As you know, it only takes a split second for a headgasket to go, you will mostlikly notice a little shake and a little extra black smoke followed by more than normal smoke at idle..However the engine will continue to run and start fine, its usually not untill your overflow starts overflowing that you notice something is not quite right in there...
The Thermostat test to check for a blown gasket.. What you do is:
*Let engine Cool *Drain The Radiator *Disconnect both Hoses From radiator..bend bottom hose up and stick T-stat inside to help keep water from coming out. *Disconnect waterpump belt *With the t-stat housing off and t-stat out, fill the engine with WATER till its to the top of the T-stat deck *Fire it up, water will spit out The side of blown gasket
My advise is that if you only have ONE blown gasket, replace both it would be unwise not to do so.
*ALWAYS have your heads Resurfaced and cleaned! *Use FINE grit sand paper and acetone to clean the blocks surface! *Study the the surfaces very carefully..as the slightest irregularity will result in a bad seal. *NEVER use the same Headbolts (especially if the engine is an 80 or earlier and still has stock bolts)..updated or not...I Strongly recommend using ARP Head Studs, as they dont break or strech..If not, be sure to use NEW bolts and throw some strong washers as big as the head will allow under them..this will help even out the torque through out the head..And Remember, you may not always catch a streched bolt..Installing a stretched Bolt will result in a blown gasket soon enough! ***(Failure to do these things can result in blowing a headgasket...again!)***
Last but not least and this is important.
Due to Tight valve to piston clearance, it is important to Use the correct gasket..There are two.
Standard fel-pro part # 8983PT (used when pistons project no more than .025" from deck..This gasket is about .030" thick) and,
.010" thicker fel-pro part # 8716PT1 (used when pistons project .026" - .032" above the deck..a gasket for resurfaced blocks in other words).
Aftermarket Gaskets such as Graphite and especially Copper, be sure to pay attention to the thickness..
...I never understood why when a stupid gasket would go, they'd throw the whole engine away...well it makes me think..dont know about you...