Head bolts
Im with coldknock on that. I would never use a set of head bolts more than twice. After that, due to the operational stresses, as well as the torquing, they tend to lose their elasticity, and while they may torque down well to the recommended specs, it wouldnt take much to stretch them out after that. I would recommend studding the heads in lieu of using head bolts, if you are planning to remove the heads frequently. Studs dont get twised when you torque them, like a head bolt does. The twisting is as detrimental to the life of the bolt as well as is the stretching, especially if theyve ever been hot.