Experimental results, together with calculations and formulas for the ultimate strength of cast iron internal threads, are presented. The calculated thread strength based on the formulas and double shear tests of cast iron samples is compared with direct tension tests on steel bolts engaged to various depths in tapped cast iron specimens. The wide range of strength values in cast iron is recognized. A chart for the necessary length of engagement for various SAE grades of bolts with different strength levels of cast iron summarizes the overall results.
