I like the fact that Mercedes specifies a stretch limit for the bolts. Some people think of reusing torque-to-yield bolts the same as putting on dirty socks. It's not necessarily so, especially if you understand the stress-strain curve.

Torque values rely on the elastic part of the stress-strain curve. Clamp loads based on these values are prone to error, such as thread lubrication, grit, and burrs. Torque-to-yield puts you past the elastic curve, and into a somewhat flat zone where the stress doesn't change much with strain. The clamp load is much more predictable and consistent in that region, thus less chance of blowing a head gasket.

The head bolts are rather long for MB heads, which gives more usable yield. The torque-to-yield recipe (52 ft-lbs + 90° + 90°) uses only a small part of the usable flat zone. The specified limits on bolt length allow for reuse, and at the same time weed out any bolts that may have had a "problem".... such as torqued twice because of interruption or other loss of concentration, or excessive thread lubrication.

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95 E320 Cabriolet, 169K