MG B Inner Tubes
An early MG B would fit a 155R14 Michelin X. But then the majority of MG B’s fitted a 165HR14 Pirelli Cinturato CA67. This Michelin inner tube is the best possible inner tube for either of these sizes and perfect for an MG B.
There is a possibility that your MG B may have been fitted with over sized tyres such as a 185/70R14, under these circumstances you will need a 14 E 13 Michelin inner tube. And the best tyre would be the 185/70VR14 Pirelli Cinturato CN36. However the best tyre is the original 165HR14 Cinturato CA67.
If you car has been fitted with some lower 65% profile tyres then you should not fit inner tubes. If your MGB does have wire wheels then you should always fit an inner tube.