Line borers special inside callipers 

Quality Moore and Wright calipers modified to enable a 50mm bore to be measured with a 40mm bar in position, max size is 150mm. 

These are our Go-To callipers for checking small bores, and we even find ourselves using them to measure the bigger bores too. 

The last photo shows the callipers being used to measure a 50mm bore with a 40mm bar in position