Click for Coverage
Exclusive Content
Original Shows, Motorsports and Live Events
Try it free for 14 days
Due to the EU’s Global Data Protection Regulation, our website is currently unavailable to visitors from most European countries. We apologize for this inconvenience and encourage you to visit for the latest on new cars, car reviews and news, concept cars and auto show coverage, awards and much more.MOTORTREND.COM
  • JP Magazine
  • Dirt Sports + Off-Road
  • 4-Wheel & Off-Road
  • Four Wheeler

From Blown to Bulletproof: Permanent Solutions for the 6.4L Power Stroke’s Biggest Problems

Posted in How To: Engine on March 20, 2018
Share this

On paper, the 6.4L Power Stroke was a definite improvement over its problematic 6.0L predecessor when it debuted in ’08-model-year Ford Super Dutys. Larger head bolts helped better anchor the heads to the block; the failure-prone high-pressure oil injection system had been replaced with common-rail; and a compound turbo arrangement—a true gift from any original equipment manufacturer—resided in the valley. Sounds like a durable, cutting-edge marvel of an engine, right?

Out in the real world, things weren’t so peachy. Aside from a plethora of emissions system failures, aggressive programming often resulted in blown head gaskets, the newfangled fuel system proved highly intolerant of any contaminant, and turbo up-pipes were notorious for cracking. On top of that, nearly every truck produced came equipped with a radiator that would eventually spring a leak.

To showcase a 6.4L-powered truck suffering from nearly all of the ailments mentioned above, we trekked over to Flynn’s Shop in Alexander, Illinois. There we found a low-mile ’08 F-250 with blown head gaskets, failed up-pipes, a weeping radiator, and its cab up in the air. For a permanent solution to each problem, head studs were sourced from ARP, heavy-duty stainless steel replacement up-pipes came from BD Diesel Performance, and Mishimoto supplied one of its all-aluminum radiators. Follow along as the guys at Flynn’s walk us through the ins and outs of fixing the 6.4L’s factory shortcomings.

