What exactly is meant by mash cap? Also how long do you pre-boil your mash water?
A mash cap greatly reduces or eliminates O2 contact with your mash. It covers the mash mixture by direct contact, ideally covering the entire interior surface of your mash tun. Depending on the shape of your mash tun, it might be pie or pizza baking sheets, or in my case a piece of 2" Styrofoam covered with aluminum tape.
As for preboiling, I boil 5 minutes then cool to 200F, add acid and water salts, then continue cooling down to strike temp.