First thing that comes to mind: did they use abrasive discs (Surface Conditioning Discs) to clean the gasket surfaces when they did the engine work previously? These are much like "scotchbrite" pads, but somewhat more coarse, and intended for use with power tools.
For that matter, did the tech working in the next service bay to your engine use them on "his" engine? The abrasive particles get airborne and travel everywhere.
Where I worked, we called 'em "cookies". They're Velcroed to a backing disc, then spun with a drill or die grinder. They're fabulously effective at removing baked-on gaskets. They're also great at removing the top layer of metal from the casting, and for spewing microscopic abrasive (aluminum oxide, I think) EVERYWHERE including into the engine, where they're carried by the oil system and destroy bearings/journals, cams, lifters, piston skirts, etc. I haven't used them for decades, in part because I came to the same conclusion that the Big Guys did: The abrasive particles are harmful to the engine.
The AERA has a service bulletin warning of their use, as does GM.
https://www.holmanparts.com/wp-content/uploads/2015/09/06_GasketSurface-Cleaning_V2.pdf
For that matter, did the tech working in the next service bay to your engine use them on "his" engine? The abrasive particles get airborne and travel everywhere.
Where I worked, we called 'em "cookies". They're Velcroed to a backing disc, then spun with a drill or die grinder. They're fabulously effective at removing baked-on gaskets. They're also great at removing the top layer of metal from the casting, and for spewing microscopic abrasive (aluminum oxide, I think) EVERYWHERE including into the engine, where they're carried by the oil system and destroy bearings/journals, cams, lifters, piston skirts, etc. I haven't used them for decades, in part because I came to the same conclusion that the Big Guys did: The abrasive particles are harmful to the engine.
The AERA has a service bulletin warning of their use, as does GM.
https://www.holmanparts.com/wp-content/uploads/2015/09/06_GasketSurface-Cleaning_V2.pdf
