Nylock nuts are no substitute for proper torque. Bolts stay tight because they stretch a little as they are tightened to the proper torque, and that depends on the "grade" of the bolt. Grade 2 bolts, the ones in the bulk bins at most of the big box hardware stores, are soft as butter and shouldn't be used in applications where there's any stress at all. They're OK for oil pans, valve covers, and other non safety related situations. Grade 5 is for medium duty situations, like water pumps, intake and exhaust manifold bolts, etc. They need more tightening torque than a grade 2 to stay tight. Then there's Grade 8- - - -cylinder head bolts, main bearings, and any component that's in a safety related location like steering linkage, universal joints, suspension components, etc. They need to be TIGHT to keep from loosening up in use. Self locking nuts just try to keep the parts from falling off once an insufficiently tightened fastener loosens up. There are published specifications for proper torque values for every type, size, and grade of fastener. Then there are the specialty bolts like for high performance connecting rods, that are MUCH tougher than graded fasteners, and need to be tightened to a specific amount of stretch, measured with a micrometer, to stay tight!
Jerry
