Engine Parts Need Proper Torque Settings to Prevent Leaks, Breakage, Experts Say
Old habits are hard to break, but engine parts that are tightened beyond specifications can be easy to break.
For many engine components, there are clear guidelines for the right amount of clamping force — or torque — that must be applied. But maintenance managers and technicians must stay informed about these requirements on every piece of equipment they repair and resist the temptation to just tighten everything with a speedy impact wrench, service experts said.
“Keeping all fasteners properly torqued isn’t so much a recommendation as a requirement for very many reasons — safety, vehicle life and performance and uptime, to name a few,” said Doug Gunter, director of warranty for Peterbilt Trucks, which is based in Denton, Texas.
And it’s truer now than it was for earlier engines, added John Phennig, director of global development for Navistar International Corp., which is based in Lisle, Illinois.
“If you go back 25 years, you’ll see many external engine parts were steel castings,” he said. “Going forward from that time, standards changed and many parts, from brackets to front covers to valve and cam covers, began to be made of aluminum. Mixing very different metals meant that proper torquing became much more critical.”
He noted, for example, that technicians learned that care must be taken when matching parts made of different materials.
“Putting an aluminum part on an iron block and torquing the fasteners the way it had been done in the past meant that the bolt head could be pulled right through the cover,” Phennig said. “Later, composite parts were adopted. Even with metal inserts, the same kind of thing could happen.”
Materials used to make gaskets also have changed, adding another level of complexity to torquing.
“You may even have to apply a sealant before mating the parts, put them together, torque the bolts to the correct rating, and then, 30 minutes later, retorque them,” after the sealant has set up, he said.
“With new seal and gasket technologies used on today’s drive components, proper torque is critical not only to the life of the components. It is just as important to assure that leaks do not develop due to improper torque.”
While procedures such as these could seem tedious, failing to follow them can cause leaks that can lead to major engine trouble down the road, said Roy Walton, senior manager of technical training for Mack Trucks.
“Oil leaks from cam covers, oil pans and front covers [have] the potential to shorten engine life,” he said. “Properly torquing these components can help ensure the oil stays where it’s needed.”
In some cases, those needs are very specific. For example, Phennig said, proper torquing of fasteners that hold a replacement fuel injector in place on certain International MaxxForce engines is critical to ensuring that the supply pipe that carries fuel through the cylinder head to the injector remains in place. Proper installation will prevent leakage at the injector and at the connection point for the supply pipe. A leak in either place could allow fuel to mix with the engine oil, he said.
Torquing these parts is an eight-step process, during which bolts that hold the injector in place are alternately tightened in three stages. Nuts holding the pressure pipe in place are installed in two stages, Phennig said.
More complex are the instructions for torquing cylinder head bolts on MaxxForce 11 and MaxxForce 13 engines. Not only do all 26 head bolts need to be torqued in exact order, from one to 26 and one by one, they must be torqued in seven stages. The bolts also must be lightly oiled to ensure proper torque.
The first four stages involve four increasing amounts of torque, from hand-tight through 221 pound-feet. From there, each bolt — still in exact order — must be turned 90 degrees, or exactly one quarter turn, in three more passes through all the 26 bolts, for a total rotation of 270 degrees past the point where the final measured torque was achieved.
Those last three turns measure the rotation of the bolt rather than the torque, as a component of proper installation is bolt stretch. The stretch is more precisely measured in terms of rotation than torque.
Due in part to the precision of the installation process, head bolts tend to be single-use items, Phennig and others said.
“A lot of manufacturers use torque-to-yield bolts, which are designed for one-time use,” he said, noting that MaxxForce engines use them.
In fact, the notion of “retorquing” bolts — or retightening a bolt after its initial installation — is a thing of the past, said Carlton Bale, director of the Customer Support Organization for engine manufacturer Cummins Inc.
“In general, we design our engines to not require torque maintenance,” he said. “Torquing is done at the time of part installation and does not require periodic maintenance checks.”
The same is true for Detroit engines, the in-house brand for Daimler Trucks North America, said Brad Williamson, manager of engine and component marketing for the company that builds Freightliner and Western Star trucks.
“Detroit engine parts are calibrated to remain at the same torque for the life of the engine,” he said. “We have sophisticated quality measures in place during manufacturing that ensure the proper torquing sequence. A properly calibrated wrench is used, and the correct pattern is always followed.”
He added, “When the proper sequence and patterns are followed, unless an unforeseen event occurs, there should be no issues and there’s no need to routinely check torque.”
But that doesn’t mean technicians needn’t worry about it. In fact, understanding the nuances of torque application — both for technicians and their managers — is vital, experts said.
“The most critical step is to make sure technicians check the service manuals to make sure they use the correct specification and sequence for the repair they are conducting,” Cummins’ Bale said. “This check is necessary because the details vary by engine and can be revised over time. There is too much variation to remember all of the details for all repairs.”
“Trade schools are doing a good job of providing training in a classroom setting, but we also provide training at our dealer network,” said John Crichton, field service director for International Trucks. “We offer training that is Web-based to all our dealers. There are also three fixed facilities in the U.S.”
“Proper torque settings are critical and are covered in many of Mack’s instructor-led classes,” Walton said. “During classes such as engine overhaul, students are looking up torque specifications and demonstrating their skills in practical application.”
Volvo’s Flynn added, “Having the proper service procedures available to fleets, including proper torque specs, is critical.”
Also critical is keeping shop equipment in top shape, and experts agreed that torque wrenches, for example, should receive scheduled upkeep.
“Maintaining equipment is an important baseline, and many shops contract vendors to perform periodic calibrations,” Bale said.
Volvo’s Flynn added, “Technicians should receive a torque wrench calibration service biannually.”
Daimler’s Williamson added that shop managers should make this work a priority.
“Supervisors should put all of the shop’s torque wrenches on a regular maintenance schedule,” he said.
It is also critical for shop managers to periodically check in with technicians, Phennig said.
“Some of the most sophisticated shops will go through and audit each technician’s performance once or twice a year,” he said. “It can also help if a supervisor spot-checks by asking techs to describe the steps, or by watching a procedure being done, [and] have charts relating to service information blown up and posted where the technician can refer to it frequently.”
