Fire-rings, Head Studs, & Screw-in Freeze Plugs - Diesel World

Solving the 6.7L Cummins’ Head Gasket Issues—For Good

The 6.7L Cummins is hard to beat in the diesel world. The common-rail inline-six features the longest stroke, the biggest bore, and the most displacement in the segment. Its ability to spool an S400 has all but killed off the need to run compound turbo arrangements, and the stock injection system can support 550rwhp or more. Unfortunately, head gasket failure is a big problem in the 6.7L world—especially on trucks that spend their lives tuned and hooked to a trailer. We’ve seen some go 200,000 miles before lifting the head while others can do it in half that time. With a second-gen turbo swap, an S467, and aggressive tuning onboard, the 6.7L fourth-gen in this article pulled off the feat in just 114,000 miles.

Luckily, the owner plans to keep his Ram for the long-haul, and as such decided to do things right the first time. Instead of sending the head off for a simple pressure test and resurfacing, it was shipped to Scheid Diesel for a few additional reinforcement measures. And thanks to Scheid’s quick turnaround, a decked head cut for fire-rings and fitted with screw-in freeze plugs was returned to his doorstep in just four business days. Between the addition of fire-rings, ARP head studs, and screw-in freeze plugs, the head is about as bulletproofed as it gets for a 550rwhp truck. For a look at what the two-day job entailed, along with a glimpse into the hot re-torque process that’s required when running fire-rings, keep reading.

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Though the climactic drama of puking coolant from the degas bottle never occurred, the owner did notice the upper radiator hose was pressurized and stiff each morning before heading into work (a telltale sign of head gasket failure). When he could finally afford to have the truck down for a week, the head was pulled on a Saturday, shipped out to Scheid Diesel on Monday and back in his hands by Friday.

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While the head was in the care of Scheid Diesel—the shop that would resurface it but also machine it to accept fire-rings and screw-in style freeze plugs—we inspected the original head gasket. It ended up letting go between cylinder numbers 2 and 3. In its early stages of failure, no rising in coolant level or puking from the degas bottle had occurred yet, but the engine would overheat as if an air pocket was present in the cooling system.

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Readying the block to accept the resurfaced head, we took a right angle die grinder and a mild Scotch-Brite pad to the deck. With all of the original head gasket material removed, we double-checked the block for trueness using a straight edge and a feeler gauge. It checked out just fine (as most Cummins blocks usually do). Then all head bolt bores were hit with compressed air to ensure no fluid or debris was present (fluid or other contaminants can throw off your torque readings).

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You might find this trick helpful if you ever find yourself performing the same job. To save time when pulling the head and reinstalling it later, both the turbo and exhaust manifold were pulled away (and held away) from the head using a ratchet strap. An existing hole in the fender was employed as the passenger side hook point.

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For utmost sealing in applications that see excessive cylinder pressure, fire-rings have long been a popular upgrade. Cut at Scheid Diesel’s new machine shop facility in Terre Haute, Indiana, the fire-ring grooves measure 0.040-inches deep and measure 4.340-inches in diameter.

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The fire-rings supplied by Scheid are made from mild steel and measure 0.105-inches thick. When the head is torqued down, they crush, forming a tremendous seal around the combustion area of each cylinder. While some believe fire-rings should be reserved for competition-only applications, they’ve proven highly reliable in street-driven, high-mile applications—so long as they’re installed properly.

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In high-horsepower applications and sometimes due to age, the traditional, press-in freeze plugs can fall out of the factory head. When they do, coolant disappears and quickly mixes with engine oil. By machining all eleven internal freeze plug bores to accept threaded, screw-in style freeze plugs, Scheid rules out this notorious failure from ever occurring in the future.

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Regardless of when a 6.7L Cummins goes down due to a blown head gasket, in almost every case ARP head studs go back in when things go back together. Such was the case here, with ARP2000 material fasteners getting the call (PN 247-4202). Even though plans to push the truck into 700-rwhp territory lay ahead, ARP’s standard studs, in conjunction with the fire-ringed head, should have no problem coping with the added cylinder pressure that will come with that type of power goal.

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After the block’s deck had been cleaned up and wiped spotless, an OEM four-layer MLS head gasket from Cummins was set in place. However, in order to accommodate the steel fire-rings, the factory head gasket was void of its own fire-rings.

