Project MCLB: 48RE Transmission Build With BD Diesel Performance

The concept behind Project MCLB Heavy Hauler is to build a reliable medium-power truck that is capable of doing everything we want. First and foremost, we must be able to tow heavy loads. Second, we want to be able to enjoy the truck and not worry about whether we can make it back home or not. Third, it must look good, be comfortable, and be fun to drive.

When building a truck, it is always a good idea to have an end goal in mind. Then you can work backward to figure out how to get there. This way, you know what major purchases you need to make and when you need to upgrade your weak links. Our Project MCLB Heavy Hauler is a 2006 Dodge Mega Cab powered by a 5.9L Cummins with a 48RE transmission behind it. The 48RE is known for its share of problems, so for help, we turned to BD Diesel Performance.

A very common failure on the 47 and 48RE transmissions is the overdrive direct clutch snap ring according to Jared Simmons of Diesel Dynamics

Weak Link

In our project truck, the weakest link is the 48RE transmission. The question is when does it need to be addressed? The truck has 138,000 miles on it and as far as we know, it is bone stock. Typically, when someone starts to upgrade their late-model truck, one of their first purchases would be a tuner. In our situation, a decent tuner (bumping the horsepower up to almost 500 to the rear wheels) would quickly lead to a broken transmission.

The factory 48RE transmission is barely able to hold the stock horsepower and just about doubling the power would certainly end its life in a timely manner. Since reliability is at the top of our list, the transmission is the first thing we are going to tackle.

Our Kickdown Band was still holding, but showing signs that it was about to fail.

To address the issue head-on, we called the guys at BD Diesel to discuss our options and figure out exactly what we needed. “Most factory transmissions fail from burned-up clutches or broken hard parts like a shaft or planetaries,” says Cam Rose technical CSR for BD Diesel Performance.

“A stock 48RE transmission typically lives somewhere between 100,000 and 200,000 miles, but I have seen them fail with as little as 30,000 miles; especially in the later trucks with more power. So, we pretty much recommend doing a 47 or 48RE transmission anytime you start increasing the horsepower,” explains Rose.

(Left) We went ahead and ordered a complete valve body instead of modifying our original. (Right) The BD pressure controller is extremely easy to install. It simply has two plugs for the transmission (one male, one female) and one plug for the knob. No power, ground or anything else needed.

Pump Up The Pressure

A complete transmission can be expensive, and there are some ways to strengthen it before a major upgrade is needed. “We have a pressure controller that is a good idea at any level, even on a stock transmission.  Especially if a guy has a mild upgrade, the pressure controller is a great idea. It is a simple plug-in product that bumps up line pressure and helps the clutches and bands hold better.

Increasing line pressure can really extend the life of the stock transmission and it helps performance transmissions as well. It is a good investment at any time. The same goes for the valvebody. Because the valvebody will firm up shifting and raise line pressure, it helps to preserve the transmission. Given you can replace the valvebody without removing the transmission; it’s very sensible and a fairly cost-effective upgrade,” Rose says.

There is only so much room within a transmission. In order to an extra clutches, the pressure plates are machined down.

What To Upgrade And When?

Many upgrade options exist for every component and it can be hard to know what you actually need to upgrade and what you don’t. Often, there are even multiple options to choose from when upgrading. BD Diesel Performance’s kits offer two different types of clutch material and many upgrades inside of the transmission.

Both materials are much better than factory clutch materials, meaning they are more durable. The difference comes into play when you are looking at building a full race truck. The BD Diesel Performance racing clutches have such a high friction coefficient, that the gears engage aggressively. The non-racing clutch material engages a little easier. Both will hold and work well. The racing clutches are intended for the 1,000-horsepower rigs and trucks that live on the track.

The upgraded billet shafts have the same splines as a factory shaft. What makes them stronger is the material, the heat treatment, and the profile of the shafts.

When it comes to the rest of the internals, Rose says “The actual guts of our transmission: the pump modifications, extra clutches, upgraded planetaries, and so forth, are the same for all of our transmissions.” According to Rose, once you upgrade, you don’t have to worry about whether you are going to burn up a clutch.

