Bomb Proof Top-End for the 6.7L Cummins - Diesel World

Fire-Rings, Head Studs, and Valve Train Upgrades for the 6.7L Cummins

No matter the power plant, the natural design of the internal combustion engine—where the cylinder head(s) bolts to the block—lends itself to eventual head gasket failure. While not quite as frequent as the failures you see on other engines (the 6.0L Power Stroke or LLY Duramax, to name a few), blown head gasket scenarios are surprisingly common on the 6.7L Cummins. Some believe head gasket failure occurs more often with the 6.7L due to it building more cylinder pressure (i.e. torque) than the 5.9L mills did. Others contend that the 6.7L’s larger bore lessens the available sealing area between cylinders and water jackets when directly compared to the 5.9L.

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Bullet Proofing

No matter the cause, a head gasket job always warrants a look at “upgrading” things so the failure doesn’t occur again. When this ‘09 Dodge Ram 3500 came into Flynn’s Shop in Alexander, Illinois, with a pressurized cooling system, the owner knew he wanted to fix the problem permanently. For that, the head would take a trip over to Scheid Diesel to be resurfaced, fire-ringed, fitted with threaded freeze plugs, and equipped with performance valve springs. A set of Scheid’s Stage 1 push rods and ARP Custom Age 625+ head studs would finish off the parts list. Follow along for the ultimate tutorial on how to bulletproof the top-end of a 6.7L Cummins. DW

This is what the 6.7L Cummins cylinder head looked like after the folks at Scheid Diesel got their hands on it: resurfaced and cut for fire-rings. The fire-ring process is something Scheid Diesel has all but perfected over the years, and when the finished product gets installed, there are no maintenance concerns after the fact. This means that—contrary to the belief that fire-rings are for competition-only purposes—fire-rings are perfectly acceptable for daily drivers, tow rigs, and of course, high performance trucks if done correctly.

The fire ring grooves in the head are cut between 0.040-inch to 0.041-inches deep, while each ring encompasses a 4.340-inch inside diameter. As for the head gasket, a factory 6.7L MLS unit is used, albeit with the center rings removed (to make way for the fire rings).

Made from mild steel, the fire rings themselves are 0.105-inches thick. Once crushed between the head and block (and sitting in their respective grooves in the head,) a much stronger seal is employed to help contain excessive cylinder pressure.

Oversized valve seats were also pressed in while the head was at Scheid’s. The larger valve seats are installed so that, when the head heats up and expands, the seat won’t drop out (which is known to happen from time to time when the factory seats are still in place).

To accompany the oversized valve seats, the valve springs were also upgraded. A set of 110-pound springs sit in place of the stockers, and should be good for elevated boost and drive pressure levels, in addition to handling higher rpm.

As an added insurance measure, Scheid ditched the factory (pressed in) internal freeze plugs in favor of the thread-in style. At elevated power levels, the 11 internal freeze plugs within the top side of the head are all fair game for blowing out. And when one does, coolant disappears quickly, while mixing with engine oil at a rapid rate. The threaded freeze plug method all but rules out this possibility and makes sense to do anytime head removal if necessary.

For utmost clamping force, a set of ARP Custom Age 625+ head studs were chosen to keep the head glued to the block. The combination of the fire-rings and ARP’s top-of-the-line fasteners should be good for boost levels approaching 100 psi (should the owner ever choose to go there). Six of the 26 fasteners are marginally longer than the rest, which signifies they be installed on the exhaust side of the head (passenger side of the truck). Tensile strength with ARP’s Custom Age studs is rated at 260,000 psi (vs. 220,000 psi for the ARP 2000 units).

Addressing the final weak link in the valve train was the replacement of the factory push rods. These Stage 1, two-piece units from Scheid Diesel are made from 4130 chromoly and utilize a 0.080-inch wall thickness.

With the head back at Flynn’s Shop, technician Jake Bosie got started by cleaning up the block’s deck surface. It’s important to note that unless the head is warped considerably, checking the Cummins’ block for straightness typically isn’t a major concern.

