I dented the top (not the stud side, don’t know if top is the right word) of the ball joint a little bit. Did it while pressing it in to the control arm. The harder metal on the outside is fine, the softer metal in the center has the dent.
Moog K90691 is the part, it’s on my 2005 Hyundai Accent.
Is that damage important, or are they still usable?
If you have not installed everything yet, I would try to see if it moves free yet.
If you have the opposite side apart, you can use that to compare it with.
By seeing if it moves free. Try sliding a deep socket with a short extension over the stud end of the ball joint. The far end of the extension should be able to be swung in an arc 306 degrees.
Compare with the other side. If any resistance or binding is felt, the ball joint is damaged.
Yosemite
Denting the steel cap will cause a leak. The ball joint will slowly leak grease out and water in. There have been recalls due to ball joint cap leaks. I would clean the the capped area and apply a rubberized coating.
I can swing it around 360 degrees but it gets slightly stiffer in the middle.
I actually dented both sides because the press I rented didn’t have an adapter small and I got impatient, so I just pressed the joint directly. So I can’t compare unfortunately.
So it will cause a leak even though it is dented only in the middle, and not where it meets the rim?
If you lay a straight edge across the cap, how deep is the dent?
Here is a picture.
It doesn’t dent inwards, just a flat spot where it’s supposed to be rounded.
I have to say I don’t know much about ball joints beyond taking them apart and putting back together again needed when servicing my Corolla’s CV joints, but I wouldn’t re-use a ball joint with that much of a dent. Suggest to purchase a replacement instead.
Any damage can compromise the ball joint cap. Clean the cap and ball joint, apply a layer of RTV (some OEM ball joints have sealant on the cap).
That looks good to me. Put it together and it won’t pop off. You will know soon enough in any case.
If it were me, I’d replaced them and be sure not to dent the new ones.
I wouldn’t want to go through all this work and maybe have to do it again in a month or two.
They are less than $5 each at Rock Auto.
Yosemite
A $5 ball joint???
That would be a red flag to me. Too cheap to be real 
Applying the force of insertion onto the cap would be a reason to replace them IMO. In this case, rushing to complete versus waiting to have the correct cup could have caused some significant damage to them internally. I agree w/Yosemite. I would R&R them again now rather than have to re-do it again sooner rather than later. Just my preference…