There is a caveat to this, once the clutches engage and hold the power, another issue can arise. Depending on the power level, the truck shafts can break. One of the best things about a diesel engine is the huge amount of torque it produces. Torque, however, is a twisting force and it has a way of breaking things.

“I use the following rule of thumb when it comes to upgrading shafts: 450 horsepower requires a billet input shaft, 550 horsepower needs a billet output shaft, and with 650 horsepower, you better do all three; input, output, and intermediate. For anything at 700 horsepower and above, you want to further upgrade to the larger billet input shaft. The larger input shaft is called a fat input shaft and there is a special torque converter that goes with that, to accept the larger shaft,” continues Rose.


With a sound idea of exactly what we need for a transmission, the only thing left to consider was the torque converter. Torque converters are a really interesting part. Their job is to slip at idle and then transfer 100 percent of the power from the engine to the transmission when accelerating. All of this is done while there are no physically connected parts between the transmission and the flywheel.

A torque converter is basically a hydraulic pump. The outer shell and stator move at engine RPM. The stator’s job is to take the fluid inside of the torque converter and direct it over to the front cover/pump. The front cover then slings the fluid over to the turbine blades, which are mounted on the transmission’s input shaft.

This action is what creates the power that drives the transmission and moves the vehicle. When in higher gears, most torque converters can “lock up”, bypassing this hydraulic side of the torque converter and directly connecting the outer shell to the transmission’s input shaft via a set of lock-up clutches.

48RE transmission

Stall Speed

“Dodge converters have a stall speed of approximately 2,100 rpm, which is really too loose for the Cummins. This is why they tend to slip a lot off the line. We bring that down a few ways. We use a CNC billet stator and furnace braze all the fins [veins] on the converter. Brazing the fins has a few benefits. They won’t chatter around and they last longer. In addition, it fills all the gaps around the veins so you get less fluid leaking through them. Of course, lost fluid is lost power and it creates more heat,” explains Rose. “With these modifications, we bring the stall speed down to around 1,800 and this makes the truck much more responsive. We do have extra-low stall speeds and high-stall speed options but, these are really only for racing.”

What We Ordered

Part # Description
1060618 Build-It Stage 4 Kit
1030423 Valve Body ’05-‘07
1600115 Billet Input Shaft
1600117 Billet Intermediate Shaft
1041210 Flexplate
1071217X Triple Torque Converter
1030348 Pressure Controller

The other key aspect is torque converter lock-up. The big three offer lock-up torque converters as standard equipment from the factory because of the better fuel economy. In the aftermarket, 99 percent of torque converters you encounter will also be lock-up. The only real exception is a few racing converters.

BD Diesel is one of the only companies to offer two versions of the lock-up. There is a single-disk lock-up and a triple-disk lock-up. “The single disc is easily capable of handling 450 horsepower and honestly, we have run them in with rigs much higher than that. But really, once you move beyond that, and start making serious power and torque, we recommend the triple disk,” explains Rose.

There may be a lot of parts in this picture, but keep in mind that they are broken up into smaller sub-sets. Each clutch pack has its own set of parts. Each clutch pack uses basically the same parts just different sizes.


Given our build plans, our 48RE transmission needed everything. Triple disk converter, full billet shafts, upgraded internals, valvebody, and pressure controller. There are really three ways to get this done. We could buy a complete transmission, we can have a local transmission shop rebuild our transmission with an aftermarket rebuild kit, or we can buy the rebuild kit and do the work ourselves. There are pros and cons to each of these. Buying a complete transmission will cost the most, but it will work right, will have a good warranty, and the truck will only be down for a day or so, while someone pulls the old unit out and sticks the new one in.

48RE transmission

If you decide to pay a local transmission shop to build the transmission, they can use many of the same parts and the downtime may increase by a day or so. This option does let you pick what parts go in. The transmission should work just fine and you will have a warranty as well. Generally, the local shop’s warranty will be a little less than a manufacturer’s warranty, but still, you get a warranty. The cost of this could be slightly less than a complete unit.

The third option, and one we really hope our skilled DIY’ers consider, is to build the transmission yourself. The 47 and 48RE transmissions are actually very straightforward units. If you have never built a transmission before, this is a good one to start with. This isn’t one of those transmissions where you have to worry about putting in an O-ring backward or anything odd.

