Thanks to everyone, what I did at the end was to use liquid vaseline (I believe that at the end it wasn't necessary) and put it in the hole from the saddle. After that, I put the insert, w/o force, taking care that it stays horizontally. Then with a clamp (like the one that
uses here IBANEZ RULES Clean and Setup
) I pressed the inserts aggain the saddle, take care not to damage the cable from the piezo.
I'm doing a small mod, w/o preamp. I go from the summing board (wich sums all of the piezos signal together) directly into the connector. Mine guitar came with a TRS (or stereo) connector so no need to change, T is for magnetic and R for piezo. As long as you use a high impedance preamp, it works fine, or you could use an external preamp or DI box for that.
Why piezos need preamps is because they act as capacitor, so if the impedance of the preamp isn't high enough, you end up with a highpass filter, removing the lower signals. Normal mixers doesn't have high impedance so DI box or preamp is needed.
Since I have a pod HD 500x which has 2 inputs (guitar and aux) I wan to use ine for guitar and one for piezo and do all of he changes between them there (even blending could be possible).