Can I convert 130mm Surly New hub from 130mm to 120mm by simply replacing with a 120mm axle? I have a 130mm hub that I'm running on a sscx bike, and want to use the wheel for a track bike with 120mm rear. I believe the cog/freewheel position is the same between 130 and 120 singlespeed/fixed hubs (so that chainline is not affected), the only difference is OLD between the two axles. Is that correct?
No, this conversion isn't possible. The 130 version has a different shell than the 120. The flanges are further apart. That's the good way for them to have done it, because if they had used the same flange spacing throughout, that's wasting lateral strength on everything but the 120. If you look at your hub you'll see that there's not really anything you can take out to get the OLD down very much.