During my presentation on the GM 8L90 transmission at Expo, I asked the group, by a show of hands, who had the special service tools that GM requires for checking clutch pack clearances. Only two or three raised their hand out of a room of 120+ rebuilders. In my presentation, I showed the group the GM clutch pack clearance procedure for the C2, C3, C4, and C5 clutches. It involves placing a specific force on the clutch, between 30 and 80lbs (depending on the clutch), with a load gauge (DT- 50903), then zeroing a dial indicator with the load applied (figure 1). Next, the rebuilder removes the load and uses air pressure through a special tool (DT- 50905) to stroke the piston using 50psi of air pressure. With the piston fully stroked, the rebuilder reapplies the load with the load gauge and uses the value on the dial indicator as the measured clearance (figure 2).
Knowing that this process requires quite a bit more time and additional tools, I was curious to find out how much these measurements would vary when using traditional methods of checking clutch clearances versus the published method using special tools and equipment. For this experiment, the “traditional method” for checking clutch clearance involved placing a slight pressure on the clutch assembly to remove any irregularities in the clutch pack (figure 3). At this point, I zeroed the dial indicator. Then the clutch pack was lifted with hand force or air pressure to determine clutch clearance. This tried-and-true method has served builders for many years.
So how does this compare to the “involved” method described in the service information for the 8L90? More importantly, could a builder use the traditional method for checking clutch clearance on the 8L90, or is the process laid out in the service manual essential to achieve proper clutch clearance? I tested the C3, C4, and C5 clutches for this experiment. The C3 was chosen (figures 1, 2, and 4) because there is no wave plate or clutch cushioning component within the clutch pack. I chose the C4 and C5 clutch (figures 3 and 5) because they utilize wave plates for clutch cushioning.
I performed these tests with a set of good “used” clutches that have no visible wear and no signs of overheating. Then, I repeated the tests on a new set of clutches and steels purchased from the dealer. First, I tested the new set dry, then after soaking the clutches in transmission fluid for 15 minutes. I repeated each of these tests numerous times to ensure consistency.
Refer to tables 1, 2 and 3 for the results. The following bullet points are descriptions for each column:
- The column labeled “clutch plate deflection by hand” indicates the amount of dial indicator movement from light hand pressure on the clutch assembly, showing the amount of “flex” in the clutch pack when pushing down on it slightly.
- The column labeled “clutch plate deflection with specified load” displays the dial indicator movement that occurred when the load gauge was applied to the released clutch, published as step one in the clutch clearance process. The load gauge establishes a specific amount of pre-compression on the clutch pack. The intent is for a more accurate and repeatable measurement.
- The column labeled “hand only clearance measurement” displays the measured clutch clearance from a “traditional method” of applying a slight hand pressure on the clutch pack, zeroing the dial indicator, then pulling up on the clutch assembly.
- The column labeled “air only clearance measurement” is the same as the “hand only,” except instead of pulling up on the clutch assembly by hand, I used 50 psi of air pressure to stroke the piston and obtained the measurement.A regulator provided the air pressure through a special tool that solidly applied the clutch assembly. Using 50 psi provided a consistent application of the clutch pack and was the same amount as the service manual procedure using the load gauge.
- The column labeled “load gauge clearance measurement” indicates the measured clutch pack clearance as outlined in the service manual with the load gauge applying force and 50 psi of air pressure stroking the piston.
- Lastly, the column labeled “fixture deflection” indicates the amount of change in the dial indicator measurement after the piston stroked under air pressure and after the load was reapplied.
- This process seemed unnecessary because why reapply a load to a pressurized and compressed clutch assembly? I presume GM has the builder reapply the load to consider any deflection in the testing apparatus, which might alter the measurement due to dial indicator clamp locations and fixture flex. The thought process is that if you are zeroing the dial indicator under load while the clutch is released, you should also be measuring the final clutch movement under that same load. As you will see in tables one through three, it did make a slight difference in some situations. Without reapplying the load, the overall clearance measurement would be 0.005” higher than the actual clutch clearance in a couple of cases. Meaning, the reapplication of the load, after the piston stroked, reduced the measured clutch clearance as much as 0.005.”
