Bore .266". There are 3 models for the ST backbores. They are numbered from 1 to 3, with 1 being the smallest. It is worth noting that these models were designed for the large tenor; it is not just a tenor backbore with a large shank.
The ST* (STAR) series is the same as the ST series except that there is no cylindrical section.
The ST* models are somewhat more free blowing than the standard models.