Originally Posted by
englishal
Your CofA doesn't suddenly become invalid and hence insurance invalid due to the failure of an instrument in the air.
If the prop is left fully forward then the RPM cold be used as per a fixed pitch prop to get you home.
I read working hardpost's as implying he was prepared to continue to fly the aircraft without a working MP guage. If you takeoff knowing that guage is no working than the aircraft most certainly does not have a valid C of A and therefore the insurance is also likely invalid.