I think that if a stretch bolt has not elongated at all. It has recovered the stretch it produced when in service. It is not damaged. Any elongation remaining evident means that it has not and should not be reused. You are too likely to work it into the true over yield area. With the engine in service. Mercedes gives a tolerance yet for this to be really valid the material those bolt where made from would have to be kept to a very tight standard. Especially for a cast iron head.


Buy a known good brand of aftermarket bolts. I think engine builders where forced to go to variable pressure bolts in service. Because of the expansion and contraction coefficients with aluminium heads. I think this is far less critical on cast iron heads.


In the grand scheme of things I would use new at 67.00 a set. At hundreds of dollars a set I would have to think about it. For a cast Iron head I would be less concerned. If they show no evidence of retained stretch they go back in. As there is then no evidence of distortion.