Years ago I got generic bushings from NAPA that were solid rubber. They were about $8 each. I assumed then that they were for SS. I didn't use them, got full LCAs instead.

 

I just recently replaced my lower control arms on my 09 SS and also ran into the the SS larger ball joint not fitting. I had to get another pair of of control arms with the smaller ball joints to get it back together. I was completely confused.

 

Unless somebody replaced the aluminum knuckles with non-SS ones (if that's even possible), the larger diameter (20.5mm) ball joints are the correct ones.

Putting the smaller diameter ball joints used on non-SS HHRs will eventually damage the knuckle and/or cause handling issues.

As hhoyt posted above in post #25, a previous owner may have used the smaller ball joints and just tightened down the split-bore bolt until it was tight. It was a lot of work for him to spread it out to the right bore again.

Steve

 

Agree, the knuckles are likely crushed down to the smaller size, like the OP experienced.