Hello. My freewheel stopped working. I can no longer coast. All the freewheel replacements I've seen come with cogs. Can I replace just the freewheel part and transfer my existing cogs, or do they only sell freewheels and gears together? Thanks.

First off, be sure it's a freewheel and not a cassette. Cassette systems have the freewheel mechanism built into the hub ("freehub").

If it is a freewheel, you'll need to buy the entire freewheel unit; this includes the cogs and the freewheel mechanism, which holds the cogs together.

Yes, but you probably don't want to. As Jive said, do you really have a freewheel, or is it a cassette type hub? The kind of bike and/or any information about the rear hub helps enormously. Read on: https://sheldonbrown.com/free-k7.html

If you do have a freewheel, it's possible that it's gotten old and gummed up. Removing, soaking in solvent, then re-oiling the bearings can restore it to usefulness.

If it has gone bad, separate freewheel bodies are sometimes available, but it's usually more cost-effective to replace the whole freewheel as one unit. Removing the cogs and transplanting them to a new body was common 30 years ago, but not anymore.

I do indeed have a legitimate freewheel, not a cassette. I tried putting tri-flow into it, but that didn't help. It makes a bad noise when I hold onto the cogs and spin the wheel. It's not a constant grinding, more like the sound of things clunking/grating periodically. Anyway, I guess I'll just replace the whole thing then.

What sort of freewheel is it?

If it's a Suntour New Winner, you can buy a NOS freewheel body for $18 from Loose Screws (https://www.loosescrews.com/index.cgi...&id=3823127217). I haven't seen any other bodies for sale, probably because few of them are as modular as the last generation of Suntours.

If it's anything else, you cannot buy a body, and the cogs are almost certainly not compatible with any other model of freewheel. Shimano, for instance, use different, subtly incompatible spline patterns across their current range (to the best of my knowledge). Older screw-on freewheels will be even weirder, and documentation is almost impossible to find. If you are able to find a similar freewheel in good condition, though, you might be able to transplant some or all of the cogs to wind up with the correct set of ratios.

Most of the current freewheel offerings that still exist are:
IRD: 13-24, 13-28, 13-32 (about $50), 7sp, with similar 6sp options
Sunrace: 13-24, 13/14-28, many others (nothing close ratio, though) ($cheap, less than $20)
Shimano: 13/14-28 (I think), 13/14-34 MegaRange (also $cheap, but not quite so)

Rebuilding freewheels is such arcane deep magic that even Sheldon Brown says not to. It might be worth doing so if you have a really high quality close ratio freewheel (like a 13-18/19) and are willing to replace an unreasonable number of bearings individually in a procedure which sounds more like open heart surgery than bicycle maintenance. Instructions are on the same page where Sheldon says to never, ever resort to this.

That should be about all of your options. Happy deciding!

Replace it with a Hyperglide freewheel for the best shifting.

Pretty sure you should be able to find 13-21 if you want it close, but I dunno about 6spd.

Here's good linkage: https://noscyclingparts.info/?s=NOS+Hyperglide

It's really not that bad, as long as you're able to get them apart. There are several different lockrings and assembly designs available.
I see it more as a question of investment vs payoff. Apart from the balls, I've never seen any separate parts available. If disassembly reveals something else than gunk, you're left with replacing the unit anyhow.

It's a 6 speed for a mid 80's Trek road bike, so I think it's Shimano. At least all the other components are Shimano. Those things are really cheap.

Replace it with an IRD, for sure. I put a 6spd IRD on an old Peugot and it ran so much more smoothly than the original.

Buy another 6 speed freewheel. and buy a new chain ,
the possibility is that the chainrings have worn significantly also,
but they can be replaced..
though you are looking at a significant budget to do all that..
but all are mechanically renewable.

dont have the money? pull the freewheel off, and soak it in solvent, like Kerosene,
It may be gooey dry, then once it breaks free, run some 90 weight gear oil in.

OK- so it's toast. It's much easier and probably cheaper to replace the whole freewheel rather than just the body. As Fiets said, you'll probably have to replace the chain, too.

The only case where a Shimano freewheel body swap would be warranted is if it were a top-end Dura-Ace, Santé, or 600EX freewheel. You'd still have to hunt down a new(er) one, but they're good for thousands of miles.