It definitely looks like the damage from skirts but probably from being too tight, not from slap. If the engine was rebuilt then for sure the pistons weren't fit properly and/or the bores where not machined properly. You would be hard pressed to find a re-builder that can come anywhere even close to BMW tolerances. Possibly the engine was overheated or run hard cold. Aren't some of the V-8s infamous for cylinder material issues ??

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)