Single speed advice.

[Turner]

I have an old Richie P-21 frame with a 1'' head tube...this is where you come in,
What cranks and cog do you run and where did you get it?
I plan on doing a build up 4 this up coming spring.
Tanks in advance!

[mtnjam]

The ratio depends on what you are going to be riding. I've got mine at about a 2:1 (32x18) and ride around here in the lower foothills comfortably.

Find some old cranks, I'm using 175mm length, but 180s also would work and find an old cog off a bmx bike or get a new one from webcyclery.

Does the Ritchey have horizontal or vertical dropouts? If you have vertical dropouts you may need to get a chain tensioner or you can screw around with getting half links(not sure if there are quarter links?) and make the drivetrain look cleaner.

[Pedro]

Do you have a wheelset already? if so there are many after market Spacer Kits, and many companies like Surly make Dedicated Cogs that will fit on your freewheel. If you use a BMX Chain, You will have lots of forgiveness if your chainline isn't straight. I just eyeballed mine.

If you have a crank already, with ramped chain rings you might want to order a single speed ring that will fit on your crank. All of this should be fairly cheap.

32 Bucks for a 32 t single speed chain ring (Make sure you get the right bolt patern)
http://www.bikemannetwork.com/biking/p/SSCH/CR4191

32 dollars for a spacer kit, but you can make your own with some pvc pipe if your handy.
http://www.bikemannetwork.com/biking/p/SSFHSP/FW9999

27 bucks for an 18t cog, there are all sorts of cogs so you can get away with a cheaper one if you like. or if you have an old cassette that you broke down.
http://www.bikemannetwork.com/biking/p/SSCS/FW2076

All sorts of chain tensioners, i ve never used one so i wont coment. Ive always had horizontal dropouts or EBB
http://www.bikemannetwork.com/biking/c/SSCHTEN

Search around there are lots of great single speed stores on the net, not a lot of LBS stock this stuff, but some are catching on.

[beans13]

I would recomend starting out with a 34-20, but that is for the terrain I ride which is VERY hilly, so that is comfortable for the ups

[m1e448]

If you have vertical drops, you'll need something to tension the cheain unless you can find the magic ratio or use a half link, the best alternative is a white industries eno eccentric hub. The hub is eccentric and tensions the chain, then maybe go with a tensioner such as a rennen, surly...

yYou can use any old cranks, the stiffer and longer the arms the better. Salsa or Surly singlespeed chainrings are nice. SS cog in the back or bmx freewheel if you go with a hub that rolls that way. Spacers can be had from your lbs. or pvc or from taking apart old cassettes, if you decide to run a normal hub in the rear with a freehub body. Most cool lbs will save the spacers from old cassettes for people making conversions and give them to you or charge you like $2.