The Starrett 640JZ Dial Depth Gages are direct reading tools, referencing from their hardened and ground bases. All bases are 2-1/2" (64mm) long. They are quicker and more convenient to use than any other type of depth gage within their ranges and accuracy. Electronic Indicators can be furnished by special order. 0-1/2" Range, .0005" Grad, 0-50 Dial, in Case.