Also, can you give the application of what this is for because it’s really hard for me to understand why you need a part to spawn inside another part, but only on it’s Y-axis.
A part positioned above another part has to be calculated by the following formuala: Position of the bottom Part + 1/2 the height of the bottom Part plus 1/2 the height of the Part you are placing on top = the Position of the Part on top.
Same with placing a Part underneath the top Part.
Top Part Position - 1/2 height of top Part - 1/2 height of Part being put underneath = Position of Part underneath.
Works for Ball Parts too.
I assumed that because he only wants the new Parts placed on the Y axis that the Part was a Block and Oriented vertically, not at an angle.
That has nothing to do with spawning inside a part only on the Y-axis. This is spawning around a part. That’s a much more complex effect and should have been shown in your original post.