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I need to determine what cog/lockring size for a wheel. I don't know much about these. I found on the hub the name Dachang. I don't know a size but it is a roadmaster bike.

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As usual Sheldon Brown has the answer, in his glossary of all places:

Fixed-gear hubs use a left (reverse) threaded lock ring to keep the sprocket from unscrewing when the cyclist resists the motion of the pedals. Fixed-Gear (Track) Hub Lockring Threading (Left-hand thread):

       English/ISO            1.29" x 24 TPI
       Campagnolo/Phil Wood   1.32" x 24 TPI
       Old French             33 mm x 1.0 mm

It's most likely that your bike takes the first of those options, and the difference is small enough that unless you have both in front of you telling them apart is hard (it's only 1/30th of an inch)

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The easy and accurate way is to get a vernier caliper and measure it yourself.

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    This answer needs a lot more detail. Can you suggest what the possible sizes are, or thread pitch? – Criggie Mar 21 '16 at 21:30
  • Welcome to Bicycles SE. We're looking for answers with more detail. If a vernier caliper caliper is easy and accurate, it's same to assume that the OP doesn't know what it is. Please consider expanding your answer to explain what it is and how to use it to solve the problem at hand. A short, one-line answer like this is likely to get downvoted, flagged for moderator intervention, and possibly deleted. – jimchristie Apr 11 '16 at 19:12
  • Welcome to Bicycles SE. Your passive aggressiveness is noted and appreciated. The suggested answer would boil down to "there are these two choices that differ by 0.03 inches" which is not helpful at all, because you need a tool to distinguish them anyway. – ojs Apr 11 '16 at 21:41

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