Bigger and Better: Replacing the Transmission on your 7.3L Power Stroke with the Legendary 4R100

DieselSite Delivers

July 2017 Feature, Installs, PowerStroke Cayd Freeman

Everybody knows that when adding aftermarket parts to the engine and creating more horsepower and torque, a reliable transmission is a must, especially when you’re dealing with a 1999-2003 7.3L Power Stroke. Figuring out whether or not to replace your transmission can usually be pretty easy to tell. Some of the complaints include: shifting hard between certain gears, overheating, delayed engagement when towing, and that’s just a handful of many more. Another sign it’s time to upgrade is when you’re going to be increasing power on your 7.3L. Experts say that if you’re going to be doing racing or towing, anywhere around the 400whp mark is when to upgrade. The stock transmission is only designed to hold so much and almost 20 years of new technological advancements since these transmissions were first built can only help the decision of an upgrade. 

Where To Start 

Out with the old... with the new

Now, finding the right transmission can be a daunting process. (Hopefully not after this article!) When we started to put plans together for a recent build, we knew we were going to need the biggest and baddest transmission out there. Not only did we have grand designs of a high-horsepower, off-road-capable rig, but the stock transmission was giving us fits. Hell, it would barely manage going into reverse! After doing some studying and finding DieselSite’s Legendary 4R100 Transmission we couldn’t have been more positive about making the right decision. Dan Shoda, director of sales at DieselSite, gave us good insight about what kind of transmission he recommends with around 400whp. He made it easy on us and recommended we also put in the Legendary Billet Triple Disc Torque Converter along with the Mag-Hytec Deep Tranny Pan. Once we confirmed everything, the transmission showed up in less than 2 weeks. 


Taking the old transmission out wasn’t hard but did take some patience and a few indispensable tools. There wasn’t a lift available so we used the handy man jack, raised the truck’s front end and put it on jack stands. Using a transmission jack we were able to lower the stock transmission easily. The tranny wouldn’t come out from under the truck while on the jack so we had to raise the transmission by hand and slide the jack out from underneath. Sliding the transmission was just as easy as we placed a piece of cardboard underneath for ease and protection. Sometimes a low-tech solution is exactly what you need! 

After getting the old transmission off, the flex plate and bell housing adapter needed a good deep cleaning. 

Following the same steps as taking the stock transmission out we knew we couldn’t fit the transmission back underneath while on the jack. We carefully placed the new trans on the cardboard and slid it under the truck. We all got into position (as best as possible considering the cramped quarters) and raised the transmission onto the jack. Once in place, we put a tie down around it to keep it stable while we got it into position. From there, we moved the jack into place and lifted the 4R100 up into the waiting cavity and aligned it, bolted it in place, and set the cross braces.

Final Steps

Once the transmission was installed and everything in place, the transmission needed fluid. For this tranny Amsoil recommends their Torque Drive Synthetic ATF. Last but not least, with the truck having a Hydra Tuner from Power Hungry Performance and an engine from Swamps Performance, it needed some new tuning. It only took two tunes and a little bit of back and forth before this 7.3L was running to its true potential. 

Since then, we’ve put more than 2,000 miles on the transmission, both on interstate drives as well as up in the mountains on trails more suited to a Jeep or ATV. It’s safe to say that DieselSite has made believers out of Diesel Tech! By leveraging today’s technology to improve on a 15-year-old transmission, DieselSite can transform the transmission into something that’s more reliable and engineered to last. It’s also designed for the utmost in towing performance, which is good, because our sister magazine Dirt Toys has some equipment we’re dying to take for a spin. Finally, the 4R100 has vastly improved shift feel, thanks to a clutch material that handles extended and elevated temperatures without burning. Plus, the dang thing comes with a two-year unlimited mile warranty.

How did it perform? Watch the video, dummy!

So what makes the DieselSite transmission different? When you receive a completely built transmission you don’t see all the hard work and design that goes into them. All DieselSite transmissions come standard with direct and forward clutches built with 120 percent of the OEM clutch surface, a coast clutch upgraded with 200 percent of the OEM clutch surface area, a custom spiral snap ring, and a fully cut snap ring groove to hold up to exhaust brakes. There’s a whole host of other improvements DieselSite makes to the 4R100. Every single clutch pack is measured and set to exact clearances; every unit is within .003 inches. They have Torrington bearings throughout, a new output speed sensor, transmission speed sensor, transmission range sensor, and thermal bypass valve, as well as high-energy 850-degree frictions in all forward gears (including overdrive), which no 4R100 has ever had. It’s a DieselSite exclusive! Finally, there are full thickness steels for maximum heat transfer from clutches; a custom in-house built valve body for shorter shift duration which give firmer shifts and longer clutch life, and every remanufactured part is run through their de-burring machine. It’s safe to say we are driving stress-free down the road in our 7.3L with DieselSite’s Legendary 4R100 4WD heavy-duty transmission.

If you are interested in getting your own or have any questions, head on over to

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