GS 160 piston sizes

I have 2 rusty 160GS barrels that will need either honing or reboring. I am sure neither have had any previous work done to them and the inside diameter of both barrels measure 58.5mm. the pistons measure 58.4 although one has 58.6 stamped on it.

The standard piston, Art.No.19402000 is Ø 58mm, and the description says 'The bore Ø always includes the size of the cylinder.'

The over sizes are: 2.o/s, Ø 58,4mm, 3.o/s, Ø 58,6mm, 4.o/s, Ø 58,8mm,

I am a little unsure what measurement I should be working to when ordering a new piston for reboring/ honing, can any one help, Thanks Andy