“Reduce, Reuse, Recycle” is a message we hear everyday in this environmentally-conscious era, but there are still a few places where reusing a part that seems good is still a terrible idea: head bolts. We all know about “torque to yield,” but a surprising number of Canadian techs roll the dice by re-using these fasteners. How serious is the warning to throw them away? Is it just marketing hype to sell bolt kits? The answer involves just a little bit of basic metallurgy.
Head bolts, like all metals (and lots of other materials too) behave in two ways under stress: elastic and plastic. In the elastic zone, at lower strains, (the rising part of the curve in the diagram) the part stretches, but springs back to its original shape and length when the stress, in our case bolt torque, is removed. Exceed a critical stress limit, i.e. over torque a bolt, and it behaves “plastically,” meaning it stretches, but it won’t spring back to its original length. This is the flat part of the torque diagram. Go too far, of course, and the bolt breaks. For a hundred years, head bolts were used like any other fastener, in the elastic range. They could be torqued, untorqued and re-used indefinitely. For the last twenty years, however, the switch to torque-to-yield head bolts made them disposable parts. TTY bolts aren’t cheaper than conventional fasteners, so why did OEM’s switch?
There are several reasons. A major one is the move to light alloy block castings which have become thinner and thinner with each engine redesign. Distortion due to uneven clamp loads is a real hazard if heads are installed carelessly. Another reason is increased bolt length. Modern OHC head designs are taller, and bolts have to reach down below thin block decks and grab more substantial metal lower in the crankcase. Longer bolts act like torsion bars, with lots of spring well below proper clamp loading. Even angle torquing may not give repeatable head clamping forces with long, thin bolts. Another issue is clamp force distribution. The bolt pockets available for washers are getting smaller and smaller as multi-valve heads are attached to shorter engines. With less surface area to spread the clamp loads, consistency is crucial. Cylinder spacing is also decreasing, requiring high-tech gaskets and special surface finishes to keep combustion pressure from blowing into adjacent cylinders and water jackets. It’s amazing that head gaskets last as long as they do.
With this difficult sealing environment, why torque-to-yield when elastic bolts are a proven technology? Put simply, clamp forces are much more consistent when bolts are angle torqued to the yield point. Why? Because the clamp force remains consistent over a broad range of tightening angles and torques once the bolt begins to stretch. Get in to the yield zone and consistent clamping is almost assured, whether in your bay or on the assembly line. Can you get as good with conventional bolts? Yes, but under special conditions. This means perfectly clean and dry threads on the bolt and in the block, smooth, flat washers and bolt pockets on the head, plus the right amount of the specified lubricant on threads and washers. If you’re meticulous, it can be done … but then there’s the retorque issue.
Retorque or not?
All gaskets compression set, which means they get thinner when clamped down, and stay thinner when the clamping force is relaxed as the engine heat cycles. On older engine designs, this compression set is addressed the old-fashioned way: retorque the head bolts. Done properly, the new gasket is thermally cycled numerous times before the retorque procedure, which requires the same care as the original job. And it’s obvious that once a TTY bolt is stretched, retorquing it will stretch it even more, possibly to the point where it breaks. The solution comes from the gasket industry. Aftermarket gaskets that are designed to eliminate retorquing (and often work with compromised deck and head surface finishes) eliminate this final resistance to TTY technology. And that’s a good thing for the bottom line too, since consumers expect the price of a head job to include any post-service checks. Retorquing is a bay clogging cash flow killing procedure that’s not necessary with modern engines. If the price is throwing away a set of head bolts that might or might not give you more service, it’s cheap insurance.