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Chevy 2500HD Allison Transmission Rebuild

PPE “Stage 6” Allison Transmission Build – Part 2

When we first rolled our 2002 Chevy Silverado 2500HD into the bay at PPE Diesel's shop in Montclair, California, we thought for sure the transmission was toast. Interestingly, upon inspection it was actually in relatively good shape despite likely having more than 200,000 miles on a truck that was pushed hard enough to melt a piston and head. This just further reinforced the notion that with regular fluid and filter changes the Allison 1000 transmission should outlast the truck.

Still, we were planning to push a fair bit of power through it while punishing everything with big tires. So, the build moved forward. At first, we figured that a Stage 4 or Stage 5 build from PPE would be enough for our needs. PPE claims that the Stage 4 kit is good for holding about 850 hp, and the Stage 5 about 1,200 hp (measured at the crank, so roughly 650 and 800 hp at the wheel, respectively). Although either would have done the trick, the team at PPE sold us on the company's top of the line Stage 6, which can hold about 1,500 hp (about 1,000 to 1,200 at the wheel). Aside from regular fluid changes, we should never have to worry about the transmission again.

There are several different ways to get a PPE transmission. You can order the parts that you need individually, by complete kits, or as finished units ready to install. However, because there are so many different options and every truck's need is different, it's best to call and talk to the pros at PPE before buying anything.

The biggest difference between a PPE Stage 5 and Stage 6 Allison transmission lies in the clutch friction material. While Stage 5 and below use Alto's Red Eagle clutches, the Stage 6 swaps them out for Alto's more aggressive G3 clutches.

Along with better clutches and steels than factory—and more of them,—the Stage 6 build also includes a valve body recalibration kit (this doesn't change shift points, instead it increases clutch holding power), an upgraded converter flow valve, and all the necessary gaskets and hard parts needed to complete the job.

Even though we went down to PPE's shop to document what goes into building a Stage 6 Allison transmission, there are many different ways to get a PPE transmission. You can buy the parts all individually, as a kit to do yourself, or have a local shop build. Or, you can buy the complete transmission ready to install and have it shipped to your door. Because there are so many options, it's really best to call up PPE and talk through your build needs.

You'll also notice that we didn't address the valve body. As part of the build, we installed a six-speed conversion kit from Duramax Tuner, which we'll take a look at in a separate story (along with final drive impressions).

As always, this isn't meant to be an exhaustive guide to rebuilding an Allison transmission. There are special tools needed to complete the build and many tight tolerances that need to be adhered to. PPE has published good directions for the do-it-yourselfer, but we highly recommend leaving performance transmission building to the professionals. Enjoy!

