I was kindly given this link :-
by Kelvin Lamport, (Thanks Kelvin!), but I'm having a few issues achieving the desired outcome. For starters, in the video, the top cap gets raised and then the path line gets extended and made coincident with that cap. I am unable to do that due to the fact that my top cap (so to speak) has a central hole inline with where the path line would meet and therefore no relation can be made. The top of my spring needs to sit flat on the underside of the top face of the valve retainer. When I have tried ways around this, I have ended up with a broken spring which now leaves me with two solid bodies (see Jpeg image).
I have attached the parts and assembly. Could people please have a play around and see if they can get this to work successfully for me or advise me on a work around.