is made of steel I found out. I just screwed and unscrewed it till the thread smoothed out.