Lol i swear that happened to me on every set of springs i've done with the dual compressor. Pissed me off so much.I have the dual spring compressor, but wish I got the single. If one set of retainer locks seats but the other spring doesn't, you need to redo both springs once you tighten the compressor back down.
The only issue with using air is that if you are removing the valve locks with a valve spring compressor and the spring compresses and pulls the valve down with it and then pops free the valve will drop. Imo the only way to do it without risk is to have each piston at top dead center while removing the springs.
I have Never needed to turn over engine when using compressed air to do springs? Been doing it for many years and have not had a problem. I guess it may be a safety precaution but unneccassary.. Whether doing 1 spring or 2 springs makes no difference. I guess if you were using a very weak compressor you might run into a problem, but doubt it...Both valves need to be closed when using air. Since you have to turn it over to do that I like to put the piston up and use air also.
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May wanna get a couple extra valve locks. Those things are a PITA to try and find if they fall when you are trying to remove/install them and the do the 'Tink Tink' down in the engine compartment.