Extensions can be added to both the 780XT and 78XT Series, enabling internal measurements in deep hole bores. (Multiple extensions can also be used.). 2.5" Length.
Internal Micrometer Extension, 1/4-5/16" (6-8mm), 2.5". The Starrett Internal Extensions for 78 Series Internal Micrometer includes 1/4-5/16" (6-8mm) sizes, 2.5" extension size, and fits models 78XT/780XT/781XT-312-375. Extension 2.5"/63mm Long for use with 78/780/782, 312-375.