How do I preset a manualWeld?

For some reason I cannot preset a manualWeld's Part1.

I have a box consisting of 6 parts. The top and bottom part have 4 manualWelds attached. The bottom part works fine, but the top part is a copied bottom part. So the manualWelds Part1 has no value and it's also greyed out.

The wiki says it will weld automatically when parts touch eachother, but after some moving and scaling they're still not welded like the bottom part is welded to the side parts.

Does anyone know how I can fix this? If you have any more questions, just ask me. Screenshots