Is there any way to repair a torn grease cup on a lower ball joint?

 

Not really without replacing it. I don't know if you can get just the rubber cover, if so you should be able to separate the ball joint, replace the cover, and put it back together. However, if your cover is torn, chances are dirt/water have gotten in there and it's only a matter of time before it is bad so you might as well replace the whole joint now.

 

I bought one such 3rd party cup but for the life of me I cannot get it to go on the ball joint. I am trying a suggestion of sealing the crack with RTV silicone gasket maker. Hopefully it will hold when I stretch it onto the ball joint.

 

It likely won't, RTV doesn't really care to be stretched much. If you were going to try this I'd put it on the ball joint, then apply the RTV...but even then I don't recommend it. New ball joint ends are what, $50 or so, an alignment can be done for less than $100? It's better to do it now then wait for everything to wear out thanks to being full of dirt.

 

When I installed the cup the RTV silicone had been curing for about 20 hours. It held up well during the installation. I don't know how long it will last, but it looks good so far. I squirted 8 pumps of grease into the ball joint and I plan to add a little more until I see the cup start to swell.
As far as the project goes, I got the psgr side buckled up last night. Just a few more items on the check list and I'll be ready to try my hand at DIY toe-in adjusting. should be able to get to that this morning.
I had watched a video of a guy using a socket to press his grease up onto the ball joint. That is what I was doing when I tore the cup. I do not recommend that method. It is better to just install the cup as best that you can by hand to begin with. you should be able to get it about 2/3 installed with just your fingers. After that use channel locks and squeeze the flange down a little bit at a time until you close the gap completely. Then I took a blunt punch and went all of the way around tapping the flange lightly with a small hammer to fully seat it. That is how I installed the other three cups after tearing the first one and that is also how I installed the repaired cup. Live and learn.