My son has a 78 build that we are frantically trying to keep alive. Two engines dismantled and major chrome chipping on both engines. I have a lead on a 100k engine. I would still rebuild, but when does the 12a housing chipping start. Or at least when is it most likely to start to surface mileage wise?

 

Each engine needs to be disassembled and evaluated on case by case basis.
Have seen 12A housing damage on as little as 65k miles and less damage on engines with 125k miles. Wear is relative to environment engine was operated in. Tear it down...

 

my sample isn't representative, but the chrome flaking seems to be worse on cars that ran a little hot for a long time. like the radiator was plugged, or thermostat was bad/lazy, not enough to overheat, just enough to be too hot.

in any case you won't be able to tell from the outside

 

I had a 120k engine with decent looking rotor housings and a 30k engine that had large swaths of chrome missing.

I think Lynn Hanover had it figured out... the flaking starts when the side housings fret against the rotor housing and peel the chrome up slightly, then it breaks off. Once that process starts happening it only gets worse. He said to take a fine file and put a slight bevel on the edge of the chrome surface all the way around and so far this seems to work okay.

 

I remember that advice. Lynn E Hanover was a real stickler for nomenclature, which I appreciate.