A few years ago, we had a project vehicle that went back and forth. Did we want an automatic or standard transmission? After some serious debate, we chose to go with a manual. Rowing gears in a vehicle has a certain appeal, and we thought a 5-speed would be perfect for driving on road trips. With that thought, a Tremec TKO600 was purchased along with everything we needed to convert the small block over from a TH350 automatic to the manual transmission.
After we had the transmission installed, we fired up the car and cruised around the block to test out our new setup. Everything felt good on the initial shakedown, so we made our way onto the expressway. We rowed through the gears in the maroon Chevy II Sedan with the windows down with the fuel-injected small-block thumping away. Life was good, or so we thought.
As we hit the exit ramp and pressed in the clutch pedal, the shifter didn’t want to come out of gear. When it finally did, we got stuck at a red light. I hit the clutch pedal again, pushed the shifter forward, and the gears made a horrible grinding noise. We had to be towed back to the garage to troubleshoot the problem.
So what happened? Unfortunately, the ordeal was entirely self-inflicted. In short, we failed to measure the distance from the clutch pressure plate to the slave cylinder. In our case, the concentric slave cylinder broke from being overextended, which blew out the o-ring and left us with hydraulic fluid pouring out from under the car.
We contacted Mike Weinberg the President of Rockland Standard Gear to get the skinny on the mysteries of a concentric slave cylinder and how to properly set them up.
Rockland Standard Gear has been in the business of building some of the best transmissions in the industry and with a 38-year track record to prove it. The company produces more than 100 manual transmissions, transfer cases, and powertrain components each week. They also offer personalized service and outstanding tech support due to their knowledgeable staff.
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. – Mike Weinberg
According to Mike, when doing any type of clutch work or replacement, the most important part is proper clutch release. Mike said, “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.”
Mike explained to us that the symptoms of release problems could be very subtle. Some of the telltale signs that can be felt are notchy shifts or grinding which may not show up in every gear. The clutch must fully release, so that power from the engine is completely separated from the transmission. If there is no break of power to the transmission, all of the engine torque will be applied to the synchronizer rings. Synchronizer rings are not designed to handle a load and will fail, which will cost you time and money.
The concentric slave cylinder is an ingeniously simple design that has eliminated parts that were used to disengage clutches in the past. This single unit has gotten rid of the clutch linkage, clutch fork, and pedal effort. The concentric slave cylinder is also tucked away from danger mounted on the quill tube inside the bell housing of a transmission.
There are a few different designs of concentric slave cylinders. Mike explains, “Some of the concentric slave cylinders have a spring-loaded return. After the clutch is released, spring pressure brings the cylinder back towards the transmission front plate. Some of the designs don’t have a 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.” Mike continues, “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 con-centric slave is fully retracted and the face of the clutch fingers are .160 to .220-inches. This distance will provide about .050 to .100-inches of air gap at the disc and will provide a complete clutch release. The average usually will be .190-inches from the face of the clutch fingers to a fully retracted release bearing.”
To figure out what size shims are required, some simple measurements are all that is needed and should be made on every clutch installation. Mike said, “After you have the flywheel bolted up to the engine with the clutch set mounted on the flywheel, you will put a straight edge across the clutch fingers. You will then measure the distance from the straight edge to the mounting face for the transmission on the engine block. Note that distance (for 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 bellhousing-to-the-release-bearing measurement (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-inches to guarantee a complete release of the clutch.”
If you are using all OEM parts, you may not need to shim the slave cylinder. However, A few simple measurements will prevent a lot of extra time and effort to take the unit back out if a shim is required even with OEM components.
If you’re planning on replacing your clutch assembly, it would be in your best interest to measure and ensure the proper function of your clutch system. If any questions come up along the way, feel free to reach out to George Kreppein, VP of Racing and Performance for expert advice and sales help.
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