I am not sure about what to do if I have the diameter of the piston cup increased with 0.18 mm (with no ovality) due to wear. Should I change the pistons and increase the piston cup diameter or should I just change the piston rings. The pistons are not worn.

I'm not exactly sure I understand the question but I will try. Resleeve the engine with original bore size sleeves, bore/hone the cylinders to get the proper clearance between your existing pistons and cylinders, install new rings and put it all back together and you should be good to go.

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Jim

What kind of engine?

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k