Although blown head gaskets aren’t as common on the 6.4L Power Stroke as they were with the 6.0L, it’s always a possibility when excessive cylinder pressure is at work. Such was the case for this 88,000-mile ’08 F-250. After tolerating the added stress from an aggressive programmer for thousands of miles, the truck was dropped off at Flynn’s Shop in Alexander, Illinois, with coolant boiling out of the degas bottle.
First things first, the cab went north and the folks at Flynn’s tore into the engine. After a full day of raising the cab, digging into the engine, and pulling the heads, we caught up with Flynn’s Shop technician Jake Bosie for the second day of the labor-intensive job. We soon learned how it pays to be meticulous when working on the high-dollar 6.4L Power Stroke.
If you venture this deep into a 6.4L, it behooves you to start with fresh lifters. After being dunked in fresh engine oil, Bosie easily installed the new hydraulic lifters thanks to their plastic guides. Once in place, the lifter guide bolts were hit with Loctite before being torqued to spec.
Knowing that the success of every head gasket job depends on quality machine work, the guys at Flynn’s rely on a distinguished local machine shop to Magnaflux and resurface their cylinder heads. Believe it or not, some novice mechanics still skip this vital step in the head gasket replacement process. Trust us, taking a die grinder to the heads and eye-balling them will not make them true—and it certainly won’t allow you to tell if they’re warped beyond repair. Most 6.4L heads require 0.004 to 0.005 inches of machining before they’re perfectly flat again.
Just as it is for a 6.0L Power Stroke, this is the ultimate solution for head gasket problems: ARP head studs. Made from ARP’s proprietary ARP 2000 material, its fasteners have a minimum tensile strength rating of 220,000 psi and are heat-treated prior to thread rolling for unmatched clamping force and optimum fatigue life. Heat-treated, chromoly steel 12-point nuts are also included, along with parallel-ground washers and ARP’s Ultra-Torque assembly lubricant.
In this comparison between a factory 6.4L head bolt and an ARP head stud, you can see how much longer the head stud is. Head studs allow for complete thread engagement in the block, not to mention that a head stud distributes clamping force throughout the length of the fastener. Even though the larger 16mm head bolt diameter employed on the 6.4L (versus 14 mm on the 6.0L) helped rule out a lot of the head gasket issues the 6.0L had, at the end of the day the 6.4L still utilizes torque to yield head bolts that will eventually stretch under elevated cylinder pressure.
Before any studs are threaded into the block, it’s vital to ensure that each head bolt bore is free of any oil or debris. Hitting each hole with a few shots of compressed air does the trick. To make certain the threads are properly prepped to accept head studs, a 16x1.75mm tap can also be run down until bottoming out in each bore.
Once Bosie had installed the cylinder head alignment dowels in the block, all head studs were threaded into their respective bores, handtight. As you might’ve noticed, the 6.4L is also similar to the 6.0L in that it utilizes just four head bolts per cylinder (10 bolts per bank), whereas competitor engines such as the Duramax and Cummins make use of six fasteners.
In recent years it’s been discovered that over-torqueing head studs on a 6.4L can lead to a cracked block—reason enough to follow ARP’s head stud installation instructions to the letter. The stud shown here, positioned near the lifter valley between the number 6 and 8 cylinders, is one area where the 6.4L block is prone to cracking. The block casting is just 0.174 inches thick here. Located next to two water jackets, once the block is fractured in this section of the crankcase it allows coolant to contaminate the engine oil.
Prior to installing the new Ford head gaskets, Bosie hits them with compressed air or gives them a quick wipe down to make 100 percent certain they’re free of any debris (namely oil or airborne contaminants). Then with the head gaskets in place on the block, the resurfaced heads are positioned over the studs and carefully lowered into place.
After methodically bathing the supplied ARP nuts and washers in Ultra-Torque assembly lubricant, they were placed over the head studs and run down, handtight. We’ll note that Bosie held each stud in place with an Allen wrench during this process (to keep the studs from turning any farther into the block).
Following ARP’s three-part torque sequence and beginning with the centermost head studs, Bosie got started by tightening each nut to 90 lb-ft. The second round called for a 180 lb-ft torque spec, and the third and final sequence required 275 lb-ft. With all 20 head studs torqued, the upper (M8) head bolts were snugged to 25 lb-ft.
Even though the heads were back on, Bosie was far from done addressing the 6.4L’s weak links. Next on his to-do list was the replacement of both rocker assemblies. Not only do the OEM rocker arms break under high rpm and big horsepower, but even under normal operation they tend to wear, prematurely, at the fulcrum ball (the ends wear down due to excessive friction). Hopefully a fresh set of rockers will buy the owner another 10 years of service.
Due to the fragile nature of the 6.4L’s high-pressure common-rail injection system, several key components should always be replaced after being removed from the engine. Such items include the high-pressure fuel lines, which Ford deems one-time-use parts. With these lines being located under the valve covers (injectors to fuel rails) and under the turbos (fuel rails to high-pressure fuel pump), it’s a laborious and potentially expensive fix if one of them springs a leak.
With the cab up in the air, it was an opportune time for Bosie to address the cracked factory turbo up-pipes before they had a chance of becoming an even bigger problem. Somewhere between the 80,000- to 150,000-mile mark, the up-pipes are infamous for developing drive pressure (i.e., exhaust) leaks in the bellows. When this happens, the driver notices a drop in boost pressure, a loss in power, and soot all over the firewall and transmission tunnel. With plenty of room to work, Bosie went ahead and scrapped the factory up-pipes in favor of BD Diesel’s 304 stainless steel up-pipes, which feature heavy-duty expansion bellows.
Bosie’s last item of business would entail installing a new radiator. The 6.4L is notorious for its leaking radiators, and the unit aboard this ’08 F-250 represents a textbook example of where they fail: right where the plastic end tanks are crimped onto the aluminum core. Some blame the 6.4L’s chronic radiator failures on chassis flex, while others see it as a quality control issue. Either way, it happens to nine out of every 10 trucks.
The factory radiator’s replacement was none other than the popular all-aluminum unit from Mishimoto. Key selling points behind this $945 piece are that the end tanks are TIG-welded to the core, it doubles the factory coolant capacity, and is a direct, drop-in replacement with a lifetime limited warranty.


Wilmington, DE 19803
BD Diesel Performance
Sumas, WA 98295
Flynn's Shop
Alexander, IL
Ventura, CA 93003

Connect With Us

Newsletter Sign Up

Subscribe to the Magazine

Browse Articles By Vehicle

See Results