Without question, the 6.4L Power Stroke is one of the most rewarding engines to modify in the diesel segment. As far as entry-level mods go, it’s hard to beat what the ’08-10 Fords are capable of right out of the box. Simply add a tune and you’re making 550-plus horsepower and running low 13s at the track. It’s no wonder why the last Navistar-built Power Stroke—a common-rail, compound-turbocharged V8—became so popular with enthusiasts. But now that these engines have been out for a decade, many of them having been tuned for the majority of that time, how exactly are they holding up? Much like you’ll find on their 6.0L predecessors, the bottom ends are pretty stout—it’s what’s bolted on up top that you have to worry about.


From inconvenient to catastrophic, cracked up-pipes, broken rocker arms and even blown head gaskets can all plague tuned 6.4Ls, while leaking radiators, plugged oil coolers, and failed injectors and high-pressure fuel pumps aren’t exactly infrequent. After an ’08 F-350 rolled into Flynn’s Shop in Alexander, Illinois, for new up-pipes, the folks there immediately discovered something the truck’s owner hadn’t: blown head gaskets. With the cab hoisted skyward, it was an opportune time to have the heads inspected and resurfaced, head studs added, and several other failure-prone areas addressed. With upgrades ranging from necessary to “you probably should,” this article details what can be done to make a tuned 6.4L hold up for the long haul.