There is a huge savings in doing the transmission yourself. The reason is that a large portion of the cost of rebuilding a transmission comes from labor. So, by doing it yourself, you can save a good amount of money (Could be a way to possibly splurge for the billet shafts). The major downside is that you are the one responsible for the warranty.




(Top) Direct Drum. (Bottom Left) Direct Clutches, (Bottom Right) Forward Clutches. Our transmission showed little sign of any potential issue, so far.

Our Choice

We opted to have our local performance shop (Diesel Dynamics in Joshua, Texas) rebuild our 48RE transmission with everything in BD’s stage-4 transmission, including a billet input shaft and intermediate shaft, as well as a triple disk converter. (We plan on upgrading the output shaft later.) We chose this option for two reasons. The first reason was downtime and the second reason was the warranty. Not only do they offer a warranty on all of their work, but they were more than happy to walk us through the process.

48RE transmission

While this tool is a specialty tool for disassembling a transmission, all it is actually doing is depressing a set of springs. A couple of well-placed C-clamps could do the same job.

“For anyone wanting to do this themselves, I recommend buying the ATSG [Automatic Transmission Service Group] rebuild book,” says Jared Simmons, of Diesel Dynamics. “Really, if you can rebuild your transmission, you can save a ton of money and the 48RE transmission is easy,” Simmons continues.

Removal And Tear Down

Our 48RE transmission had 138,000 miles on it, and even though the magnet in the pan had a lot of metal on it, it appears there isn’t much wear. (Bad transmissions would have a ton of stuff on the magnet.) The valvebody comes out as one unit. So, only some of the bolts get removed. If we were installing a shift kit, the other bolts would be removed, but we have a complete valve body so we won’t need to go into it. Next, the pump has a threaded hold, so you can screw in a bolt and work the pump out. It may feel pressed in, but it is just a good fit and will come out with a little persuasion.

48RE transmission

The first major assembly to come out of the transmission is the direct drum. The direct or front clutches are located inside the drum and it is mounted against the Forward drum. The forward drum, yes you guessed it, has the forward clutch assembly. In our transmission, everything looked ok.

To finish the direct drive assembly, Simmons uses a specialty tool to depress the piston springs down, so he could remove the snap ring. At home, this can be accomplished with two C-clamps.

Once the final assemblies were removed, the transmission case was almost ready for a good cleaning. Just the two servos and the intermediate shaft support had to be removed.

The next major group of parts to be removed were the shell, rear planetaries, reverse band, and reverse drum. With the main internals removed, Simmons knocked out the intermediate shaft support, and removed the kickdown and reverse servos. This emptied the main case for cleaning, so Simmons picked up the extension housing to start disassembly. The first set of clutches to be removed was the overdrive clutches. These showed some signs of overheating and wear.

48RE transmission

But upon removing the overdrive clutches, Simmons pointed out a very common issue. The snap ring that holds our overdrive direct clutches in place was actually broken. When Simmons took the overdrive assembly over to the press to remove, he noticed that a few of the pieces were pushing out. In fact, the snap ring was in four pieces. He believes we only had a few thousand miles until failure.

48RE transmission

(Top) The entire overdrive assembly comes out in one big section. (Bottom) The snap ring that holds in the overdrive direct clutches was broken in 4 pieces. If we had done a tuner first, this would have failed even faster.

To compress the spring inside the overdrive hub, Simmons uses a press. Once the pressure has been removed from the snap ring, it can be removed. If you don’t have a press at home, you can take this assembly to most shops and they can do this for you. This is really the only specialty tool needed to rebuild this 48RE transmission.


Just like you never assemble an engine without greasing up the bearing, you should never assemble a transmission without soaking the frictions.

48RE transmission

(Top) Simmons just cut a square out of a coolant container and filled it with ATF+4 fluid to soak the frictions. (Bottom Left) When assembling a transmission, make sure you use transmission lube and not bearing grease. Simmons uses Dr. Tranny lubegard. The difference is, transmission lube will dissolve once the tranny has been started. Bearing grease won’t and it will prevent the transmission from working properly. (Bottom Right) Here is what the direct drive piston looks like disassembled. These nine springs are what need to be compressed to install the snap ring.

