The new model's air tube has the same ID as the old model from the '70s but they're both smaller than a factory air tube from a '60s Model 1894. The 1894 factory air tube ID was said to be a #40 drill bit (0.098"). I'm currently using an ID of 3/32" (0.094) on the factory air tubes. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.