Building A Better 48RE Transmission With Randy’s Transmissions

When the 48RE transmission was released in 2003, this four-speed automatic transmission was found behind the 5.9-liter Cummins Turbodiesel. The 48RE was considered a strong improvement over the 47RE with a torque rating of up to 570 lb-ft. That was then, but this is now.

Although the 48RE was an improvement in execution compared to its predecessors, it was not without issues. However, it can be built into a very capable transmission with just a few changes. And since the 48RE has been around for quite some time, there are plenty of upgrades available. In fact, we got to follow along as the guys at Randy’s Transmissions dug into one of these gearboxes to build one of their 48RE Stage-2 units.

48RE transmission

Randy’s Transmission’s Stage-2 48RE is not a simple replacement transmission. Quality parts are used to build each unit and make this a must-have upgrade.

According to Randy’s, the Stage-2 48RE transmission is the company’s most popular build for the 2003 through 2007 5.9-powered Cummins trucks. This upgraded unit is rated for up to 650 horsepower because it is built with heavy-duty components. It’s no secret that including nearly indestructible parts like a billet input shaft, upgraded clutches and bands, and a modified valve body make the 48RE much more durable and reliable. But what other heavy-duty parts are used?

48RE transmission

A reliable transmission is only as good as the parts used to assemble it. Inside, you’ll find Raybestos frictions throughout.

Inside The Upgraded 48RE Transmission

For starters, all new Raybestos frictions and new steels ensure precise clamping in each gear. And since high-horsepower output can twist an input shaft quickly, Randy’s uses a billet input shaft to stave off that concern. Another problem-prone area of the 48RE is the direct clutch drum. These are known to fail because of insufficient clutch capacity and a very weak cast-aluminum piston. When the direct drum fills with oil, this aluminum piston tends to crack.

Randy’s 48RE transmission alleviates this concern by developing and using what they call a Power-Packed Overdrive and Power-Packed Direct Drum that utilizes a custom pressure plate. I asked Randy’s to clarify and explain what this upgrade includes, but they said, “that’s proprietary information”. However, they were willing to tell me that it increases the clutch count and clamping ability

You will also find a TCS Super Servo, a billet band-strut, a billet wedge, and a billet accumulator. By changing out the light-duty OE cast aluminum parts and replacing them with these billet parts, durability is exponentially increased as they ensure longevity that the OE parts could never offer. Next up we’ll discuss the 48RE valve body.

While a stock 48RE utilizes many cast-aluminum parts internally, Randy’s upgrades them with better-than-OE billet parts. Take for instance the billet servo and Sonnax input shaft. Both are used to ensure you have a transmission that can take the abuse you will probably serve up.

Valving Your Shift

It is widely known the cast-aluminum valve bodies used in many automatic transmissions are plagued with a few problem areas that present issues when trying to build a transmission capable of working behind any engine that has an increase in horsepower. For instance, sufficient line pressure is paramount when talking about automatic transmissions.

What Is Line Pressure

Automatic transmissions use pressurized transmission fluid to control which clutches and bands are engaged at any given time in order to select the desired gear. The fluid is typically pressurized by a pump which is turned by the engine, while the Transmission Control Module sets the pressure at a predetermined level. The line pressure has to be high enough to prevent any unwanted slippage of bands when in gear. It also impacts the speed of the gear shifting. The higher the line pressure will result in a faster and harsher shift. Lower line pressure results in a smoother shift, as the internal clutches slip more during the engagement of the next gear.

Unfortunately, if the 48RE develops more than 170 psi. It will produce trouble codes. This happens because the governor pressure system sees too much pressure (above 170 psi) and cannot adjust as needed. The problem arises because this transmission uses control and return voltage with a transducer to help coordinate shift dynamics. When it sees too much governor pressure, the code is triggered.

Randy’s Baby Daddy valve body makes sure your transmission shifts and performs as it should. it eliminates torque converter slip or shudder, band and clutch failures, and provides short clean shifts.

Randy’s Stage-2 48RE comes with a custom Baby Daddy valve body that offers a more aggressive pressure curve and a maximum main line pressure set to 185 psi in all forward gears. This offers as much as a 75-percent increase in torque-holding capacity for all the clutches. Also, a proprietary valve body separator plate allows you to achieve torque converter lockup operation in all forward gears.

Torquing Up Randy’s 48RE Transmission

Since we mentioned the torque converter, let’s dig a little deeper into that piece of the puzzle. Inside Randy’s Stage-2 48RE is a triple-disc torque converter that uses a billet base to create a solid foundation, a Sonnax turbine hub that is TIG-welded to the turbine, and a welded sprag retainer inside the stator with upgraded bearings. All of this adds up to creating a robust torque convertor that stops warping and slipping from taking place over the factory spec/built converter. When a customer gets into stages 3 and 4, they’re automatically upgraded to the XHD Series Quad Disc converters and the input shaft increases to a 32 spline count.

“We rigorously test what we sell,” Randy’s affirms. “We do not bring a product to market without one of us beating the crap out of one of our built transmissions to test its absolute limits. We don’t go off what we project a transmission will hold based on data sheets; we back it up with real-world testing. Whether that means drag racing, towing, taking it off-road, putting it on a dyno.”

48RE transmission

A Few Final Things To Consider

If you’re on the fence about whether a Randy’s transmission is right for you, there is more to consider. “We will not leave the customer high and dry. Even after their warranty expires,” Randy’s states. “If they find their transmission needs attention and the warranty has since expired, we will offer them a refresh — rebuilding the transmission with new clutches, steels, and seals for a fraction of the cost of a new unit.”

On top of that, if a customer has a Randy’s transmission and decides to add more power to the truck, they can easily upgrade to the next stage for only the difference in cost. This is a great option, as you are not needing to purchase a whole new unit. This only applies to a customer who has a Randy’s built unit, and any upgrades or refreshes apply to the original purchaser and the truck the transmission was installed in.

Each transmission is hand-assembled in the USA and Randy’s stands behind every one of them.

Randy’s finished by saying, “we offer the 48RE in stages 1 through 4, and our Stage 4 is rated at 1000-plus horsepower. With stages 3 and 4, customers have the option of ordering with a full manual valve body. Our competition truck, Red Delicious, has our production Stage 4 with the full manual valve body and just laid down 2,778 horsepower. We also have proven builds for the 68RFE, and Aisin AS69RC transmissions as well as the Ford 5R, 6R, 10R, and the Allison 5, 6, and 10 speeds.”

If you ever wondered what goes into a replacement transmission, now you have an idea of what it takes to make a unit that will survive the abuse that just about all of us throw at our trucks every day.

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About the author

Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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