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Replacement components for 7-Speed Shimano STX drivetrain(1994 Specialized Rockhopper
Hi all, first post here. I recently got back into mountain biking after dusting off the 1994 Rockhopper Sport I purchased new when I was a teenager. I've been doing repairs along the way, learning as I go. My background is in auto repair, so learning how to wrench on bikes has quickly become a new hobby for me.

The bike still has it's original Shimano STX drivetrain(aside from a chain that was replaced years ago), and I want to replace/upgrade the cassette, freehub, and chain. The rear hub is a Shimano MC30. I have already serviced the rear axle and freehub(thanks to the repair guide videos on this site), and it's working, but clearly tired after 25+ years of service.

Before servicing, the bike made an audible rotational clunk in the rear axle while coasting, bad enough that I could feel it in the seat while riding on the street. The noise only occurred while coasting without load on the drivetrain. Backpedaling made it worse. I put the bike on the stand and noticed a significant wobble in the cassette while the rear wheel was rotating. Long story short, during the servicing, I accidentally left out what I now know is called a cassette cog spacer. It's 1.2mm thick.

Since the spacer has been absent from the rear hub, the noise is gone, the wobble is significantly reduced, and the rear derailleur shifts perfectly. Is this thing necessary? The bike seems better off without it, and I've been pushing it pretty hard on the trails(as hard as my skill level will allow, anyway). Without it installed, there is a tiny bit of side-to-side play when I use a screwdriver against the chainstay to push the cassette towards the wheel. Without actually measuring, I would estimate 0.5mm or less. I went searching for a thinner spacer to take up that slack, but I haven't found one that looks the same. Rear axle is the original 135mm QR, and it appears perfectly straight when checked against a straightedge.

Now that I've ridden the bike for a while, I'm also thinking about replacing the worn original STX 7-speed 11-28t cassette with a 7-speed 12-32t cassette(I'm looking at the Shimano Alivio HG200, but I'm happy to take other recommendations here) for a little more hill climbing power with less shifting on the front derailleur(chainrings are 24/34/42t). I want to replace the original freehub too, as it has some radial scoring that looks like the cassette may have slipped at some point. However, I'm a little confused on how to select the correct replacement part. The cassette lock ring says Hyper Glide - C, so I assume I need a Shimano compact freehub, but I run out of knowledge after that. I haven't had any luck finding a direct replacement, but obviously I want to find one that will match the STX hub in my wheel and the HG200 cassette. I also need some guidance on the best chain to use with these components. It currently has a Shimano HG narrow chain marked 3K on it with a KMC CL573R quick link. I've been looking at Shimano HG40 and HG71 chains, but I'm a little unclear on whether I should use a quick link or install using pins.

Recap of questions -
Is the cassette spacer necessary? Can I(should I?) install a thinner one to correct the wobble in the cassette?

Will the HG200 12-32 cassette be compatible with my bike? If not, what should I use? What freehub and chain should I use with it?

And a more general question - where's the best place to find replacement/upgrade parts for a "vintage" bike like this?

Thanks everyone

1994 Specialized Rockhopper Sport
could be something for you? @Painkiller
Yes it should be on there for a snug proper fit, it spaces the rivit ends of the cluster, some wobble is normal. If you need to replace the hub body this link may help..https://www.parktool.com/blog/repair-help/freehub-service
I would go with a Sram cluster and sram chain with the quik link
ebay is a good place for these types of parts parts
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"

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