Clutch and Release

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We’ve all been through the thrash to get the car ready for Saturday night. As often happens, basic maintenance morphed into a major overhaul with a new clutch. But somehow the hydraulic throw-out bearing assembly was overlooked, and now at the track, the clutch won’t release. The driver is frustrated, the car owner/ crew chief is upset, and nobody’s having a good time. Sound familiar?

The best parts will never cooperate come race day if the setup’s not correct. Small details like making the time to adjusting the hydraulic release bearing for the new clutch makes a big difference between a clutch working like it’s supposed to – or not doing its job. It’s a fine line we intend to walk right here to make it right.

Our setup is a typical budget-oriented package starting with one of Quartermaster’s affordable 7.25-inch diameter, 3-disc V-Drive clutch assemblies. Our previous unit could have been revived, but the cost associated with resuscitating the old parts compared with new parts didn’t justify the effort. We combined the new clutch with a Quartermaster 710-Series hydraulic release bearing.

We had multiple issues with our existing clutch system including a way-out-of-spec bellhousing that literally was so messed up due to a backyard engineering mid-plate installation that it left very little of the original dowel pin in the bellhousing. This elongated the dowel hole and allowed the bellhousing to move 0.075-inch out of alignment. When you consider that the spec for alignment is 0.006-inch, you can see we had a serious problem!

All this pointed out the necessity of setting up the clutch release system as accurately as possible by verifying all the details. We started by measuring our new Quartermaster bellhousing to ensure it was within spec. Then it was just a matter of following the directions to ensure the hydraulic release bearing had the proper clearance.

The plan is to create as close to 0.100-inch of clearance between the clutch fingers and the release bearing with the clutch engaged. This ensures the bearing will have a slight amount of free play yet still deliver sufficient stroke to release the clutch. If it sounds simple, that’s only because it is. The work involves accurately reading a dial caliper and some simple subtraction. If you can do that, you can set up a hydraulic clutch.

Making the time to blueprint proper clutch release will pay off with a system that will deliver hundreds of trouble-free laps and should also free up some time to fine-tune the car for quicker lap times. That’s how the fast guys get there.

Configuring a hydraulic release bearing isn’t difficult, but it does require some simple measurements and attention to detail. The payoff is a reliable clutch that works as intended, every time.

We decided to start with a fresh system which in this case is a steel Quartermaster bellhousing kit complete with a 7.25-inch V-Drive clutch and pressure plate system combined with a hydraulic release bearing and all the necessary hardware, including hydraulic fluid.

After removing the trans and old clutch, we noticed several warning signs of distress. Note the bluing on the input shaft that probably emanated from the clutch. We then discovered the pilot bushing was wasted and the original bellhousing was grossly out of spec. Starting with fresh parts was a good idea.

To begin, we installed the 153-tooth Quartermaster flexplate and 7.25-inch flywheel, also called a button. Quartermaster emphasizes not to use anti-seize on any of the fasteners or on the input shaft as this stuff is famous for depositing on friction surfaces, which is the last thing you want on a clutch.

Using a fine-spline dummy shaft, we installed the V-drive clutch assembly making sure to use a star pattern when tightening the bolts to prevent tweaking a lever. Once the pressure plate is fully seated, inspect all the release fingers to ensure all are at an even height. If even one arm is not even, the clutch will drag and not release properly.

We had previously measured the bellhousing to verify the center bearing inlet and the face of the housing were in spec. If anything relating to the bellhousing is modified, you should recheck alignment. With the housing in place, measure the distance from the pressure plate fingers to the bellhousing’s mounting face. This is called the set-up depth which, in our case, was 4.210-inches.

We next measured the distance from the top of the bearing retainer bolts to the mounting face of the trans as shown. For the release bearing we were using, the Quartermaster spec was a minimum height of 0.800-inch. We stacked six 5/16-inch supplied washers under each of the four retainer bolts to achieve this distance. The caliper reads 0.803-inch. Make sure the retainer bolts are long enough to fully engage in the transmission case.

We slid the release bearing over the bearing collar and measured the overall bearing height, which in this case was 4.070-inch. Adding the necessary 0.100-inch of desired free play clearance, this produced 4.170-inch overall height.

By subtracting our 4.170 from the transmission set-up distance of 4.210, we came up with 0.040-inch. This distance must be accommodated by adding a shim over the release bearing collar as shown. The closest shim from the supplied shims was 0.030-inch. This gave us an overall free play clearance of 0.110-inc, which is well within Quartermaster’s guideline of 0.100 to 0.0125-inch free play.

The reason for the 0.100-inch of clearance (free play) is to compensate for clutch wear. As the clutch discs wear and become thinner, the entire stack height will shorten, which will raise the pressure plate fingers, reducing the free play. A good maintenance program would be to occasionally measure the set-up distance and compare that to the release bearing distance, which should not change. When the overall clearance falls below 0.100-inch, we can remove the shim underneath the release bearing to compensate.

All that’s left is to locate the release bearing over the pin in the bellhousing and install the transmission. We like to use dowel pins in the top two trans mounting bolt holes to guide the trans into place and not upset the clutch alignment. With the trans flush with the bellhousing, remove the studs and attach the trans. All that’s left is to connect the hydraulic line to the release bearing, bleed the system, and you’re ready to race.

Hydraulic Release Bearing Set-Up
In this example, subtracting A minus B equals the initial clearance of 0.140-inch. Then subtract 0.100-inch of minimum clearance and the result of 0.040-inch is the thickness needed for the shim stack height. Adding a shim of no more than 0.040-inch will create the minimum 0.100-inch free play clearance. By choosing a 0.030-inch shim, we created a total operating clearance of 0.110-inch which is well within Quartermaster’s suggested range of 0.100 to 0.125-inch.  (All measurements are in inches)

  1. Set-up Depth:                              210
  2. Overall Bearing Height:            070
  3. Initial Clearance:                        140
  4. Minimum Clearance:                 100
  5. Shim Stack Height:                    030
  6. Total Operating Clearance:      110

Typical Bearing Travel
This chart specifies the typical release bearing travel necessary to fully release the indicated clutch packages.

Quartermaster Clutch                     Bearing Travel
4.5” Clutch                                        0.120”
5.5” Clutch                                        0.150”
7.25” Clutch                                      0.140”
8.5” Clutch                                        0.180”

Parts List

Description PN Source Price
Quartermaster 7.25” 3-disc clutch kit 43039890 $1,629.00
Hydraulic Release Bearing – 5.5” clutch 710200 $175.99
Hydraulic Release Bearing, 7.25” clutch 710105 $234.47
Quartermaster bearing and sleeve 710103 $54.77
Quartermaster hyd. bearing shim kit 710102 $18.16
Quartermaster o-ring kit 710007 Summit Racing $4.94
Quartermaster spline tool, 26-spline 660091 Summit Racing $53.07
Quartermaster spline tool, 10-spline 660080 Summit Racing $53.07
Quartermaster SBC pilot bushing 110011 Summit Racing $12.89


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