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 fun to drive. Last, we need to have enough power that we can have some fun with it.

When building a truck, it is always a good idea to have an end goal in mind. Then you can work backwards 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 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 horsepower 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.

To address the issue head on, we called the guys at BD Diesel Performance 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,” said Cam Rose Technical CSR for BD Diesel Performance.

“A stock 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,” explained Rose.

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

(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

With how expensive a complete transmission can be, there are someways 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 for performance transmissions as well. So it is a good investment at any time. Same goes with the valve body. Because the valve body will firm up shifting and raise line pressure it helps to preserve the transmission. Given you can replace the valve body without removing the transmission; it’s very sensible and a fairly cost-effective upgrade,” Rose said.

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

What To Upgrade; When?

Transmissions are one of those assemblies that consist of numerous complex parts. 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 a component. Like with BD Diesel Performance’s kits, they offer two different types of clutch material and many upgrades inside of the transmission.

Both materials are much better than factory clutch material, 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. Where as 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.

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, of 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.

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.

“I use the following rule of thumb when it comes to upgrading shafts: 450 hp requires a billet input shaft, 550 hp billet output shaft, 650 hp you better do all three; input, output, intermediate. Anything at 700 hp 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,” continued 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 100 percent 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.

Stall Speed

“Dodge converters have a stall speed of approximately 2,100 rpms 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 the veins. Of course lost fluid is lost power and it creates more heat,” explained Rose.

We bring the stall speed down to around 1800 and this makes the truck much more responsive – Cam Rose

“With these modifications, we bring the stall speed down to around 1800 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,” continued Rose.

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% of torque converters you will encounter will, also, be lock-up. The only real exception is a few racing converters.

BD Diesel Performance 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 hp and honestly we’ve had  run 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,” explained Rose.

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


Given our build plans, we need everything. Triple disk converter, full billet shafts, upgraded internals, valve body, 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.

If you decide to pay a local transmission shop to build the transmission, they can use many of the same parts and the down time may increase by a day or so. This option does let you pick what manufacture 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 a warranty. The cost on 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 straight forward. 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 backwards or anything odd. It is straight forward and something we feel most of our readers can do. There is a huge savings in doing the transmission yourself. The reason is that a large portion of the cost in 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.

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. So, really, there are only about 12 sets of parts and that is identifying the torque converter, flywheel, oil pan, and valve body as separate parts..

Our Choice

We opted to have our local performance shop (Diesel Dynamics in Joshua, TX) rebuild our 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 down time 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.

“For anyone wanting to do this themselves, I recommend buying the ATSG [Automatic Transmission Service Group] rebuild book,” said Jared Simmons, Master Technician at Diesel Dynamics. “Really, if you can rebuild your transmission, you can save a ton of money and the 48RE’s are easy,” Simmons continued.

Removal And Tear Down

After getting our parts from BD Diesel Performance, we headed to Diesel Dynamics. Simmons likes to drain the transmission fluid before he even starts to pull the transmission. This helps to reduce the mess and can give him a decent idea of the condition the transmission is in.

With the fluid draining, Simmons made quick work of removing the driveshaft, transmission cross member, cooler lines, torque converter bolts and bell housing bolts to drop the transmission.

With the transmission on the bench, the torque converter was set off to the side and the extension housing of the transmission was pulled off.

Simmons then laid the transmission on its side and pulled the valve body off. Then he removed the pump.

The transmission had 138,000 miles on it. Even though the magnet has lots of metal on it, it appears there isn’t much wear. (Bad transmissions would have a ton of stuff on the magnet.) (Left) The valve body 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 on the bench, but we have a complete valve body so we won’t need to go into it. (Right) 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.

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.

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

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

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

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 servo and reverse servo.

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

This pretty much emptied out the main case. Simmons set the case off to the side for cleaning and picked up the extension housing.

The first set of clutches to be removed was the overdrive clutches. These showed some signs of overheating and wear.

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 4 pieces. He believes we only had a few thousand miles before failure.

(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 used a press. Once the pressure has been removed from the snap ring, it can be removed. Ours kind of fell out because of how many pieces it was in. 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 transmission.


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

(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 that transmission lube will basically 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 9 springs are what need to be compressed to install the snap ring.

Simmons starts the reassembly 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 like in this 48RE, there is a very slight difference in thickness between the direct clutch steels and the forward steels. If you get them backwards, 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 had to really look twice at the steels to ensure they were right. Once assembled, the clutch packs should have the same tolerance.

With the case cleaned and painted, the components are re-assembled 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.

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

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

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 transmission.

The last component on the 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.

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

One issue that Simmons says he regularly sees on the Cummins engines are 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 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 not bound. If it stops spinning, this is most likely caused from not getting the torque converter fully seated. Tightening the bolts more will not seat the torque converter, but may crack the bell housing.

The final task was to install the pressure controller. It was simply a matter of connecting to the two wires 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 are. 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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