Original Shimano Deore components all around. Shimano MF Z012 5-cog freewheel. Biggest cog is 30 teeth is worn out, chain skips, and that cog needs to be replaced. Rest of cogs are ok. Are compatible individual 30 teeth cogs still available as new parts? Source? If not, what's the recommended solution?
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If the 30 is worn to the point its slipping and jumping, I would my guess the others must be due for replacement, along with the chain.
Individual cogs tend to only be available in high quality gear, so a new cluster will probably cost less than a single cog.
Replace the cluster and chain if you can afford it. If not, the cheapest way would be to get a beater from the tip, side of road etc, and take the 30/32 off that. Its rare that the largest cogs are worn out more than the others (more teeth per rotation, lower torque) so even a really old bike is likely to have a useable large cog.
Maintaining a freewheel (especially the body) is a PITA and finding the right cog to do it is more trouble than its worth unless you're really attached to it for some reason. You could just not use the 30t cog, or throw on a new freewheel. If you really want to pursue this avenue, Sheldon has some words of advice (I'd probably start with grinding the teeth if it looks like it'd work).
The solution is to buy a new freewheel. Sunrace and Falcon make a cheap 14t-28t freewheel, which should get you along nicely. IRD also makes an expensive 13-32t and a few other options (looks like it was originally spec'd with a 14t-30t). Note that you should throw on a new chain while you do that as well. If you really must have the same freewheel (maybe because you like the taller teeth or something), you can find used ones in relatively good condition on eBay.
For what its worth, I'd probably get the Sunrace one and hope that I could get the old freewheel off without having to do something reckless - Falcon's freewheel remover doesn't have enough purchase. And IRD is pretty damn expensive.
Agree with other answers on replace the freewheel and chain.
You should also look at the chain ring(s). Replace if worn.
If you are wearing out the biggest gear then you are spending the most time in that gear.
You may want adjust the range like 14-32.
If you replace the front chain ring then maybe a little smaller to put you more in the middle of the freewheel for your most common gear. This gives you room up and down from your most common gear.
If you are running the larger front chain ring and wearing out the 30T then spend more time in the smaller front chain ring.