Once all of the previous head gasket material was removed from the deck surface, Bosie shot compressed air into each head bolt hole to clear it of any oil or debris. From there, the block was wiped clean using brake cleaner and a specific shop towel that Bosie prefers. Then it was time to install the head gasket.

To work in conjunction with the fire-rings, a factory four-layer MLS 6.7L gasket is used. The only difference is that this head gasket has the center rings removed in order to accommodate the 0.105-inch thick fire rings.

With the head gasket aligned using the factory front and rear dowels in the block, Bosie set each fire-ring atop its respective cylinder. The alignment dowels in the block play a key role in the head’s reinstallation. Without them, the entire process is much more difficult and could easily be compromised.

You can never be too careful in a job like this. Taking care to ensure all passageways were free of any leftover oil or debris, Bosie shot compressed air into every orifice in the head once it was chained to the cherry picker.

Inch by inch, the 125-pound head was slowly lowered onto the block. With the fire rings resting freely inside the head gasket, it was important to set the head correctly on the first attempt (so as not to move the fire rings or damage the head gasket).

As an added guide for the head, two exhaust side ARP studs were inserted with minimal thread engagement. This way there were essentially four guides (two head studs, two alignment dowels) aiding the process of setting the head flat on top of the fire rings and gasket.

Once the head was positioned on the block, a telescoping inspection mirror and flashlight were used to confirm it was in fact sitting in the alignment dowels. Then all of the Custom Age 625+ head studs were installed.

First, all head studs were threaded in by hand, and then Bosie turned them a little more using a 0.25-inch ratchet. The key here is to hand-tighten, not torque them down. After each stud had bottomed out in its respective hole, the rest of the ARP installation hardware was unwrapped.

Prior to installing the washers and 12-point nuts, they were coated with the supplied ARP Ultra-Torque assembly lubricant. The ends of the head stud threads were also bathed in a little lubricant as well.

With all nuts installed, Bosie began the torque sequence. Working from the center out and torqueing in three separate steps, he got started with a 50 ft-lb. specification. The second round of torqueing down the head called for 100 ft-lbs. Then for the third and final sequence, ARP’s 150 ft-lb. spec was met. Bosie then went over each nut once more.

To free up the airflow side of things, an X-Intake Grid from BD Diesel was installed. The air inlet, which does away with the factory grid heater, is said to be 35-percent larger than stock.

Next, Bosie fitted the injectors with new O-rings and reinstalled them, the injector hold downs, injector feed tubes (which were matched with the same cylinder they fed prior to teardown), and the fuel rail. After that, the Stage 1 push rods from Scheid were installed, along with the rockers.

Then it was time to reattach the exhaust manifold to the head, complete with six new manifold gaskets. By only disconnecting the exhaust manifold where it bolts to the head (allowing the turbo and exhaust to support it) reconnecting it was a piece of cake.

Reinstalling the 6.7L’s factory rocker box calls for a slight modification once ARP head studs are in the mix. To clear a head stud’s nut at the back of the cylinder head, Bosie simply had to notch out a small area on the rocker box with a dye grinder.

One requirement that is absolutely necessary when settling fire-rings is the hot re-torque process. After the first heat cycle, all head studs need to be retightened to the 150 ft-lb. specification. It’s not uncommon for most nuts to require 0.25 to 0.5 of a turn. The hot
re-torque process is as follows: 1) Idle the truck up to operating temp (allowing the thermostat to open), 2) Shut the truck off and promptly tear down until all head stud nuts are accessible, 3) Re-torque all head studs to 150 ft-lb. Notice the intake rockers have to be removed to access the head studs during a hot re-torque, while the exhaust rockers don’t.

Before buttoning everything back up (this time for good), Bosie ran the valves. Full disclosure, the valves are run prior to initial start up, but it’s very important to run them again after the hot re-torque has been performed. Once Bosie adjusted the valve lash to 0.010-inch on the intake and 0.026-inch on the exhaust side, the truck was 100-percent ready to go.

Sources:
ARP Studs
800.826.3045
ARPDiesel.com

BD Diesel
800.887.5030
BDieselPerformance.com

Flynn’s Shop
217.478.3811

Scheid Diesel
800.669.1934
ScheidDiesel.com