How NOT To Blow Your Gasket (Head Studs)

May 2011 News, Cummins

While experimenting with a Smarty PoD tuner on the LBMC recently, we found that by using custom parameters we could put a lot more power to the ground a lot faster than we were used to.

We dialed individual settings in that are tailored around the modifications we currently have on the truck. By turning the torque management off and turning rail pressure up, we saw turbo boost pressures spike quickly. With the throttle at one-quarter open, boost pressure pegs 60 psi (max on our gauge) and the duallys start scratching ruts in the pavement.

With boost levels getting a little aggressive, we decided that we had better replace the stock head bolts with a set of head studs before the bolts stretch and allow the head to lift from the engine block.

We went to the authority in engine fastener technology, Automotive Racing Products (ARP), for a set of head studs. If you want to read some cool stuff, check out their website and catalog at arp-bolts.com. These guys are crazy about the science of keeping stuff together. There's a ton of technical information about their fasteners and the gospel of clamping force.  

First of all, what's the difference between a head bolt and a head stud? Obviously, a bolt has a fixed head on top of it. A stud is a rod that's threaded on both ends. One end threads into the engine block. The gasket and head slide down over the protruding studs and on top of the engine block. The head is clamped into place with washers and nuts that thread onto the top of the studs.

Bolts work great on a stock OEM equipped engine. They're engineered and installed to provide the right amount of clamping force to keep the head and head gasket sealed to the engine block. However, if you're going to build more power and run higher engine pressures in the top of your engine, high performance head bolts or studs are needed to keep your head down when it's under pressures that exceed the OEM's master plan.

Head studs are the optimal choice over bolts because when the fixed head on the bolt comes into contact with the surface being clamped (the cylinder head in this case), it puts a binding force on the bolt. Not only is the bolt being stressed vertically, it's now getting pressure horizontally as it contacts the head. This twists the bolt and compromises physics involved with getting the perfect amount of clamping force on the head. When studs are used, the stud is threaded into the engine block by hand until it's seated at the bottom of the thread bore. When the washer and nut are tightened on the top of the stud, the nut is able to spin independent of the stud. This allows the stud to focus all of the tension vertically, which is best for uniform clamping.  

Studs and bolts are like springs. In order for a fastener to work the way it's designed, it has to be tightened the prescribed amount for that particular fastener. The bolt or stud actually stretches to achieve the right amount of clamping force, which is needed to hold the head to the block. For example, if 18,000 lbs. of clamping force is needed to hold the head down, you must torque the studs or bolts to 125 ft. lbs. This equation is dependent on the material being used for the fastener and the amount that the fastener will stretch under the pressure applied to it.

One of the most important things to remember when installing head bolts or studs is that it's crucial to torque the fasteners to the correct specification, in the correct order and pattern for that engine. If the fasteners aren't torqued enough, they won't keep the head down and you'll develop leaks. If they are torqued too much, they stretch more than they should, compromising the strength and integrity that has been engineered into the fastener, which can lead to a failure.  

Lubrication is key. The correct torque and precision is necessary to obtain even and uniform clamping force over the surface of the head. To get precise torque on the fasteners, we must eliminate or minimize other factors that can throw our torque measurement off. Friction, binding and thread galling will trick the torque wrench, despite its good intentions.

The factory uses motor oil as a lubricant. Moly (Molybdenum disulfide) has also been widely used as a lubricant, which is significantly better than motor oil. ARP came out with a new lubricant called ARP Ultra-Torque, which is even better than Moly for this application because it contains no metal (like Moly) and withstands temperatures up to 365 degrees F. It is proven to provide uniform load dispersion under torque.  

We chose ARP's strongest available studs for the LBMC. They're made out of a super-alloy called Custom Age 625+. They are completely resistant to atmospheric corrosion and oxidation and have a 260,000 psi tensile strength. We're also using the ARP Ultra-Torque lube, which means we'll be able to reach the right clamping force on our first installation and tightening of the studs, while normally, they should be installed and tightened several times to achieve the same clamping force.
1.      Remove the composite upper valve cover.
2.      Remove the valve cover gasket, carefully removing the wire nuts from the top of each injector and unplugging the wiring harness plugs from the outside edge of the gasket.
3.      Unbolt and remove the aluminum lower valve cover from the top of the head. This cover looks similar to the valve cover structure on the 24-valve engine (Second Generation).
4.      Unbolt and carefully remove the rocker arms from the top of the head. Take care not to disturb the tappets that bridge between the intake and exhaust valves. When removing any internal from an engine, lay the parts out so that you'll be able to assemble each component exactly in the position it was removed. This prevents creating new and abnormal wear patterns in the metal.
5.      There are six 6.26-inch studs (longer than the other 20) that need to be installed on the exhaust manifold side of the engine.

6.      Evenly coat the threads on each end of the studs as you install them one at a time. Be sure to get even and full coverage of the threads. Install the studs in the block so that they "bottom out" in the block. Hand tighten only. The real force comes when we torque them later.
7.      Because we have not disturbed the head gasket between the head and block, we will install the studs by removing one bolt at a time. After one bolt is removed, we immediately install a stud in its place and torque it to 150 ft. lbs. Bolts and studs should be installed in a circular pattern, beginning with the center bolt and moving in a clockwise circle, outward on the head. Check a repair manual for the specific sequence of your vehicle.
8.      After all 26 studs have been installed and torqued to 150 ft. lbs, start at the center of the head and re-torque them to 150, following the same sequence pattern. Gently click the torque wrench several times until you see that the nut will no longer move further down on the stud at that torque setting.
9.      All of the studs have been installed and torqued.
10.  Reassemble the engine in the reverse order that it was disassembled. Check the valve lash clearance after reassembling the rocker arms with a feeler gauge. Adjust valves if necessary.

11.  In order for the aluminum lower valve cover to seat completely over the rear center stud (behind the number 6 cylinder), part of the aluminum valve cover must be ground down with a die grinder and filed smooth. Be sure to thoroughly clean the cover with brake and parts cleaner so that no metal shavings or contaminant are present.
12.  If a new head gasket has been installed, drive the truck until normal operating temperature has been reached, then disassemble the valve covers and rocker arms and re-torque the studs to 150 ft. lbs. This is necessary to settle and compress the new valve gasket.

  

Sources:  

ARP

www.arp-bolts.com

800.826.3045 

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