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LONG-HAUL DURABILITY, 700-HP CAPABILITY

CAPABILITY INSIDE FLEECE PERFORMANCE ENGINEERING’S NEW CUMMINS STREET HEAD

The trickle-down effect from the competition diesel segment has a unique way of impacting the rest of the industry. Although the parts and pieces on display at truck pulls and drag races are used to support obscene amounts of horsepower, they’re also engineered to stand up to extreme heat, pressure, and stress. After these components prove themselves in diesel’s harshest environments, it’s no wonder a lot of the same technology makes its way onto our daily drivers. With the cylinder heads they were building for their pulling and drag racing customers proving to be rock-solid reliable, the folks at Fleece Performance Engineering decided to bring the same proven components and techniques to the masses with the release of their new line of Cummins cylinder heads.

Coined its Freedom Series heads, two versions are available: Street or Performance. Each Street head is rated for 700 hp at the wheels, while the Performance version is good for 1,100 rwhp. This time, we’re walking you through the in-depth process that unfolds when you order a Street version for your ’07.5-present 6.7L Cummins. Using state-of-the-art equipment and processes, each unit is thoroughly inspected and deep-cleaned, and then it’s fitted with new valve guide liners, oversize valve seats, and fresh valve springs. After that, the factory weak links are permanently addressed and the head undergoes a series of validation tests before being allowed to leave the premises. Stay tuned for Part 2, where we’ll detail the extensive upgrades performed on Fleece’s Performance cylinder head.

