Just throwing this out there, not saying it applies to 420a's. I built a motor for a focus (2.0 zetec) with arp studs and cometic gasket and the head gasket leaked at 8psi of boost. I called up ARP and they said to tq the head down to 80ft/lbs in one step, none of the 20-40-60ft/lbs stuff. Just hand tight and straight to 80ft/lbs following the stock sequence. 10,000 miles later and its still holding 20psi of boost.

I didnt do that when i built my 420a so i dont know if its a good idea or not for our motors or not, but something to think about.