When we left off last time, we had the Allison transmission almost completely disassembled. The final piece to disassemble, and the first part to be modified and put back together, was the pump housing.
Nothing changes inside the actual pump itself, however this is a great time to give it a full inspection. It's not uncommon for the pump to fail in glorious fashion.
Working with the stator, the regulator spring is removed, and a shim added.
For early transmissions like ours (2001-2003) the converter flow valve needs to be replaced. The original is on the right and the new is on the left.
Also replace are the torque converter relief valve springs. The factory springs are shown on the left, while the new ones are on the right.
It's hugely important to not mix up or misplace any of the pieces of the stator or oil pump. But, really, did that need to be said?
The final modification is the enlarging of one that is found on the stator adaptor plate. But don't worry, the drill bit is provided, and the instructions show exactly which hole to enlarge.
Using the input shaft as a guide, the oil pump, adapter plate, and stator can be reassembled.
The stator to pump bolts get torqued to 20 ft-lbs in a crisscross pattern.
The first of the new clutch packs to be installed were the C1 clutches and steels. Placed next to the factory clutches, you can see the difference in friction material between factory and the new Alto G3 clutches. The new C1 pack uses 16 single-sided clutches: eight with external splines and eight with internal splines.
After installing the bottom apply plate against the apply piston, the friction plates are installed beginning with an externally splined friction with the material facing down. The remainder of the clutch plates are installed, with the material down (friction material against steel), alternating internal and external splines.
The finished C1 clutch stack, with the top pressure plate attached, should look like this.
A spring clip is used to secure the clutches in the drum. It's crucial to ensure these clips are fully seated and care is taken not to launch them across the shop.
Pictured here is the new PPE Stage 6 C2 clutch pack (right) next to the factory clutches (left). If you haven't noticed by now, all five of the clutch packs are unique in some way, so installing them incorrectly is nearly impossible.
Once the C2 clutch hub has been installed in the drum, the C2 clutches can go in. Unlike the C1 clutches, these use a double-sided friction material with Koleen steel plates in between. In fact, the C1 clutches are the only single-side friction plates.
Moving back to the transmission case, assembly work begins from the backside (bellhousing facing down). After applying assembly lube to both the case and the C3 piston, the piston can be gently slid into place, noting the bleed hole that needs to be oriented at the 12 o'clock position.
After installing the P1 ring gear and planetary gear, the new apply plate can be installed on top of the piston. Note that the springs will be facing up.
On the bench here are the C4 clutches and steels. Factory are on the left, and the new PPE Stage 6 (Alto G3) are on the right.
Prior to installing any of the new clutch plates, they should be coated in a copious amount of good, clean, transmission fluid. Some builders like to simply dunk the new plates, while others choose to soak them for an amount of time. There appears to be no wrong answer other than putting them in dry.
If you're starting to see a theme here, it's that the clutches are placed into position alternating between friction and steel plates. The C4 pack is no different.
After the C4 clutches are in, several steps occur before the next clutch set is installed. These include installing the pressure plates along with the P2 and P3 gear sets.
If these clutches look a bit different than the others, and vaguely familiar, it's because they are the factory C5 clutches that were removed in the previous installment. While replacements are available for these, it's incredibly rare that they need to be (or even are) replaced for street trucks making less than 1,000 horsepower.
Once the C5 clutches are in, the final pressure plate and set of return springs are installed. At this time, the intermediate (or main) shaft is also installed. The intermediate shaft can be installed earlier in the build, but it isn't necessary.
With the P3 planetary gear and the output shaft splined together, the assembly can be lowered into place. After installing the output shaft assembly into the case, the shaft spacer and shim can be installed, along with smaller items such as the park pawl.
After installing the C5 piston in the extension housing, and after applying a new gasket, the extension housing can be bolted to the rear of the transmission housing. The extension housing only installs one way.
The extension housing bolts get torqued to spec in a crisscross pattern.
With the extension housing bolted into place, the transmission gets flipped over to complete the assembly process. Seen here is the P1 thrust spacer, lubed and ready for installation.
The last set of clutches to be installed are the C3s. The C3 clutch pack is comprised of six friction plates and five steel plates.
The C3 clutch pack gets lowered into the housing, alternating between steel and friction plates.
Note the difference in size between the factory (left) C3 pressure plate and the new one provided by PPE (right). This size difference helps to account for the additional friction and steel clutch plates that are added.
After the C3 clutch assembly is completed, the C1/C2 drum can be lowered into place.
Next, the pump assembly can be installed. First, place a new gasket on the transmission case, followed by the pump, and then another gasket. Then, gently lower the bellhousing into place.
Care needs to be taken in replacing the bellhousing bolts, as there are several different kinds that need to be returned to their specific holes. Some bolts have gaskets, some have washers, and there are a few different lengths.
The bellhousing bolts get torqued to spec in a, you guessed it, crisscross pattern. If you're working on a bench (not on a transmission assembly stand) you'll want a second set of hands to steady the transmission for this step.
This is what the power take-off (PTO) gear looks like with the cover removed. If someone wanted to add a PTO-driven accessory, such as a hydraulic pump for a snowplow, it would attach here. All Allison 1000 transmissions are equipped to accept PTO accessories.
An often-overlooked part of an Allison transmission build is the power take-off (PTO) cover. It might seem like an insignificant part, but installing a heavy-duty cast aluminum cover adds strength where it's needed in the center of the transmission case. It also helps prevent seepage from the gasket when the factory stamped steel unit contorts. The PTO cover bolts get torqued to 32 ft-lbs.
We reinstalled our PPE deep transmission pan and torqued the bolts to 20 ft-lbs. The PPE deep transmission pan adds an extra four quarts of fluid capacity. Also, of note, Allison transmissions have a reusable pan gasket.
Before installing the torque converter, it's best to fill it with a quart of transmission fluid. Starting the engine with the converter dry can cause damage in the time it takes for the transmission's pump to start loading it with fluid.
The torque converter needs to be installed in the transmission as square as possible to help prevent damage to the input shaft seal. It's also important to make sure that the converter is fully seated before installing the transmission in the truck. Each converter manufacturer will give a measurement of how deed the converter needs to be seated to be considered correct.
With a deep pan, an Allison transmission will hold between 18 and 24 quarts of fluid, typically. Don't start the engine until the transmission has at minimum 18 quarts (4.5 gallons) of fluid in it, and be sure to check the level frequently for the first few weeks as air bubbles get worked out of the system.
Before driving the truck with the fresh transmission, it's necessary to perform a transmission relearn. This process is initiated with a GM Tech II scanner (or equivalent scan tool). The process is needed so that the transmission controller can properly adapt to the new clutch pack thicknesses. As the Allison transmission wears, the controller compensates, and if left alone one errant shift can destroy a new build. In addition, it'll take several hundred miles of stop-and-go driving for the controller to fully adapt, so odd or rough shifts during this time are not unexpected.
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