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With a core program that brings in all kinds of different candidates, some heads show up looking like this—reason enough to justify why a thorough cleaning takes place before each unit can be properly evaluated. A core charge of $700 is incurred by the customer with the purchase of Fleece’s Street head (the head itself running $1,836.45), but is fully refunded once the unit they’ve sent back passes inspection.
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Once a core cylinder head has been stripped, cleaned, and qualified as reusable, the remanufacturing process begins. Here, three pallets of heads (approximately 30 units) are about to begin circulating around Freedom Racing Engines (the machine shop and engine building side of Fleece’s Indianapolis area operation), where they will either be transformed into a Street or Performance version of their former self.
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The day we sat in on the machining and assembly process, our guides were the head of Freedom Racing Engines’ Cummins cylinder head program, John Benshoof, and machinist and assembler, Brandon Smith. To initiate the Street head build, Smith placed the usable core head in Freedom’s Haas VF-4SS vertical machining center to be decked. During resurfacing, between 0.003 and 0.005-inches worth of material is removed—a number that varies depending on how untrue the head is to begin with.
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Next, the head is flipped over, affixed with a template, and machined to accept screw-in freeze plugs. The 11 steel internal, pressed-in freeze plugs installed at the factory (valve cover, exhaust manifold side) are notorious for blowing out and allowing coolant to mix with engine oil. This insurance measure is included in every head Fleece produces, not just the Street version.
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Once out of the Haas VF-4SS VMC, the Cummins head’s cast-in valve guides are fitted with K-Line bronze valve guide liners—liners that are specifically approved by Cummins. A spiral pattern is present in each new valve guide liner in order to facilitate additional lubrication.
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Expanding the size of the valve guide liner during the broaching process, a hard-chromed precision tool is employed. After this step, the guide liner is effectively locked in place within its respective cast-iron valve guide. Later, the excess protrusion into the top side of the head is cut down, flush with the valve guide.
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With the valve guides reconditioned (guaranteeing valve seat-to-guide concentricity) and the head machined to accept thread-in freeze plugs, the valve seats become the next area of focus. Prone to cracking over time, the valve seats have long been one of the most potentially catastrophic issues present in all factory Cummins cylinder heads.
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To ensure the valve seats never drop out of the head, Fleece installs oversize versions. The oversize valve seats are also heat-treated to resist cracking, are radius’d on the head mating side, and are pressed in deeper than the factory seats.
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Aboard Fleece’s Newen CNC valve seat machine, the valve seat counterbores are treated to a 5-angle valve job. Using a precision, single-point cutting tool, each counterbore takes just 50 seconds to complete.
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Here, John Benshoof verifies the correct valve depth has been achieved. Though the Newen CNC machine he’s using is dead accurate (for utmost quality control), Benshoof still makes it a point to check each counterbore right after it’s been machined. While proper valve sealing is highly important in any engine, it’s especially true for a diesel that sees considerable boost pressure.
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Once the machine work is complete, all 26 of the head’s freeze plugs are installed. The 11 freeze plug bores that were machined earlier to accept threaded replacements are now filled with pipe plugs. The threads of each thick steel plug are treated to a coat of pipe thread sealant before being snugged up and ultimately torqued to 100 ft-lbs.
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In addition to both 1.5-inch diameter intake shelf freeze plugs being installed at this time (shown), so are the remaining 1-inch freeze plugs. Each traditional freeze plug is hit with Loctite retaining compound before being pressed into place.
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At both ends of the head, Fleece installs a 235-to-240- degree coolant temperature indicator. Should the head ever be resold or sent back to Fleece, these indicators can help clue the recipient in as to whether or not it’s been exposed to excessive heat in the past. Placing a temperature indicator near the No. 6 cylinder can be especially telling because of this cylinder typically seeing the most heat.
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Before the valve springs, retainers, and locks are installed, the head is subjected to a leak test. To perform the test, a steel test plate with a rubber liner on the fire deck side is fastened to the head via the use of fasteners in the head bolt holes.
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With a coolant pressure sensor threaded in along with a custom-made adapter that sits in place of the thermostat and accommodates an air hose attachment, the cooling system is checked for leaks using compressed air. From here, the head is sent to the paint booth for a fresh coat of Fleece’s signature black paint.
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In its Street head builds, Fleece regrinds and reuses the original valves (where possible). Here, you can see the stock exhaust valves from a high output version of a late model 6.7L Cummins. They’re made from Inconel, an alloy which has no problem surviving hundreds of thousands of miles in a high EGT environment.
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The valve springs employed in the Street head are stock pieces as well, but Fleece starts with brand-new springs straight from Cummins. The factory springs have approximately 80 pounds of seat pressure, which proves adequate for most moderately modified trucks and engines that see 40 psi of boost or less.
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After fresh valve seals and the aforementioned reground valves are in place in the head, installation of the new valve springs is next on the to-do list. Once each valve spring and its corresponding retainer is in place, a CNC-machined spring compressor and a 7/8-inch wrench cinches them down so the locks can be set in place.
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The factory 6.7L Cummins valve springs have proven capable of supporting considerable horsepower before valve float or valve creep becomes a problem, hence they’re use in the 6.7L Street head, rated for 700 rwhp. Beyond 700 hp, you’ll need to step up to one of Fleece’s fire-ringed Performance heads. We’ll go into detail on the Performance head in our second installment.
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As a final means of testing their work, Fleece performs a vacuum test on the head. This last verification process (and the fact that they vacuum test each valve individually) ensures valve seal is satisfactory and adds to the guarantee that the customer is getting a head that exceeds factory Cummins specifications.
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For that little something extra, Fleece even runs a tap into each exhaust manifold bolt bore to clean up the threads. Whether you’re taking delivery at your shop, a garage, or a private residence, this 100-percent, ready-to-bolt-on head is designed to minimize your downtime.
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An ID tag is applied to each Freedom Series cylinder head, not only to track its steps through the machine shop, but to track its future movements. With parts passed around so often in the diesel industry (especially performance components), should the owner of the head ever change, this ID system serves as a way for Fleece to give that person the exact specs on his or her head.
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Placed on top of Instapak and in a specially-built wood crate, each Freedom Series head tips the scales at a hefty 208 pounds. In addition to the heavy-duty crate helping with return shipping of their core, Fleece has established a partnership with Fastenal to provide an affordable, convenient freight service for residential customers, which runs approximately $100. Even in terms of the shipping logistics, Fleece has thought of everything surrounding its new line of heads.
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Born from the competition side of the diesel world and encouraged by its own drive-in service department, Fleece aims to provide high-quality, direct-bolt-on and ready-to-go, off-the-shelf cylinder heads for ’98.5-present Cummins owners (yes, VP44 trucks will soon be supported, too). Both its Street and Performance heads are currently available through big name distributors such as XDP, Premier Performance, Meyer Distributing, and Turn 14, as well as directly through Fleece.

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