I need to replace a cassette, and the last time I did it, which was the first time, I noticed the cogs all come apart. So my question is, why can't I just flip the cogs around, so the wear is on the other side of the teeth? I have already ordered a new cassette, but I thought maybe this is a way to get a little more life out of one? And I did notice grooves in the cogs, on the outside side, that they all have. Do these serve a purpose? I'm talking about the ones to the left of the teeth number in the photo.

I think I have a SRAM cassette, but I don't know for sure. Came with the bike, I'm at work, at the bike I'm talking about is at home.

Short answer= no

Those little groves you see help the chain move for shifting. Brad's answer is correct. Buy a cassette and enjoy your bike.

The splines aren't all the same so there's only one way the cogs will slide on.

Thats so the "ramps" line up for faster shifting

In theory, if you modified the splines, it would work. Your shifting performance would suffer, likely considerably, but it would 'work'. I have a 8 spd bike (1x8) that is my beater commuter bike and when my chainring wore out, I simply flipped it and it mated to a new chain no problem. I have also run front chainrings that have had no ramps or pins, and it will shift, but it is quite sluggish. I have never tried it on the rear as you would have to take the time to file down the one big spline so that the cassette would fit on the freehub body. I suspect it would work best with friction shifting, as it's a bit easier to "overshift" to help it changes gears.

Try it, and let us know how it works.

Old Uniglide cassette's cogs are flippable, although that's not what you have

As mentioned, the resulting cassette would suck mightily, and thus would be of less overall quality than a $20 cassette. You'd also probably need much more than $20 of labor to do it. So probably isn't worth the effort.

Not to mention that the smaller cogs (read the ones that wear out first) often have the spacer integrated into the cog... so even if you went to the trouble of grinding down the splines to get it to fit on backwards you'd still have an issue.

It'll shift backwards if you flip the cogs.

HAHA. Not a bad idea actually. It'd solve the ramp problem. Can't imagine it'd fit by too many seat stays though.

In theory, you could work a bit to make it "work": it's fairly easy to file the narrow notch a bit larger and flip the cogs. You would loose the ramps, which will make indexing a bit harder. Not impossible, just harder; for instance, you'll need to ease off pressure on the pedals to shift adequately, so shifting while going uphill will be a bit harder. How hard? Probably still easier than with a 1970s drivetrain.

However, don't expect miracles from flipped cogs. Worn off cogs are worn because the U-shape teeth flatten. Flipping cogs won't solve that problem completely. The chain won't be slipping off the cog, but will still "stretch" faster because cogs are worn.

???? Are you crazy???

Generally speaking, cogs will have some amount of information stamped on them (like "12T"). Still generally speaking, the side with the writing on in should face to the outside (i.e. drive side) of the rear wheel. This is good for current Shimano, Campy, Mavic, and SRAM cassettes. Don't flip them around. Replace them when it's time. I replace cassettes every three to five chains. I check them with a Rolhoff cassette checker. And, any time I replace the cassette, I take a really hard look at my chainrings. I know which chainrings are getting the most usage and I try to replace them when they need it, also. Most of the time, when you replace the cassette or the chainring/s, you'll see a dramatic improvement in the quality of the shifting. It will often surprise you (how much better it gets).