Speaker, teacher and race engine builder Joe Mondello says that paying attention to how you bolt an engine together goes a long way to ensuring reliable performance–and conventional–rebuilds.
Often, he says, engine failures are mistakenly blamed on bolts that snapped because they have been pushed beyond their torque limits. “Most rod failures are because they are under-torqued, not over-torqued. I’ve tightened rod bolts to 95 ft.-lbs. and haven’t broken one yet.” Most of the time, says Mondello, rod bolt breakage occurs when a bolt has not been tightened sufficiently, works its way loose, and then hammers itself to pieces.
“Torquing is still something that a lot of people don’t understand. They know it’s a number you tighten the bolt to so that the bolt will work,” but little else, says Mondello.
“Torquing is overcoming friction,” he adds. “When you overcome friction, then your bolt will start to stretch to yield. All bolts are made to have a maximum clamping force. Where you have problems is that with today’s sophisticated fasteners and sophisticated gaskets, you have to have good, straight machine work.
“Table, fixture, and cutter head all have to be parallel to each other, not shimmed with beer cans and shim stock and toilet paper and newspaper, but dead level.
“I’ve been to a lot of machine shops and I see you guys all giggling, but you guys all know that there’s a lot of stuff out there.”
Mondello says that he likes to cycle bolts as many as six times to ensure that they will retain the correct clamping force.
“I cycle all of the fasteners four to six times. A lot of guys who write articles will tell you that if you tighten and loosen them that many times they’re really not good any more but that’s not true. It’s only when the heat is applied to it in the engine that the fastener starts to weaken slightly. You can torque to yield and torque to stretch five or six times.”
He says that often it is the fastener that takes the rap for engine failures when the blame belongs elsewhere. This can lead to incorrect decisions about what is going to fix the problem. One of these is to go to a bigger head bolt.
“Going to a bigger bolt does not mean you’re going to have more clamping force. Usually you get less clamping force because the amount of energy needed to stretch that bigger bolt is less. Between a 7/16-bolt and a 9/16-bolt, you have more clamping force with a 7/16. So if you’re saying you’re going to go to a bigger bolt to eliminate your head gasket problems, you’re going to be replacing a lot of gaskets.
Usually (the cause is) too much timing, too much spark plug, the deck’s not flat, or the bolts weren’t fastened. If you get into a whole lot of valve spring tension, that really multiplies the problem because you’re trying to jack the head right off the cylinder head gasket.”
Mondello says that simply attending to the details can prevent the majority of engine failures. Using good quality fasteners, paying attention to surface finish, assembly techniques and honing and machining procedures, and proper assembly procedures will reduce failures to negligible levels, says Mondello.
“Most stuff is common sense, the process of elimination.”