Originally Posted by conspiratemus1

I'd echo PastorBob here. How do you know it's toast? Sachs-Maillards are pretty good freewheels -- they are still the backbone of my commuting bikes. The ratcheting mechanism in the innards rarely wears out because the parts move against each other only when you're coasting. Under pedalling loads, the pawls engage the ratchet and the thing turns as a single part. Freewheels, unless meticulously overhauled, often do make grinding popping noises when you coast -- I always took that as a spur to "Pedal! Stop coasting!" But my Sachs coast more stealthily than a cassette free-hub. As long as the ratchet engages when you want to pedal, and slips to allow you coast, it's probably not toast. I've never owned a freewheel that I couldn't get working better just by pouring thin oil into the top until what comes out the bottom isn't rusty....except a few that have a seal where the outer bits move against each other. Then you have to do some thinking.

Now, it's possible that one or more cogs are worn so badly that the freewheel is, indeed, toast. This may not be obvious until you mount a new chain and it skips over the teeth when you pedal instead of engaging to drive the wheel.