hey there i have been reading up on how to turn my beautiful 1974 raleigh grand prix to a fixed gear by using a track cog and bottom bracket lock ring...my question is do i have to use some type of threaded track cog or do these Splined ones work?
the bike is in very good shape and all original other then being striped down and has an atoms freewheel thats been make to a fixed gear by welding it...

im looking for a low tooth count one around 10-13t as my chain ring is only 40t and im a strong rider looking for a work out i found this cog but dont know if it will work... where can i find the spacers/ needed cogs? thanks!!

https://www.amazon.com/Dimension-12t-.../dp/B0013G4YDY

You need a threaded track cog(like this: https://www.retro-gression.com/product/track-cog) and a BB lock ring. What you looking to do is referred to as a "suicide hub" and is generally frowned upon, but many people haven't had issues with theirs. You'll want to apply some red loctite to the threads and check the cog and lockring often to make sure they're staying snug.

You may have a hard time finding a decent cog that's super small like you want. You'll likely be better off buying a new chain ring(44-48t or so) and a normal sized cog.

If you are using the original threaded freewheel hub, splined cogs will not work. Also, be aware that 13T is about the smallest threaded cog that will fit on a freewheel hub.

No need for duplicate threads.

/this one.