I just bought a fixed gear bike with a flip-flop hub, but it has absolutely terrible gearing, something like 87 gear inches. I know how many teeth I need on the new cog, but I don't know what kind of cog I need. I've gone on Jenson USA and they have track cogs and BMX single speed cogs from Surly. I don't know what kind I should be getting and any links to the right cog with 21 teeth would be greatly appreciated.

https://www.universalcycles.com/shopp...&category=1704

dont forget the lock ring!

21 teeth is kind of a tall order. I know they make em', but you should also look into a new chainring, so you'd have more options for gearing lower or higher. Most places around where I live don't carry much over 17t cogs.

+1. Get a chainring that is in between 42-46. Then change cog accordingly. I know they are more expensive, but itll pay off in the long run. My LBS likes to stock dura-ace, and those only go up to 16. Sometimes they have other ones in, but not often

Be a Monster

The chainring is a 52t and has 3 bolts instead of 5, and I need to get the bike rideable for as cheap as possible.

The 3 bolt thing sounds like your crank is one of those cheap steel cranks with the inner ring removed.A large quality cog(like EAI)is going to cost about $30.For $10 more you could get a bulletproof crank.Those are decent low end cranks and usually come with a 48T.If that is still too big,you can easily find cheap 110 chainrings.I think i payed $9 for a 44T @ the Lbs.

https://bikeisland.com/cgi-bin/BKTK_S...ls&ProdID=1113

I'm definitely replacing the cranks eventually as there are literally no 3 bolt chainrings available and I was wondering if these cranks would work if I removed all the chainrings and keep the 42t on. The rear cog is 16t so it would be about 70 gear inches and have 8 skid spots which is fine with me. I figure I could sell the extra chainrings as well if I need to. I'm not 100% sure what the BB spindle is, but if it's square tapered my plan should work, right?

EDIT: Here's a pic of the current cranks.
https://www.flickr.com/photos/sabotag...7608178142674/

Keep the crank, it's at home on that bike. Just get a 19t cog if you don't want to be a monster & develop Ironman legs.

sounds like an old school 1970's Nervar steel cottered crank or similar. A bit heavier, but still servicable. They usually came as a 52/40 double, so 40T inner rings are available.

On the other hand, Surly makes 21T cogs. you will have to add a couple chain links, but it's still fine.

I run 19T on my TT Pro and it's fine.

52/19 sounds pretty good and to be honest I kind of like the look of the crank. So I need a track cog, correct? Would I need a longer chain since it is 3 more teeth? Here's the best pic I can find that may help you figure if I need one or not.
https://www.flickr.com/photos/sabotag...7608178142674/

noobie question, will every x Hole crank spider fit every x hole Chainring? I'm looking to replace the stock one on my Windsor.

the dura ace cog is for sure worth the money

DA cog is no bigger than 16t___FYI

Will the current chain fit if I switch from 16t to 19t? I know I need a track cog.

Yes. If the cranks are XX BCD, you can put any XX BCD chainring on there. Ideally one that doesn't have ramps and pins (which would be more intended for multi-gear situations).

Do you know the BCD on the crank?
I have a few chainring around I might be willing to part with for cheap