The engine has not done a lot of miles,one of the value seats burnt,so i re-cut all value seat and put back together job done all ok.Some time so no theosat fitted runing ok.COUPLE OF DAY RUNNING OK. THEN started to show signs of air bubbles in rad .DO you think as i do that the heads have a small crack in them or has it just damaged tha gaskets again? have got older engine for spares ,should i fit the heads of this engine or not?