[SAJ]

LI have a piston expander called a Koetherizer. Invented by Emil Koether in the Early 1930's.
You clean the piston internally by blasting with walnut shells in the right hand chamber of the machine. Then insert the special squeeze bolt through the gudgeon holes, tighten the bolt against its ballrace thrust washer and nut, squeezing it until it has expanded to the size you want across the thrust faces, using a micrometer across them. Up to 0.005 to 0.007 and maybe more can be gained depending on the design and weight of the piston.
Then put the piston, still under compression with the squeeze bolt in place, into the steel shot blasting chamber and blast all around the inside, using the automatic piston rotator mechanism to blast every part at about 100 psi with compressed air and steel shot.
This relieves the compression stresses in the aluminium alloy, so the piston holds the new shape and size after the squeeze is removed.
The original operator who did many thousands of piston still comes to relieve new modern racing pistons for friends who race modern cars, for better longevity at high revs. Mainly as a favour to old friends.
There was a category “piston expansion and automotive” in the NZ classified ads years ago in the newspapers, that dealt with Koetherizing and “micropeining” using tiny swinging hammers inside pistons. Koetherizing was said to be more gentle and kinder to piston.
SAJ in NZ