Brett.w... The rule of thumb is when running a lower nitro fuel, you run a hot plug... ie... OS P3 or P6... Anything 20% or below in nitro you should run a hot plug... Sometimes weather conditions/temps may also have you use a different plug type... Safe bet, like I said, if you're running 20% or lower, run a hot plug. As for your head shim question, if your motor is running fine the way it is... Leave the stock head shims the way they are... Taking shims out... Adding to many or not enough head shims can cause more head-aches... If it's not broken... Why fix it...
RC_Alan