After noticing chalky white residue all over the degas bottle, the guys at Flynn’s Shop pulled the cab and tackled the head gasket job first. Once the heads were removed it was evident that the driver-side gasket had failed between cylinder number 6 and 8 (shown). While head gasket failure is much less common with the 6.4L than it was on the 6.0L (thanks in large part to its use of 16mm diameter head bolts), lifting a head is never out of the question with a 300hp tune in the mix.
At the machine shop the cylinder heads were magnafluxed (inspected for cracks) prior to being resurfaced. After they checked out crack-free, 0.004 inches of material had to be removed to true them up.
With the cylinder heads on their way to the machine shop, the block’s deck surface was cleaned up via a die grinder fitted with a mild grit disc. Per the advice of Flynn’s Shop, the trick here is to focus on removing the remnants of the factory head gaskets and not on removing material from the block.
Any time the guys at Flynn’s dig this deep into a 6.4L they like to start with a fresh set of lifters. The new hydraulic units from Ford (PN 8C3Z-6C329-B) were coated in engine oil and installed four at a time, per the plastic guides that come with them.
Once each head bolt bore had been cleared out with a shot of compressed air, an ARP head stud was threaded into place in the block and left hand-tight. ARP’s head studs (PN 250-4203) are made from proprietary ARP2000 material, which is considerably stronger than high-strength 8740 chromemoly, a material known to suffice in most racing applications. ARP2000 studs are also capable of achieving a clamp load of 220,000 psi. To make a long story short, these fasteners are overkill for the 6.4L application—and that’s OK with us.
Thanks to four thin sections in the block casting near the lifter valley (4.5mm), the head stud torqueing sequence is a fairly delicate process on a 6.4L. If you over-torque the studs in these areas (located next to water jackets) you can crack the block. And if you crack a block it’s game over—we’re talking big bucks for a new short block from Ford. Needless to say, a lot of shops have their own trade-secret methods of installing head studs in a manner that is the safest possible for block longevity, and Flynn’s Shop is one of them.
Along with replacing the leaking factory up-pipes with BD Diesel’s stainless steel units a set of the company’s exhaust manifolds also made the cut. The BD manifolds come coated to resist corrosion, feature heavy walled elbow connections to prevent them from burning out, and a pre-drilled port in the driver-side unit to accommodate an EGT probe. BD also includes a new low-pressure A/C line with an offset adapter to seamlessly integrate the passenger-side manifold.
With all 20 studs threaded into the block (10 per bank), the cylinder head alignment dowels were reinstalled. Then the guys at Flynn’s checked to make sure the new Ford head gaskets (PN 8C3Z-6051-B) were free of any debris and set them over the head studs and onto the block.
Before the driver-side head was installed it was fitted with a fresh Mahle exhaust manifold gasket (PN MS19312) and one of the aforementioned BD exhaust manifolds. However, due to limited real estate, the passenger-side manifold had to be attached to its corresponding cylinder head separately.
Making sure the heads were perfectly aligned with the dowels in the block, a helping hand was used to lower each head into place. After that, the 12-point chromoly nuts and parallel-ground washers supplied by ARP were hit with a liberal coating of Ultra-Torque assembly lubricant and then installed.
With all washers in place and each nut hand-tightened onto its head stud ARP’s three-step torque sequence could begin. Once an initial pass of 90 ft-lb had been made, things ramped up to 180 ft-lb for the second round. Then for the third and final sequence each nut was torqued to 275 ft-lb.
Even though this job entailed the installation of new lifters and rocker assemblies, out of sheer habit the guys at Flynn’s placed each original pushrod in the same location it was pulled from during teardown. On jobs where the lifters and rockers aren’t being replaced this tactic helps keep the valve train in perfect sync and all points of wear the same.
On high-mileage engines the ball ends that ride in the rocker bridges are a major point of wear. It pays big dividends to thoroughly inspect each rocker any time you or your mechanic ventures this far into your 6.4L. Remember that in a worst-case scenario a failed rocker can wipe out your camshaft. In high-rpm, big-horsepower applications—where the rocker arm bodies have been known to break—many enthusiasts take the extra step of getting them cryogenically treated.
Being proactive, the truck’s owner elected to start with a fresh set of glow plugs as well (PN 8C3Z-12A342-A). Although the 6.4L isn’t known for eating glow plugs, with 100,000 miles on the clock this $80 expense is a worthwhile preventative maintenance measure.
With rocker failure being a fairly widespread occurrence on both higher horsepower and higher mileage engines, it pays to replace them before they become a problem. Here you can see a brand-new rocker arm and pedestal assembly (PN 8C3Z-6564-D) going on the passenger-side head. All rocker assembly bolts were eventually torqued to 45 ft-lb.
Moving on to the injection system, the factory O-rings and nozzle sealing washers were removed from the injectors and a complete inspection was performed before the injectors, rails and new high-pressure lines were installed. In general, Flynn’s techs looked for any signs of contamination (specifically from water). Any rust or metal particles found in the fuel drained from a 6.4L’s fuel system components warrants more in-depth inspection and possibly even complete replacement of the injectors and high-pressure fuel pump, not to mention the dropping of the fuel tank to flush it clean.
Prior to being reinstalled in the heads each injector’s O-ring was bathed in a light layer of fresh engine oil. Then each injector was installed in its respective bore, but the hold-downs were left loose. On a 6.4L, there is a specific tightening sequence applied to the injection system—and the injector hold-downs aren’t the first item to be torqued down.
Because Ford specifies that its high-pressure fuel lines are one-time-use items, all lines were replaced. Here you can see that the new high-pressure fuel lines spanning from the injectors to the rail are in place and are receiving their first torqueing sequence of 18 in-lb.
Once the rocker boxes were reinstalled the fuel rails were positioned within them (note that the fuel rails actually bolt to the rocker arm assemblies). Also notice that the fuel rail and injectors have been capped off. With the 6.4L’s injection system being so intolerant of contaminants (and so expensive to replace) cleanliness is top priority when handling these components.
Next, the injector hold-downs were torqued to the required 28 ft-lb spec. After that, it was back to the fuel rail, where its mounting bolts were snugged to 23 ft-lb.
For the second-to-last step in the injector, high-pressure fuel line and fuel rail installation process, the high-pressure lines were torqued to 106 in-lb. Then the final tightening sequence called for a 120-degree turn of each high-pressure fuel line fitting.
Due to the tight clearance that exists between the high-pressure fuel line that feeds the high-pressure fuel pump at the back of the driver-side head and the intake manifold, the intake manifold was installed first. With new intake manifold gaskets positioned on the heads so that their locating tabs faced upward (toward the center of the engine) and the factory intake manifold fresh out of the parts cleaner, it was reinstalled.
In this photo of the high-pressure fuel lines being torqued to spec at the high-pressure fuel pump you can see why replacing the HPFP is so laborious. It’s located at the back of the lifter valley and buried under the turbochargers and the HPFP cover.
A fair amount of care has to be taken when reinstalling the factory turbocharger arrangement. Being that the high-pressure turbo’s oil drain tube gets its seal via two O-rings, the assembly must be correctly positioned when lowering it into place (note that the turbo system incorporates a lifting bracket just for this occasion). Once the turbos had been set, the folks at Flynn’s checked their work with a mirror to ensure they hadn’t rolled one of the O-rings.
After two full days of work the cab was back down on the frame and the truck was ready to be primed and taken for a test drive. Thanks to the head studs, quality up-pipes, revamped valve train and freshly sealed injection system, a substantial amount of reliability has been built into this tuned 6.4L. It should be rock-solid for many years to come.
As we mentioned earlier, the truck’s original up-pipes (which had cracked at the bellows) were replaced as part of BD Diesel’s up-pipe and exhaust manifold kit. The larger diameter, 304 stainless steel, mandrel- bent up-pipes feature heavy-duty bellows that are said to hold up much better than the OEM expansion joints do.





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