Rear Hub Issues - getting it fixed

So I got a Trek 2nd District to use for mooching around on to and fro collage and mates houses. However it came with just a single speed cassette and I would like to get it fixed. I used a surly 16t cog (pretty standard) and some lockrings that were lying around. The cog went on no issues but the lockrings (I tried two) just dont seem to go on, they bite and wont go any further. I would hate to but some welly into it with a spanner but shamfering the thread would rite off the whole hub, and thats pricey. So any ideas whats going on? I wanna get my bike fixed soon (pun intended)

Cheers guys

You probably need someone who knows to have a look. Could be at least 2 things,
one, the threads on the lockrings are different to the one on the hub, I guess this is a singlespeed hub and you have the wrong lockrings
second is it a proper fixed hub? A fixed hub has two different threads, one for the sprocket, one for the lockring which is threaded the opposite direction. There are at least three different threads for fixed hub lockrings
But if the sprocket is on nice and tight it is rideable but not skidable.

It is a proper flip flop hub so it has the fixed on one side. Thanks for the advice tough, I will look for those threads

There are 3 locking standards but only 2 are common. Sheldon is your man.

http://www.sheldonbrown.com/gloss_l.html#lockring

FWIW I had exactly the same problem a couple of years ago. I couldn't work out why the locking I'd been given with the hub wouldn't go on properly until I found out that there are 2 sizes and I'd been given the wrong one.

You know lockrings are left-hand threaded, right? As in, turn them anticlockwise to tighten. Apologies if I'm patronising you!t

Good point.

Spent ages phoning around Trek etc. found out its a Durace Lockring I needed. Just thought I may put that up there for anyone else who has this trouble, it cost about £6 and I can boast I have durace components!