Breathing new life into the 318 in the Scamp!

Since you do not have an inside ball micrometer.
Cheap trick.
Cut a metal coat hanger to length, radius the ends. (Very important).
Slowly grind one end down until it slides equally down the ID of the bore.
Measure with you caliper.
Time consuming yes but it will work.
The reason you cannot get an accurate measurement on an ID of a bore is because of the thickness of the caliper jaws.
Try it and see what measurement you get.