THIS ITEM IS A SET OF S.G. CO THREAD RING GAUGES, A PRECISION INSTRUMENT DESIGNED FOR CHECKING EXTERNAL THREAD DIMENSIONS. THE SET INCLUDES TWO GAUGES WITH THE FOLLOWING SPECIFICATIONS: ONE 5/8-11 UNC-3A/GO PD .5660 AND ONE 1-8 UNC-3A/GO PD .9188. THESE GAUGES ARE IDEAL FOR PROFESSIONAL SETTINGS REQUIRING ACCURATE THREAD VERIFICATION.