Simmons starts reassembling our 48RE transmission by installing a new bearing into the direct drum and then installing the supplied piston plate.

Next, the direct clutches go in. Anytime you are installing a clutch assembly, one steel (metal disks) goes in, then a friction, then a steel, and so on until you are finished. One thing Simmons recommends is to keep all of your old clutch assemblies together. This way, you can tell which steels go where. Sometimes there is a slight difference in thickness between the direct clutch steels and the forward steels. If you get them backward, the tolerances won’t be right.

The forward clutch pack is the next to be assembled. This clutch pack has the same OD and ID as the direct clutch pack. So, it is easy to get the steels mixed up. Even a seasoned pro like Simmons has to look twice at the steels to ensure they were right. Once assembled, the clutch packs should have the same tolerance.

With the 48RE transmission case cleaned and painted, the components are reassembled in the order they came out. So, the Intermediate shaft support, then the reverse drum, band, rear planetaries, shell, and forward planetaries were installed.

Next, the direct drum and forward drum assembly with the new billet input shaft goes in. This ended up being a two-person job. One turned the intermediate shaft as the other turned the input shaft and wiggled the drum assemblies to get the friction teeth to line up with the direct hub.

With the drum assemblies in place, the kickdown band was installed and the pump body went on to finish the front case assembly.

48RE transmission

Simmons turned his attention to the valvebody. He pulled the electronics off the old body and installed them on the new BD Diesel valvebody. After installing the servos, he installed the valvebody.

48RE transmission

Another key part that we opted to upgrade was to a larger aluminum transmission oil pan. These are great for reducing transmission temperatures. Not only do they hold more fluid, but they are made of thick aluminum, which helps to remove more heat from the fluid.

Simmons then completed the overdrive-direct-clutch assembly and took the entire assembly over to the press to install a new snap ring.

48RE transmission

With the snap ring in place, the entire assembly was slid back into the extension housing and the overdrive clutches were installed. Then, the two sections were reconnected to make one complete 48RE transmission.

The last component of the 48RE transmission was the new triple-disk torque converter. When you are installing one of these, make sure you hear/feel it fully seat. Turning and gently wiggling it will usually align it to the “second” seat. It will click into place as it moves back. Then it is truly in.

48RE transmission

Anytime a torque converter is installed, be sure to fill it with transmission fluid beforehand.

One issue that Simmons says he regularly sees on the Cummins engines is broken factory flywheels. He says that your money is well spent to buy a good aftermarket one that won’t break. We went with BD Diesel’s SFI billet flywheel.

Once the flywheel was installed, it was just a matter of bolting the 48RE transmission back in place and tightening everything. As you tighten the transmission bellhousing bolts, make sure the torque converter is able to spin freely and is not bound. If it stops spinning, this is most likely caused by the torque converter not being fully seated. Tightening the bolts more will not seat the torque converter, but may crack the bell housing.

48RE transmission

The final task was to install the pressure controller. It was simply a matter of connecting the two wire connectors in line with the transmission throttle valve actuator (TTVA) motor. We mounted the module under the hood and ran the control switch to the dash.

Final Thoughts

While this isn’t a factory rebuild kit, everything does fit together like a factory kit, and all of the tolerances were well within spec the first time. It should probably be noted that BD’s tolerances are actually tighter than Dodge’s specs. We should also mention that some of the wave snap rings are replaced with flat snap rings.

Between ramping up the transmission pressures and removing some of the cushioning, this transmission does shift a little firmer than a factory transmission, but daily driving this truck is pretty much the same as with a factory transmission. Once we turn the dial on the pressure controller over to race, the shifts are much more firm. When we are at the track, everything just feels right and the shifts are exactly what we want.

Stay tuned and keep checking back as we take this Project to the next level!

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About the author

Chad Westfall

With diesel running through his veins from childhood, Chad has more than a decade of experience in the automotive industry. From editorial work to wrenching, there isn’t much he hasn't conquered head-on. When he’s not writing and shooting trucks and tech, you’ll find him in the shop working on turning the ideas floating around in his head into reality.
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