[billytheshoe]

A guide plate will at the least will locate the holes that need to be drilled out. I am new here with some close tolorance mechining experience (7yrs)& 25 yrs experience as a mechanic. I would suggest that this proceedure of drilling out the threads be done not using a hand drill. Its true if you have the angle right to start drilling the old treads will cut away just fine and leave you a "square" bore adequate for tapping. Sometimes a drill will take its own course in alum. and dig out at an angle you dont want. This would result in a cut thread that is a few degrees off of a square-perpindicular to the head fastener. This could distort the head,the block, head gasket and clamp the head to the block in a unequal manner. The other thing that comes to mind is head torque. If you over torque a bolt in alluminum you are a thousand, maybe a million times more likely to over stress the cut threads on the alum. block than the bolt. In my opinion I would only do this proceedure to a the block mounted on a machine with the angles squared up. Is it possible G.M. over torqued these head fasteners to begin with?