Mitutoyo Bore Gage Extension Rod This 6" Extension Rod will help you reach deeper into bores for measurements with the following Mitutoyo Gages... Mitutoyo Borematic Mitutoyo Holtest Specifications... 150mm / 5.9" in Length To be able to reach up to 12" deep, you can combine 2 of these extension rods. Note: Mitutoyo does not recommend using more than 2 extension rods with 1 gage. For use with 2"-12" / 50-300mm Mitutoyo Bore Gages only.