Taking the Mystery Out of Clutch Adjustment with Concentric Slave Cylinders - Transmission Digest

Taking the Mystery Out of Clutch Adjustment with Concentric Slave Cylinders

Proper clutch adjustment is a must for correct operation and durability of the clutch components. For many years, clutch engagement and release was performed with a manual linkage or an external hydraulic system. The manual linkages were adjusted by threaded rods that were set to provide proper engagement and proper clutch release, which was relatively simple to achieve. External hydraulic systems used an externally mounted slave cylinder that actuated the clutch fork and were adjustable through a threaded rod from the slave cylinder to the clutch fork. Hydraulic systems have their own challenges, which mainly revolve around proper bleeding technique to ensure that there is no air trapped in the system.

Up to Standards

  • Author: Mike Weinberg, Contributing Editor
  • Subject Matter: Concentric Slave Cylinders
  • Unit: Various
  • Vehicle Application: Various
  • Issue: Manual Clutch Adjustment

Proper clutch adjustment is a must for correct operation and durability of the clutch components. For many years, clutch engagement and release was performed with a manual linkage or an external hydraulic system. The manual linkages were adjusted by threaded rods that were set to provide proper engagement and proper clutch release, which was relatively simple to achieve. External hydraulic systems used an externally mounted slave cylinder that actuated the clutch fork and were adjustable through a threaded rod from the slave cylinder to the clutch fork. Hydraulic systems have their own challenges, which mainly revolve around proper bleeding technique to ensure that there is no air trapped in the system.

When doing any type of clutch work or replacement, the most important point is proper clutch release. If you have an improper clutch application, you will have slippage which will damage the friction material quickly and negatively affect the durability of the clutch. That said, the only damage that will occur is to the clutch. A bad or improper clutch release will cause serious damage to the transmission. Clutch release problems can be very subtle and manifest themselves as notchy, draggy shifts, blocked out shifts or gear clash (and it doesn’t have to show up in all gears). In short, the clutch must release fully so that engine torque is disconnected from the transmission. If there is no interruption of power to the transmission, all of the engine torque will be applied to the synchronizer rings, which were not designed to survive that much power. If the clutch cannot disengage fully, the synchro rings will fail. The resulting inability to speed up or slow down the speed gears will result in immediate damage to the engagement teeth on the speed gears, damage to the shift forks and a really nasty driving experience. A good release is critical to keeping the transmission from having very expensive internal damage.

As designs improve, we now have the concentric slave cylinder which is mounted on the quill tube surrounding the input shaft and internal to the bell housing. The brilliance of this design rid the system of a number of external parts, of which the clutch fork is principal. It reduces pedal effort for the driver, and shields the components from road debris and weather-related issues.

There are different designs of concentric slave cylinders. Some have a spring-loaded return on the cylinder so after the clutch is released, spring pressure brings the cylinder back towards the transmission front plate. Some designs have no spring load to return the cylinder. Some designs are bolted directly to the transmission front retainer and other “float” with no fasteners to secure them. Regardless of which design you are working with, there are dimensional distances which must be maintained in order to get enough travel so that the release bearing can move the clutch fingers far enough to get a complete clutch release. This means that many of the concentric slave cylinders will need to be shimmed between the back of the slave cylinder and the front of the input bearing retainer on which they ride. The optimum distance between the face of the release bearing when the concentric slave is fully retracted and the face of the clutch fingers is 160-thousandths to 220-thousandths. This distance will provide about 50/1000 of air gap at the disc and will provide a complete clutch release. The average usually will be 190-thousandths from the face of the clutch fingers to a fully retracted release bearing.

To get the correct-size shim requires some simple measurements that should be made on every clutch installation. With the flywheel installed and the clutch set mounted on the flywheel, put a straight edge across the clutch fingers and measure the distance from the straight edge to the mounting face for the transmission on the engine block. Note that distance (example: 3.10 inches). Now put a straight edge across the front of the bell housing flange that will mount to the block, and note the distance from the straight edge to the face of the release bearing on a fully retracted slave cylinder (example: 7.80 inches). Now subtract the block-to-clutch-finger measurement from the bell housing to the release bearing (example: 7.80 – 3.10 = 4.70 inches). Using 1.90 as an optimum amount of travel, subtracting 1.90 from 4.70 tells us that we need a shim behind the slave cylinder of 2.80 to guarantee a complete release of the clutch.

If you are using all OEM parts you may not have to use any shim at all but be aware of manufacturing differences between one manufacturer and another and do the math. A few simple measurements (which only take minutes to do) will prevent a lot of extra labor to take the unit back out if, in fact, a shim is needed.

One of the big trends today is aftermarket power adders that customers install. A super charger, turbo charger or other internal engine mod, or a computer chip that boosts horsepower and torque usually will be more than the stock clutch components can handle. This means that the clutch set will usually be replaced with a higher-torque single disc, twin disc or triple disc clutch set. Since most manual transmissions are only found in “muscle cars” such as Mustangs, Camaros, Firebirds, Vipers, etc., the customer invariably will want to maximize his vehicle in the power department. It is more fun to drive, impresses the girls and makes for “bragging rights”. There is always money for the toys. Once you understand how important a proper clutch release is for transmission function and longevity, the little time invested pays off in more profit due to less wasted labor.

Mike Weinberg is president of Rockland Standard Gear.

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