My little bit. A brand new factory block, the bore has a taper to the bottom, this is why you see a ring gap measurement from the bottom. once the cylinder has been bored this takes the taper out and is now close to a perfect tube. Now you can take the measurement from the top if you like. I like using cast-iron rings, as the gapping tolerance is the same from vehicle to vehicle measured From bore size. however original M10 gaps are quite large I suppose that is the composition of the ring material.