I have a chris king stainless cog on a shimano freehub which is a little worn down after many years of use. I was wondering if it's a good idea to flip it, i.e. make the side facing the hub face the outside of the bike? Will it extend the life of the cog? Is it a good idea? The cog visibly looks good (to me), but new chains need a break in period, before they stop vibrating under load, which is anoying.

I never tried it but in theory it should work out just fine. Do it and report back

If it is a hyperglide notched, then you may need to do some file work on some of the notches.

But, if the teeth were originally symmetrical, then it should work.

It's not notched, teeth are symmetrical. It does work. I'm just wondering if it's a good idea, i.e. will it extend the life of the cog?

If it all works, and fits, and is "single speed", then it should be fine.

I'm not sure about single speed, but on multi-speed bikes, as the chain stretches, it causes the chain to ride high on the sprockets. This in turn angles the tops of the teeth.

When one installs a shorter chain, it slips up high on the teeth, which lengthens the circumference of the sprocket, and so, riding high, the shorter chain is too short to properly wrap around the sprocket at the higher place, and thus doesn't fully engage all the teeth... with each successsive link being higher until it finally goes over the top, thus skipping.

Reversing your sprocket, the length of channel between the teeth is irrelevant as the teeth are still spaced precisely every 1/2 inch.

But, the shape of the tooth profile could potentially better hold the chain in the proper place.

Thus.. flipping is likely GOOD.

Keep in mind, wear without climbing high on the teeth is probably just fine.

CliffordK thanks for that thoughtful explanation.