A local shop replaced the head gasket on my '69 C8 in 2011. I removed the head today to clean the chamber and exhaust valve and found that the head bolt at the exhaust manifold end (of course) has metal in some threads and a new bolt won't go in (not trying to reuse the old bolt which also won't go in even after cleaning the threads). I am planning to use a thread chaser to clean the threads in the jug. Never used one before. Any watchouts? All other bolts are OK and I used anti-sieze at reassembly.