The Fowler / Bowers Holemike 40" Extension is designed for use with Fowler / Bowers bore gages, giving you precise accuracies over a extended range. This extension is ideal for measuring deeply recessed features, giving you reliable and repeatable measurements. Features: 40" / 1000mm in Length This extension fits the following Fowler / Bowers Holemikes... 2.000" - 8.000" Gage Head