I have been a transmission rebuilder for most of my automotive career. I enjoy it best because solving the problems these marvelous puzzles present brings me satisfaction. Fixing mechanical, hydraulic, and electrical challenges on the bench is a meticulous craft that all successful professional rebuilders have mastered. Yet, despite our mastery, build-related issues sneak up and catch us off guard.
The 6L80 and 6L90 units have quirks that keep us questioning the unit’s overall health when one lands on our bench. Routine repairs and preventative processes address common problems. However, one area of concern that recently came to the forefront is the gasket that seals passages between the pump stator shaft and the pump stator body. This sealing area can spark a lively conversation amongst the best builders in the country on how it should be handled. We will look at this sealing area closely and examine ways to ensure a failure does not occur.
WHAT’S THE BIG DEAL?
For decades, GM has used stator tubes for passages for oil to feed rotating shafts and drums. The 6L series transmission is no different. However, some changes were made to how the oil is routed. The 1-2-3-4, 3-5-R, and 4-5-6 clutches are fed through the stator assembly. Instead of having slotted holes drilled and machined into the actual stator shaft, the 6L series unit uses passages in the body castings to route oil from the valve body to ports in the stator (Figure 1). These ports are fed from the aluminum stator body casting to the steel stator shaft. The transition point, of course, must be sealed. A gasket is necessary since the two parts are made of dissimilar metals.
GM uses a basic paper gasket. It’s nothing fancy, but they claim that it is not serviceable. The OEM gasket comes with a new stator. ATRA addressed this concern in our seminar material in 2017. Sources from GM engineering indicate that the stator and stator body are assembled at the factory by applying a torque load that simulates engine input torque through the torque converter. The six bolts are then torqued while the load is still applied.
After hundreds of thousands of these units have been assembled in the aftermarket by builders worldwide, real-life practice indicates that duplicating this process is not necessary. All builders I have spoken with do not use a torque load when securing the stator to the stator body. A centering dowel pin is included in this assembly, effectively making loading unnecessary.
THE STATOR ASSEMBLY
Builders agree to disagree on various building practices when it comes to different sealing surfaces. The 6L series stator gasket is one of them. Since GM put out the notification that the stator gasket was considered not serviceable, some builders refuse to attempt to replace the gasket. They bench-test the assembly to verify it works.
After gathering information on this practice, I contacted sources that monitor comebacks related to stator gasket blowouts. Slightly less than 10% of all gaskets failed, either coming into the shop for the first rebuild or after placing them back in service. While this does not seem like much, you must ask, can my shop survive with a 10% comeback rate? As a builder, I would be embarrassed if I had a 5% comeback rate! So, ATRA recommends that you change this gasket on every rebuild.
THE PROBLEM
While this seems like a concise conversation with a simple solution, there have been issues with servicing this gasket. Several builders have experienced the stator bolts coming loose after changing this gasket. Driveability complaints range from slipping and flared shifts to no move and burnt clutches. Since the 6L series transmission is so common, we decided that we need to take a closer look at the parts being used and the assembly process.
THE PARTS
Replacing the stator gasket can be very routine; however, the process is critical, especially when using aftermarket gaskets. So, I contacted suppliers who could send me gaskets they had in stock. There are four variations of this gasket, including the OEM version (Figure 2). Although they may appear significantly different, all of them are fully capable of sealing critical fluid passages when handled correctly.
The securing bolts cannot be taken for granted they will work! If the base of the bolt is damaged, it will not maintain the correct contact to provide the needed clamping force to keep the gasket secured at that location. When the bolts are used to pull the stator and stator body pieces together, they can become overloaded and stretch. The stretching causes the metal in the bolt to fatigue and makes it prone to break. It also compromises the clamping force of the bolt even when adequately torqued. If the bolt is questionable, replace it!
THE PROCESS
First, if you have never removed the stator from the stator body, it is very easy and does not require a lot of force. In fact, if you remove the bolts prior to placing the assembly in the parts washer, the heated parts come apart with a few taps of a dead blow hammer. Otherwise, you can use a hydraulic or arbor press.
After reviewing the recommended assembly procedures from different resources, there was agreement on the following procedure:
- Install the included O-ring on the stator shaft. Be sure to lubricate the O-ring.
- Place the gasket on the stator using the alignment dowel to position it correctly.
- Align the stator with the pump stator housing again using the alignment dowel to position it correctly. Be careful not to damage the O-ring while positioning the stator.
- Use a press to fully seat the stator into the housing while again taking care not to damage the O-ring. Do not use the bolts to pull the stator tight to the pump stator housing!
- Apply a medium-strength thread locker to the bolt threads and torque them in a three-step sequence following a star pattern:
- First, torque all the bolts to 60 in-lbs
- Then go back and torque all the bolts to 80 in-lbs
- Finally, torque all the bolts to 120 in-lbs
- After an hour, go back and recheck the bolts to make sure that they are still torqued to 120 in-lbs.
The key areas to focus on during assembly are the necessity of using a press to install the stator, using a thread locker, and the entire torque sequence. The process gives a good outcome every time, regardless of which design gasket is used!
Never attempt to draw the stator down using the bolts. I did this using a speed handle and felt the uneven draw. Even though it seems harmless, the stator can (and does) get loaded and can become jammed in the stator body. If jamming occurs, the O-ring may be damaged, and the stator will not seat completely, causing the gasket not to seal.
THE RESULTS
Numerous remanufacturing companies use the procedure outlined in this article with reliable success. Even though their statistics give me the confidence to relay this information to the rebuilding public, as a technician, I like to prove things for myself!
I secured a stator assembly to see how well each gasket performed in a static test. I measured the thickness of all the varieties mentioned before and after installation to compare. I also noted the dimensions of the original OEM gasket under both conditions (Figure 3).
While all gaskets provided excellent sealing, the metallic core gasket raised concerns. Due to the thickness when compressed, it acts like a shim, reducing the unit endplay by about .014 inches. While this doesn’t seem like much, if it is not taken into account when making endplay adjustment calculations during unit disassembly, you may end up locking the unit up (no endplay); however, most of the time, you need to make up for added endplay due to stator and pump body machining processes needed for rebuild. Remember that we want a total unit endplay measurement of 0.004″ to 0.006″.
As transmissions progress through their life cycle, additional repair parts and procedures become necessary to return them to service. You can fall victim to inadvertent no-gos and comebacks if you are unaware of changes. ATRA is here to help keep you in the know so you can confidently deliver the goods to your customers!
A special thanks to Gerald Kosub of Kosub’s Transmissions for supplying the stator assembly and Transtec and Precision International for providing the gaskets and related technical information for this article!