FINDINGS:
Only the C5 clutch exhibited a significant difference when testing with the load gauge versus the traditional methods using either hand or air application. The C5 clutch clearance was as much as .008” less than the actual clearance using the traditional method when the frictions were soaked for 15 minutes before assembly. 
But overall, the traditional method of checking clutch clearance seems to work just fine. Ideally, the traditional measurement would be in the middle or slightly to the lower end of the clearance spec since the loaded measurements were either the same or somewhat higher than the traditional measurements. As shown in table 4, the selective snap rings are available in about 0.013” increments and are inexpensive (around $2.50 each). Therefore, it would be worth keeping various sizes on hand so you can manipulate the clutch clearance to the desired range.
Some might argue that all this measuring is overkill and unnecessary, but it’s important to remember that this transmission is a clutch-to-clutch unit and that for each shift, the TCM releases a clutch and applies a clutch. The timing is critical for a smooth shifting transmission. Therefore, the TCM expects each clutch volume to be within a specified amount. The clutch pack clearance determines the overall volume of fluid needed by each clutch assembly, and the clearance spread between maximum and minimum is as low as 0.019” for the C3 clutch equipped with four frictions. Most of the other clutch assemblies have a clearance spread of 0.026.” That’s still a tight target, and because of that, GM offers a variety of snap ring sizes to allow for adjustability. With all the shift quality issues the 8L90 transmission exhibits, it’s essential to give the unit a fighting chance by ensuring the clutch clearance is within spec.
There is also some misleading service information regarding the published procedure for checking the clutch clearances on the 8L90. Early publications and training packets instructed techs to place a load on the clutch with it released and then directly apply air pressure to stroke the piston (without instructing the builder to remove the tension on the load gauge). This process isn’t possible without removing the load gauge. If you don’t remove the load gauge, the clutch piston would potentially try to stroke the load gauge’s piston up to 0.100” of an inch, which would not happen because the gauge would max out. Later publications and revised service info state to remove the pressure on the load gauge, stroke the piston, the reapply the load.
MYTH BUSTING:
Many feel the purpose of applying a load during the clutch pack measurement is to compress and flatten the wave plate found in the clutch assembly. I’ve heard builders state that instead of using the load tools, they’ll just measure the clutch clearance without the wave installed and then subtract the thickness of the wave plate from the measurement. So, I experimented to see at what point the wave plate started flattering. The following experiment uses the C2 wave plate from the 8L90. The C2 wave’s “waviness” measured about 0.022”, and the plate was 0.106” thick. I took a flat surface plate and placed it in my arbor press. I then used a magnetic dial indicator attached to the surface plate and zeroed it on the surface next to the C2 wave plate (figure 6). Next, I used a 1-2-3-4 6L80 pressure plate (it matched the size of the wave plate perfectly) and placed it on top of the wave plate, and then I slid the assembly under the dial indicator. As seen in figure seven, the dial indicator displayed 0.041” (actually, it was 0.441” because it included the wave plate and the 6L80 plate), which was considered baseline with no wave compression.
I stacked several thick plates on top of the 6L80 plate to provide an even load from the arbor press. I measured the weight of these plates beforehand because they became part of the load. The combination of the two plates equaled 16.25lbs. Notice that even with the addition of 16lbs, the dial indicator didn’t change at all and still read 0.041” (figure 8). Now, with about 65lbs of additional load applied, the dial indicator still didn’t change at all and continued to read 0.041” (figure 9). 
I increased the force on the clutch pack until a significant change occurred, and as you can see in figure ten, 475lbs of force only flattened out the C2 wave plate about 0.004”.
The load applied to the clutch pack during measurements exists only to remove the slight waviness and irregularities that occur from the manufacturing process. Removing the wave and subtracting its thickness from the measured clearance will not result in an accurate measurement. On this C2 wave plate, even with 475lbs of force, the plate would still need to flatten an additional 0.018” before it would become flat.
CONCLUSION:
Many of the special tools for the 8L90 are on backorder and have expected shipping dates that are months out, but don’t let this deter you from rebuilding this unit. The traditional methods of checking clutch clearances are plenty accurate. The aftermarket is already making improvements and fixes for the valve body, and parts and kits are readily available from your supplier or dealer. Now that many of these vehicles are eight years old be prepared to start seeing them come into your shop!
