So I have been riding single speed and bought a new wheel with a flip flop hub today. I was thinking that I could just take off the old wheel and use my new one and everything would work fine. The problem is that my current chain does not fit the cog that came with the wheel. It seems that my front crank and new rear cog are different. I know this is probably a stupid question but what do I need to make this set up work? Some pictures are below so you might understand better what I am talking about. Thanks!!

If the chain physically won't fit the new cog is 1/8" and the old drivetrain is 3/32". Your options include:

-switch chain to 1/8" and have a slightly more noisy drivetrain
-switch to all 1/8" drivetrain (new chainring == $$)
-switch over 3/32" cog to new wheel (cheap)
-???
-profit

The pictures don't show much, but peabodypride is right

I can't switch over the 3/32 cog off of my old wheel because it is just a BMX cog installed with a spacer kit and lockring on a freewheel hub. Do track cogs come in both 1/8 and 3/32 and if so which am I more likely to find at my LBS? If they only come in 1/8 then I guess I will just replace my chain and deal with the noise.

Also is the 1/8 and 3/32 the actual width of the chain or the spacing between the individual links. Below is a picture of the actual problem, you can see in this how the chain doesn't fit in the cog.

Just try to find a cog that matches your chainring. Your LBS should carry both sizes.. if not, you can always order one.

https://www.ebikestop.com/kmc_z410g_18_gold-CH4085.php
$5.95 KMC GOLD 1/8'' chain, just for an idea if you wanted to go that route.

LBS should have both 1/8" and 3/32" but I find 1/8" more commonly. IINAME (I am not a mechanical engineer) but the pitch (distance between each link in the chain) is always 1/2", the cog "size" refers to the width of the chain. The chain is sitting on top of the cog teeth because it is too narrow to engage them.

Not a chain war but I recommend stepping it up to the $20 level for new chains. The quality between a KMC "Z" chain and an Izumi Eco or similar is tremendous for just a $15 difference. The Izumis simply feel much more safe and built well out of the box. Just my preference and my recommendation. I know Z chains are aplenty but I've heard horror stories and have had one break on me while riding.

Okay so the easiest and cheapest way to go would just be to replace my 3/32 chain with a 1/8 chain and the 1/8 chain will fit both my front crank and rear cog?

Z chains don't last very long - they tend to stretch out pretty quickly. I personally like the Sram pc-1 which is like $10-15 and resists street crud better than a higher end chain. In any case, go for a 1/8" chain with your current chainring and you should be fine (and it'll be a little more forgiving / quiet if your chainline is a little bit off).

All modern chains are 1/2" pitch; you'll occasionally see an old inch-pitch drivechain on a vintage track bike but there's not much fear of accidentally buying one.

yup. the system anecdotally makes more noise that way but lube the new chain with tri-flow and you will minimize it. again, peabodypride recommends chains in the $20 range like an Izumi Eco.

This is your most practical option.

you can get a cheap bell 1/8 chain at your local walmart for like 3 bucks. its gold too

Generic 1/8 chain = $5

Thread over.

I would buy just buy a 3/32 cog. Will cost you the same a s a new chain and then you have a completely 3/32 setup which will be quieter and marginally lighter.