Q: I followed your advice and tightened down the screws in the back of my Stratocaster to pull the bridge back against the body of the guitar. But it still goes out of tune when I try to drop the low E string down to D.
A: Our guess would be that you only have three springs installed for the trem. Even though you have them tightened down, there’s not enough tension to completely counterbalance the pull of the strings. Add two more springs and tighten the claw screws down as much as you can. This should make the bridge sit tightly against the body of the guitar and maintain the tuning when you detune the low E.
If you already have five springs installed and tightened down, then you will need to block the bridge to make it even more solid.