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Distributor's Link Magazine Fall 2021 / Vol 44 No 4


30 THE DISTRIBUTOR’S LINK Larry Borowski President GREENSLADE & COMPANY INC. 2234 Wenneca Avenue, Fort Worth, TX 76102 TEL 817-870-8888 FAX 817-870-9199 EMAIL WEB HOT DIPPED GALVANIZED STRUCTURAL BOLTS AND NUTS REQUIRE SPECIAL THREAD GAGE SIZES Many structure bolts and nuts are ordered with heavy coatings on them to increase their resistance to corrosion. The applicable American Society for Testing and Materials (ASTM) standards provide special thread allowances to accommodate the heavy coatings. These heavy coatings can be a major source of assembly problems if the special thread size allowances are not understood and used by bolt and nut suppliers. ASTM publishes the requirements for inch and metric structural bolts and nuts used in the United States. The structural bolt standards are ASTM A307, A325, A325M, A490, and A490M. The structural nut specifications are ASTM A563 and A563M. The specification numbers ending in “M” cover metric bolts and nuts and those without the “M” cover inch bolts and nuts. Within these standards, the thread class specified for the uncoated inch bolts is 2A, and for Metric bolts, it is 6g. The thread classes specified for nuts to be used with the uncoated bolts are 2B for inch nuts and 6H for Metric nuts. Thread sizes are of all coarse thread series. When ASTM A307, A325, and A325M bolts are ordered coated the most commonly specified coatings are hot dipped galvanized per ASTM A153 or mechanical zinc per ASTM B695. When bolts are ordered coated, it is a good practice to order the ATM A563 or ASTM A563M nuts that go with those bolts with the same type of finish. ASTM standards A490 and A490M do not allow any coating to be applied to these bolts as a precaution against Hydrogen Embrittlement. Galvanizing and mechanical zinc coating deposits alter the sizes of the thread characteristics such as Pitch Diameter, major diameter, and minor diameter by several thousanths of an inch. To prevent possible thread interference between the bolt and nut at time of assembly, the ASTM standards provide a clearance allowance for both in order to accommodate the size changes resulting from the thicker coating. These coating allowances are provided in a table of each standard, but only ASTM A563 provides the exact thread size limits when the allowances are added. In all other standards the supplier or user must calculate these special thread sizes if they with to order thread gages to inspect their coated bolts and nuts. To make thread gage ordering easier and more consistent for those wanting to inspect coated structural bolts and nuts, the following two charts are provided. TECHNICAL ARTICLE CONTINUED ON PAGE 159


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