How often do hg fail on the 04 discos ? If you replace it once are they good for a while after that ?

 

I think generally they will last anywhere from 80,000 - 100,000 miles. However, there are a lot factors that can create failure sooner.

 

Random question: My (new to me and totally unknown history) 1999 disco has 140k miles. Is it either A: safe to assume that the head gaskets have been changed because it's gotten up to 140k or B: safe to keep driving it regardless and just wait/hope that they are fine?

Is this a maintenance/service type thing that needs to be done or only if it's broken deal with it?

 

The preventative maintenance on a head gasket is to change and run good coolant (don't leave it in there forever), not Dexcool; and to operate at reasonable temperatures, not extreme. So if your scanner or Ultra Gauge shows you are driving around at 221F, just about any cooling system fix will be less than a head gasket job. The factory heat gauge is designed to show one position for a very wide range of temps, so you can't see things heating up over time. The soft spring 180F thermostat is also, IMHO, a great mod as it drops temps across the board.