Crankshaft. Performance Crankshafts from XKs Unlimited have been prepared for the rigors of high performance use. By heat treating and tuftriding, the molecular composition of the steel is hardened and strengthened. These processes yield a rank suitable to withstand 7500 RPM and more. This is a rebuilt part with a core charge. For more on core charges and our rebuilt parts policies, click the Documents tab and double click on Core Information Sheet 8 10 2015.