Unlike the 5R110 TorqShift found in the trucks that superseded theirs, ’99-03 Super Duty owners don’t have the luxury of the stock automatic transmission—the 4R100—holding up to 500 or more horsepower. In fact, the factory slushbox can even fail at the stock power level if it’s subjected to heavy towing on a day-in, day-out basis. This makes beefing up the 4R100 inevitable for any ’99-03 7.3L Power Stroke owner seeking maximum reliability while towing or chasing horsepower.

Bob Riley, owner of DieselSite, has been in the 7.3L Power Stroke game since the beginning of the diesel performance revolution. He has an uncanny ability to produce high-quality components that solve problems, improve performance, and seem to last forever while doing it. In addition to producing the fi rst bolt-on electric fuel system for ’94.5-97 trucks, he’s also the man behind the renowned Adrenaline high-pressure oil pump, the high-fl owing Wicked Wheel 2 compressor wheels, and now offers the “Legendary” line of heavy-duty 4R100 and E4OD transmissions.
To eliminate the well-documented failure point in the 4R100’s coast clutch drum, DieselSite replaces the factory open-ended retaining ring with a custom, spiral snap-ring and a corresponding fully cut snap-ring groove. In addition, while the coast clutch is normally only used for deceleration (engine braking), DieselSite uses it for First through Third gears, which helps evenly distribute load across the transmission. Beyond the custom, spiral snap-ring, extra clutch plates are added in as well, making the coast clutch drum much more suited to accommodate an exhaust brake.
Using a proprietary material exclusive to DieselSite, the clutches used in the Legendary 4R100 are designed to handle elevated operating temperatures without burning.
Both the Overdrive and intermediate clutches encompass 133% of the OEM clutch surface area within the transmission. The coast clutch assembly offers a whopping 200% of the factory clutch surface area—twice the area and therefore considerably more holding power.
As for the forward and direct drums, DieselSite’s clutches provide 120% of the OEM clutch surface area for improved holding power. While 20 percent may not sound like a big increase, it’s quite substantial, especially when you factor in the fact that the valve body will be configured to quicken upshifts (i.e., limiting clutch slippage). To avoid stripped splines or cracked hubs, customers can opt to upgrade to DieselSite’s 4140 billet-steel forward clutch drum for higher horsepower applications.
Prior to being installed, all stack-up tolerances are measured for each clutch pack. Riley tells us that each clutch pack is measured and set to exact clearances, and that if the need arises, the pressure plates are machined once more until everything is within the required 0.003-inch specification.
Instead of treating its transmission cases to a fresh coat of paint, DieselSite bead blasts them. Along with removing any old dirt, grime, and debris, this fi nishing method is employed because it lasts much longer than any paint will.
Here you can see the Reverse gear and its respective clutches ready to be installed in the transmission case. We’ll also note that along with all other planetaries, the reverse planetary is made of steel, not aluminum.

With tens of thousands of 7.3L Power Strokes still out there on the road, built 4R100s remain in high demand. Unfortunately, only a handful of reputable 4R100 builders exist, and the wait time to get your hands on one can be a bit lengthy. With its heavy-duty line of Legendary 4R100 transmissions in stock and ready to go, DieselSite is looking to change that. Proclaimed by the company as being “engineered to last,” DieselSite’s version addresses a host of factory shortcomings, increases holding power, improves shifting, and utilizes custom billet internals where they’re needed most.

