I recently bought an old mountain bike that I want to convert into a single speed bike for winter commuting. The trouble is, it's so old that it doesn't have the cassette-type Freehub system for the back wheel, it has an old Suntour freewheel. So instead of simply taking a bunch of gears off of the (nonexistant) cassette and putting spacers in their place, I was under the impression that I would have to buy a brand new threaded freewheel, a BMX freewheel, like this one (?) over at Performance Bike, that only has one gear on it, respace the axle to get a good chainline, and (eventually) redish the wheel.

But when I spoke to the LBS mechanic today and asked him if they sold BMX freewheels, he told me there was no such thing as a BMX freewheel, and that you have to use a bunch of spacers and a single gear to get a one-speed bike. Was this some sort of communication error (him thinking I had a freehub), or is the BMX freewheel I just linked to really a freewheel with two fat spacers and a cog in the middle? Or is it something else entirely? It would be easier if I could see this thing in person, but I can't. Any help would be very much appreciated. I'm very confused. Thank you!

sounds like he was talking about a cassette fixed conversion on your hub. just buy a bmx freewheel and some spacers and mess with your crank to see if you can get the chainline right.

You were absolutely right about what you need to do. Either the guy who spoke to was clueless, or though you were talking about a cassette like g4dhatch2 mentioned. I don't have a lot of faith in the performance mechanics from my experience. Find a small shop that specializes in bmx bikes and you'll be fine.

You are correct. Singlespeed (BMX) freewheels do exist in great quanties and they do thread onto a freewheel hub.

Thanks guys. As always, you've been extremely helpful (and quick!).

I raced down to their shop an hour before they closed for the weekend with the sole purpose of buying several tools and the BMX freewheel, because I wanted to get started on this thing tomorrow morning (my day off). I guess I'll order everything online now, and I won't be able to start for at least a week...

*sigh* Their loss, and my loss, too.

Thanks again.

It also might (theoretically), be possible to just take off the cogs from your existing freewheel (minus one), depending on the kind of freewheel it is.
I've never actually tried this, but I can't see any reason why it wouldn't work.

because it's a freewheel, and it's all one piece. (you are right, if it were a cassette, but you'd still need spacers)