usually it's one of the solder joints in the core that goes bad, or a plugged passage causing overpressurization...i've blown out three of them in the past, the second one looked like a balloon. then finally i discovered the combination of a parts store heater core and a flowkooler high volume pump do not a reliable heater core make....so i swapped to a normal water pump, my temps only went up 10*, and i'm still running with the same heater core i changed about two years ago with no issues
Put a Flowkooler in my 85 and the original heater core lasted about one hour. Replaced it with a stock Ford replacement and haven't had any problems ..... yet.
It only took me like 2 hours total my first time to pull it and put it back in. I didnt really think it was very hard at all. More of a PITA than anything but not hard by anymeans.
yea taking out the dash the first time was a biotch.. but putting in back in was very simple.. there are sooooo many damn screws that arent really necessary.. after putting my dash back in, it took about 30 minutes.. i had over 1/2 of the screws left that were supposed to be used lol.. u dont really need em all heh