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Bolts are torqued to yield in automotive engines to fasten aluminum alloy cylinder heads to cast iron cylinder blocks. This tightening method provides a reliable, even, and controllable clamping force that prevents warping of the soft cylinder head and ensures a leak-tight joint. It has one drawback: Since the bolts yield during engine assembly, they work harden even as their ultimate strength decreases. Upon reuse, yield stress more closely approaches ultimate strength. Fastener geometry and mechanical properties, now changed, supply changed joint-clamping force. For reasons such as these. Ford Motor Company recommends that bolts torqued to yield be replaced each time an engine is rebuilt. Cost savings, however, could be realized if engine remanufacturers could reuse these fasteners even once. In this article, reuse of SAE 10-mm cylinder head fasteners is simulated in the laboratory for a Ford I.9-liter four-cylinder engine. Based on tensile test data of yielded bolts, recommendations are made discouraging their reuse.

Recommended Citation

D. Lee, J. Foyos, O. Kossuth, M. Smith, & O. Es-Said. "On the Reuse of Bolts which have been Torqued to Yield,” J. of Communications in Reliability, Maintainability, and Supportability, Vol. 2 (2), 1995, pp. 35-39.