Originally Posted by
EDML
The MAX doesn't have the pickle fork problems because Boeing changed the manufacturer of the forks with or shortly before the MAX.
The new manufactures keeps the tight tolerances for the drilled holes so that the holes don't wear out and cause cracks.
My understanding is the original process in question was to force each hole to expand after each hole is drilled so that when the force is removed the inside surface of the hole is left in compression. Being in compression offsets tension caused by loads carried through the fasteners and this offset delays the onset of fatigue cracking.This is independent of the hole diameter variation. Similar processes include shot-peening. The problem was changing from a good process which had a one-time use sleeve to protect the hole and transmit the force from an expansion plug to some other sleeve that was cheaper and either damaged the hole in some way or failed to produce the same deformation.
Drilled holes have all sorts of tiny defects that can lead to the initiation of fatigue cracks; holes can be reamed to removed those, but the holes are still susceptible to load induced fatigue.
This is why expansion rivets are so popular - they force the surrounding material into compression. But with bolts, that isn't available, so the holes need a secondary treatment.