Is it a good idea to re-torque head bolts on an old engine?

Re-torquing head bolts can prevent head gasket failure. Some manufacturers have issued technical service bulletins describing the procedure. (See Saab example here: http://www.saabnet.com/tsn/members/gallery.html?memberID=112&do=show&id=369 ) Reducing the tension on bolts one at a time (But not all the way.), then tightening in sequence is often done, particularly on engines with aluminum heads. Once the gasket has failed re-torquing is less likely to stop the leak. No. Torque specs are calculated on a new head gasket. If you are experiencing a leak in a head gasket, you would be safe to assume that the gasket has already burned a hole in the gasket.