Extensions can be added to both the 780XT and 78XT Series, enabling internal measurements in deep hole bores. (Multiple extensions can also be used.). 3" Length.
Internal Micrometer Extension, 3/8-1/2" (10-12.5mm), 3". The Starrett Internal Extensions for 78 Series Internal Micrometers includes 3/8-1/2" (10-12.5mm) sizes, 3" extension size, and fits models 78XT/780XT/781XT375-500. Extension 3"/75mm Long for use with 78/780/782, 375-500.