Also exclusive to the Legendary 4R100 is the use of high energy, 850-degree-rated frictions in all forward gears (including Overdrive), which are said to yield 20% less heat. For maximum heat transfer from the clutches, full thickness steels are utilized as well (shown).
Because the input shaft handles the brunt of the shock load from the engine, it’s always wise to upgrade this shaft in any performance-oriented transmission build. DieselSite offers 300M billet-steel shafts as upgrade options for all its Legendary transmissions. For those unfamiliar with 300M, it’s a form of through-hardened alloy that’s revered for its high fatigue strength and great ductility.
With plans to send big torque through our 4R100, the intermediate and output shafts were also upgraded to 300M units. A standard diameter 300M intermediate shaft—available as an option on DieselSite’s Legendary 4R100—is shown on the right, while a larger diameter 300M intermediate shaft can be seen sitting next to it. The larger intermediate shaft features deeper splines, calls for a larger center support hub (with dual bushings), and is reserved for DieselSite’s top-of-the-line, competition-ready Hercules 4R100.
As previously mentioned, all DieselSite transmissions (even the Legendary E40D it builds) come with steel planetaries for utmost strength. Along with being stronger, a steel planetary resists heat more effectively than aluminum (aluminum units are known to soften and even crack).
Precise machining of the center support tightens up its tolerances considerably. And to prevent any internal leakage from occurring, the center support is sealed to the transmission case itself. A spiral snap-ring is also utilized over the center support, along with a center support bearing.
Once the new and improved center support had been installed, the center support bolts were threaded in and torqued to spec. Torquing these bolts to the proper specification is crucial. If they back out, the result can be excessive heat, loss of Reverse, and even the loss of the transmission’s forward gears.
Not unlike the transmission case, the transmission pump can become warped over time (mostly due to heat and wear), so the transmission pump is resurfaced prior to being modified to provide increased line pressure. Then both halves of the pump are joined together and torqued to spec.
Using a host of custom parts, the folks at DieselSite build each valve body in-house and set them up for firmer shifts to increase clutch life. Simply put, the shorter the shift duration, the less wear there is on the clutches. We consider this the icing on the cake with DieselSite’s transmission, as it eliminates the sloppy shifts that factory 4R100s are notorious for.
After assembly, each valve body is tested on a vacuum test stand before being installed. This hydraulic pressure test ensures that no internal leakage occurs within the valve body.
In stock form, the 4R100 utilized an external cooler bypass tube, which redirected transmission fluid from the pump back to the transmission’s return line instead of routing it through the cooler. This was done to bring transmission fluid up to adequate operating temp sooner. The problem is that the bypass tube is prone to failure, which prevents flow to the transmission cooler, culminating in an overheating scenario. On top of that, even when operating perfectly, the bypass never fully closes—meaning full fluid flow to the transmission cooler never happens under any operating condition. DieselSite fixes this common problem with its transmission cooler bypass delete kit, which allows for removal of the bypass tube and includes a banjo bolt that’s void of a spring and check ball (the key restriction in fluid flow to the cooler).
An ID tag complete with a serial number signifies the final touch on each Legendary 4R100. Every unit DieselSite builds receives this badge of honor on the lower rear portion of the transmission case.
With the transmission case chock full of custom, proprietary, and upgraded components, the tail housing was fitted over the billet output shaft and bolted into place. Keep in mind that our transmission is a two-wheel-drive version—which has a different tail housing and output shaft than 4×4 units—so it may not look exactly like the one underneath your truck.
The job of transferring all power from the engine to the transmission is left in the hands of one of Dieselsite’s Legendary billet torque converters. For standard transmission builds, this converter is rated to handle up to 550 hp, but in our case (a performance application) we were given a converter that was fully splined and void of damping springs. Without the damping springs, this triple-disc unit can support 1,000 hp.
In addition to offering billet shaft upgrades, a Mag-Hytec high capacity transmission pan can also be included in a Legendary 4R100 build. These deep pans increase fluid capacity by 8 quarts (for added cooling ability), come with all new hardware (bolt kit), a magnetic drain plug, and an O-ring seal machined into the cover.
To verify that each transmission it builds is 100% ready to go, DieselSite tests all units on its state-of-the-art, in-house transmission dyno. Equipped with a BlueReach Automation data acquisition and control system, the dyno can duplicate real-world driving and load conditions, as well as upshifts and downshifts.
Now for the bottom line: What does a Legendary 4R100 cost? Surprisingly, pricing begins at $2,799 without a torque converter (or shipping costs) factored in. Adding a Legendary triple-disc converter rated for 550 hp to the equation only increases the price point by $700. At $3,499, this makes DieselSite’s Legendary 4R100 one of the most affordable high-quality tow performance automatics on the market today. For competition use, DieselSite offers a built-toorder Hercules 4R100 option right). This transmission is fitted with exotic components such as a billet forward planet assembly and forward clutch drum, Aermet billet input shaft, a larger diameter billet intermediate shaft and billet output shaft, and the aforementioned fully splined, dampless triple-disc torque converter rated for 1,000 hp.

We recently stopped by DieselSite’s Homosassa, Florida, facility to witness one of its Legendary 4R100s going together. This particular unit now resides in our very own 7.3L-powered ’00 F-250, which recently cleared 596 rwhp on the chassis dyno. The following build showcases the components and techniques DieselSite employed to turn our factory 4R100 into one that could handle heavy tow duty and as much as 700 hp.DW

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