Mark,

Those fingernail marks on the head surface are from the machine shop plaining the head. Those are normal and the headgasket just fills in the minor surface marks....check your block closely!!

I've seen this kind of problem before and the only way to fix it is to find out what the problem is - as Brandon wrote. You must check the head and block for warpage - no more than .003 in. over the entire length using a precision flat and feeler gauges (my flat is made by Snap-On and is flat to .0001" along its length). Cracks and/or leaks from the liner area, or some other imperfection causing the cooling system to stay pressuriezed should be looked for. A crack in an exhaust port in the head or the combustion area of the head would also allow hot combustion gas into the cooling system causing it to pressurize for the length of time you're talking about - and will also cause the coolant temp to increase a lot when the engine is running since the coolant sensor is located in the head's coolant passages and would read the increased coolant temps. caused by the leak. These cracks and imperfections can be so small that they can be seen only with magnifying glasses (10-20 power) and/or dying techniques (best way) that will reveal the cracks/imperfections. M-B calls for sealing the head, pressurizing the head, immersing the head into water, and then looking for leaks.

The machine tool marks in the head may or may not be OK - a fingernail catching on the tool marks does not sound good. However, without seeing the tool marks I cannot say absolutely this is the problem.

Also, and this is very important, this is 20 + year old technology that required the head to block mating surfaces to have a certain roughness (measured in Ra units like the finish cylinder walls are honed to) so the head gasket will properly seal - the machine shop doing the work should have this info and sets their machines accordingly to achieve the proper finish/roughness. Using sand paper or Scotch Brite or anything similar to clean the mating surfaces should not be done because the resulting surface will be too smooth and the head gasket will not seal. Also, sanding can cause small areas that have too much material removed that will cause the head gasket to not seal properly. If the surfaces have been sanded too smooth, then the cure is to have the mating surfaces milled to achieve the proper roughness and eliminate the uneven mating surface. Sanding with rougher sand paper to try and correct a too smooth problem will only make the problem worse, in my opinion, since you're removing more material and faster with the rougher sand paper - .003 in. ain't much!

To clean the surface you should use a gasket scraper (I prefer very sharp wood chisels) and if the gasket material is really stuck you can use paint stripper (like Strip Eze, my spelling may be wrong) that is painted on the old gasket with an acid brush (like a paint brush). I don't like spray on paint stripper because it gets into places that are hard to clean. Let the paint stipper sit for a while and the old gasket will peal right off. If a thick layer of old gasket is stuck and only some peals off, then another application of paint stripper can be done. Paint stripper will not harm the metal.

Also, thermostats can be defective - even new ones. Check for opening temp. by immerisng in water that is the temp the thermostat is rated for full open position (your stat use 200 degrees F). The stat should open fairly fast (about 1 min.). M-B has been known to get a batch of defective stats - look for a thread by leathermang about going through all stats the dealer had because all were defective - and I've also seen new defective stats more than a few times and always check them before installing.

However, the stat is not the cause of your problem - only one last thing to check before re-assembling your engine after finding the real problem causing the cooling system to operate hot and stay pressurized for hours after shut-down.

Good Luck!
Tom

__________________
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
Shop Foreman;
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.

Mark

If you do that bubble test before tear down, it would help you find the area after you get in there.

Just a thought, good luck

Harry
86 300 SDL