Many stock countershaft sprockets have a Rubber bush in built to the sprocket. Why? Sound deadening. The rubber reduces sound in a mid range frequency that is offensive to Sound meters during type testing. (in the USA its EPA and DOT, who do drive bye and idle test to certify a bike is below a db level. Over the years the Japanese have learned a lot of tricks to quiet their bikes.
A cheap aftermarket sprocket will not last long, will wear out your new chain in half the time of OEM Yamaha sprocket. The Yamaha stock sprocket is steel also. I've found Suzuki stock steel sprockets (original OEM) for less then any decent aftermarket ones. There are some decent aftermarket sprockets: AFAM, Renthal, JT. NONE, imho, as long lasting as a stock Yamaha sprocket, and all are likely the same or more than Yam OEM. You don't need the rubber damper, but it does make a subtle difference. Most can't hear it.
I don't know what chain Yamaha uses on your XT. But I'd recommend the DID X-Ring VM or VM-2. Expensive? Yes. Lasts forever. Much longer than stock chain which is not bad but not the best.
I have no idea what other sprockets might interchange with the XT660's.
Keep asking around, you may get lucky. Fit the best chain/sprockets you can afford. Will pay big dividends down the road. My DR650 chain is now at 22,000 miles. Still hanging in.
Last edited by Mickey D; 8 Sep 2010 at 20:58.