I heard the OBD2s got wider coolant passages in the headgasket, this is what @LINUS said in another thread.1997 with something like 265k on original head gasket. I have read that Toyota made some improvements in the head gasket during the last year or two of production. No idea if that is true or not as the part number didn't change.
All this visible from looking at the engine while the head was on?
^^^^^230k on the OG head gasket here.
I heard the OBD2s got wider coolant passages in the headgasket, this is what @LINUS said in another thread.
I'm not sure there were any changes made for the 1997 model year though. In fact I remember seeing a post by @Ozark cruiser where he said everyone he's opened up has needed it. Apparently the material itself will just degrade over time, especially near cyls 1 & 6, and there won't always be any symptoms.
Idk if we can say it's a design flaw if you hear about them lasting 300k+ miles, though. I suppose a lot of it has to do with how the truck was driven throughout its life i.e. how much heat cycling, long or short trips, etc.