Check you have the correct part.

Another thing I learned yesterdat evening (or should've known as it isn't the first time something similar happens to me..)

Always check the replacement parts are absolutely correct before installing them.

I'm restoring a Bally Future Spa. This pinball machine has a row of 4 droptargets behind each other. The first one was broken. So yesterday evening I decided to replace it. Had a droptarget bank from a Stern machine that was parted out.. the parts seemed similar/identical enough. Paid a lot of attention to the bottom part of the droptarget, to make sure the shape where it mounts and where the screw goes through was correct.

Replacing droptarget isn't a task you do in a few minutes.. you have to disassemble half of the metal frame where they mount into. First removed a target to use as replacement part. Made sure I didn't lose the spring or C-clips involved.
Then I disassembled the droptarget bank on the Future Spa. Removed the broken part.
Put in the new part. The drop target was identical at the bottom, but the way the metal rod that holds it and pushes its up again is connected was different, as here the droptargets are behind eachother, and on the original part they were next to each other.

All goes well, about half an hour later I'm ready and want to screw it into the playfield again...
and then I notice it doesn't fit ?!
The new droptarget I used is a bit wider at the top and doesn't fit through the hole in the playfield ! :-(

So there's half an hour of my life wasted.. next task is to do it all over using a droptarget that is a bit smaller..