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Before the head could be lowered onto the block, each fire-ring was set in place within the head gasket. Various shops recommend gluing the fire-rings in place with some sort of adhesive so they don’t move when the head is installed. We didn’t opt to glue the fire rings in place, but we did take special care when dropping the head down.

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Before the head could be lowered onto the block, each fire-ring was set in place within the head gasket. Various shops recommend gluing the fire-rings in place with some sort of adhesive so they don’t move when the head is installed. We didn’t opt to glue the fire rings in place, but we did take special care when dropping the head down.

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For optimum accuracy when torqueing down the head, each stud’s threads were hit with a coating of ARP’s Ultra-Torque. Both sides of the included washers, along with the bottoms of each 12-point nut, were also bathed in ARP’s fastener assembly lubricant.

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Next, it was time to reattach the Steed Speed exhaust manifold to the head. New multilayer exhaust manifold gaskets were sourced directly from Cummins for a leak-free seal. Thanks to leaving the manifold attached to the engine’s S400 throughout the duration of the job, no T4 gasket was required.

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With the placement of the head on the block checked and then re-checked to confirm all was well, we began dropping the rest of the ARP head studs into place. It’s important to note that the six studs measuring 6.625-inches in length install on the exhaust manifold side (all other 20 studs are slightly shorter in length). Once each stud’s threads had been started into the block, we ran them in using an electric impact and a 5mm Hex socket, but were careful not to take them past the point of being snug.

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Following ARP’s torque sequence (and working from the inside-out), the head studs were torqued in three rounds. However, and due to the use of fire-rings, we deviated slightly on the torque specs listed for its ARP2000 studs. On the first round, we went with 50 ft-lb, while using 100 ft-lb for the second. On the third and final round, we took things to 150 ft-lb.

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Any time you break the seal on the 6.7L Cummins’ injection system, it behooves you to start with fresh O-rings and even feed tubes when necessary. This meant the external O-ring on the body of each injector had to be picked off, replaced, and eventually lubricated using fresh engine oil prior to installation.

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It goes without saying that the injector combustion seals (i.e. nozzle washers) should always be replaced any time the injectors are pulled. To finagle the seals all the way up the injector nozzle, we used the socket trick (a 9mm).

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We reused the factory injector feed tubes, but made sure to reseal them with the appropriate O-rings. Then they were installed and torqued to spec. When the injectors are upsized in the future, the owner plans to start with brand-new Bosch units.

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Upon making sure the injectors were spotlessly clean and after having lubed each unit’s body O-ring, they were reinstalled. The next step was tightening the injector hold down bolts to spec: 93 in-lb.

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To keep the valvetrain in perfect sync, each pushrod, valve bridge, and rocker was returned to the same spot it was pulled from a few days prior. Once the rockers were torqued it was time for a quick valve adjustment.

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Because valve lash adjustment has to be performed twice with fire-rings in the mix (before and after the hot re-torque process), they were left loose (ish) on the first go round. Later, during the final running of the valves and after re-torqueing the head studs, the intake valve lash was set at 0.010-inch (the middle ground between the 0.006-inch and 0.015-inch allowable variance) while the exhaust valve lash was adjusted to 0.026-inches.

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Continuing on with the reassembly, the intake grid plate, fuel rail, and high-pressure fuel lines were reinstalled next. After that, the existing Glacier Diesel Power intake elbow was bolted back in place above the grid heater.

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In order to clear the rearmost driver side head stud, the 6.7L Cummins rocker box has to be notched out. Once we’d tackled that, we installed a fresh O-ring in the rocker box groove and bolted it to the head.

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With everything buttoned up (and per Scheid Diesel’s instructions), the engine was fired up and allowed to idle until the thermostat opened. As you can imagine, this takes some time, but you don’t want the head gasket to be exposed to any amount of boost until the hot re-torque sequence has been completed. After the engine has seen its first heat cycle and been re-torqued, you can throw the book at it.

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Tearing back into the engine as quickly as possible upon shutting things down, the torque wrench (still set to 150 ft-lb) went back on each head stud nut. During the hot re-torque process, we were able to get anywhere from a quarter of a turn to a half-turn out of many of the fasteners. Then, and as previously mentioned, the valves were run once more, and the job was finally complete.

SOURCES

ARP
800.826.3045
arpdiesel.com

Scheid Diesel
800.669.1593
scheiddiesel.com