Alrighty, so I've unioned the engine block with the 4 jets.

And the 4 jets have a circular hole in them, where you can see through to the back of the engine block.

So I've selected the circles of each of the 4 engines, and used the Planar tool to create a circular plane to cover these holes.

This is good, except that these 4 circular planes are not attached to the engine...

And I don't know how to attach them. Union just removes the circles, and join won't work on objects.

So I'm wondering how I can do this?