So I bought a second hand b18 bottom end to rebuild for my b16 head and box, found out the motor has done roughly 250,000kms I haven't pulled it down yet but I'm wondering with that many kms what else apart from rings, bearings, piston s and all necessary gaskets would I need to replace or have checked? Would I need to get the crank balanced? Are the rods going to be ok? Or is this basically a boat anchor? Thanks for any help/advice!

 

All of that requires a visual inspectino.

 

What B18 bottom? 250000km isn't bad (condition depending many factors), but it probably needs overhauling anyway. Visual inspections and cylinders, pistons, crankjournals etc. measuring is only way find out.