I never had a problem keeping the valves closed while changing springs.
If you're removing the cams (with heads on or off), this is the best spring compressor I've found:
https://www.euroexportinc.com/ford-modular-4-6l-5-4l-3v-valve-spring-compressor/
Thanks. Honestly, you can just rotate the engine so the piston in the cylinder you're working on is at TDC, and the valves can't physically drop into the cylinder. No air pressure required. You can also take some nylon braided rope and feed that through the spark plug hole if you want cheap...
Here's an old writeup I did a million years ago... hope it helps http://www.moddedmustangs.com/forums/2005-2010/220707-comp-127500-spr-cam-install-pics-video.html
Zombie thread, activate! So, I'm diving back into this after not much time working on it. After much research I was convinced it was the chain tensioner. Took timing cover off to find no obvious damage or wear. I'm at a loss here. Here are new videos...one without the belt and one with the...