Has anyone ever heard of using a BMX cog to convert a bike to single speed? Is this possible or am I just bat shirts crazy? If the later, so be it. If not, what would one have to do to make this work?

I'm trying to build my first single speed road bike on the cheap using spare parts from my friends local bike co-op I volunteer at. We have a TON of road bikes from the 70's that no one really wants and we were think that we might Ger folks interested in them If they were easy to ride, easy to maintain single speeds. For some reason gears and drop bars deter a lot of people.

i think all single speed bikes use bmx freewheels. you just need a single speed hub on the rear wheel and horizontal drop outs.

As long as you mean freewheel there should be no problem at all. And I don't think the drop bars are too big a deterrent. At least, I still like 'em.

Depends. Some BMX freewheels have a lower tooth count than is possible on standard road/track hubs, and thus won't fit on standard threads.

there's no reason, except as mentioned, some BMX singlespeed freewheels with smaller than 15-16 teeth, i think, need correspondingly smaller diameter threads on the hub than those on an old fashioned road bike rear hub.

however, whether or not you are "bat shirts crazy" is independent, i would think, of your success .

As long as you stay away from the metric thread models, a BMX freewheel will work fine on a standard threaded road hub. That's what I used on my first conversion, many years ago. BMX freewheels also tend to be less expensive than the boutique single freewheels.

So what do I do about spacers and how would I properly space one?

You may have to re-dish and then space the wheel properly. Good info here:

https://www.mtbr.com/ssfaqcrx.aspx#ssConversionsHowTo

https://www.sheldonbrown.com/singlespeed.html

i'm assuming you are referring to the freewheel, and spacing it to get a better chainline. if this is not what you mean, ignore this...

it's possible to move axle spacers from one side to the other in order to get a better chainline. the wheel will need to be redished, assuming it was aligned properly to begin with, if axle spacers are moved around.

and it is also possible to put a spacer over the threads before threading on the SS freewheel. there are more than enough threads there to do a proper job. harris cyclery sells these spacers.

Could someone show me photos of the exact parts I'll need including what the hub should look like as well please?

Could someone show me photos of the exact parts I'll need including what the hub should look like as well please?

I think casual cyclists are put off by them. Townie/risers are a lot less intimidating.