RD collides with cassette.
 

I am rebuilding a mid 80s Schwinn High Sierra mountain bike. I am setting up the RD and I am having it collide with the cogs on the rear wheel. I have changed nothing about the bike except the shifters. Why would there not be enough clearance now between the cogs and the RD?

Parker Anderson......from my phone

Check the B-screw adjustment.

Parktool on B screw: Rear Derailleur Adjustment | Park Tool

Did you really change only the shifters? If it cleared before and doesn't now something else has changed. Chain length? Cassette? Cable slack?

As noted, the B-screw is the probable fix.

Everybody already said B-screw, that is the specific means for adjusting RD/cassette clearance. No idea why it might have cleared before but be hitting now.

If screwing in your B-screw all the way is not sufficient, take it out, go to any hardware store, and buy another longer bolt with the same threading.

I didn't remove the B screw during cleaning so I wanted to check to see if anyone else had this problem and it was not a B screw adjustment issue. I think the spring inside the RD may be shot. Trying a longer screw next.

I don't think so -- if the spring were shot, the RD would sag downwards away from the cassette. If it's pushing up, then you've got a spring working against gravity.

If you changed the shifters, did you mess with the cable? The rear housing loop acts like a spring that increases the force of the B-knuckle spring. If for some reason you removed the wheel, that could also be an issue since horizontal dropouts are used to adjust the gap as well.

Edit: Wrote A instead of B. Learn the difference, it could save your life.

rear housing loop as add'l spring, I like that hypothesis.

Is the chain sized correctly? It looks like it might be longish?

Looks like you're in the big big combination. If so your chain is a couple of links too long. A shorter chain will pull the derailleur down more when you shift to lower gears. (Easier)

It's hard to count exactly but that looks like a 32T big cog; I don't think it is original to a bike from the '80s. Not sure of the specs of your rear derailleur but it might not clear a cog that big.

It's hard to tell from the blurry pics but the B-screw looks bent, like the derailleur was installed without cocking the derailleur so the B-screw didn't clear the hanger.

Shortened housing....swapped to smaller cassette...voila!