I posted a message here about the Merge command but the board appears to have mixed it up with another thread.Sometimes when I Merge a new part into a scene I intend to replace an old one and select Delete Old, the part replaces the old at the same location and sometimes it does not. Why?This is driving me crazy assembling my aircraft! It takes time to position those objects.Steve

I've never heard of this but I would make sure the pivot point is centered on the part you are trying to merge into the scene. I know this use to happen to me when I cloned and mirrored a part, because I didn't have the pivot centered on the part...Rick Sasala