We have had a terrible time during the last few years dealing with the C4 MN6 CHS Master Cylinders (P/N 19158668) and Slave Cylinders (P/N 12509313) components in regards to meeting operational requirements of the ZF S6-40 gear shifting operation. Our livelihood at ZR51 Performance, remanufacture of the ZF S6-40 transmission, is profoundly impacted by problems incurred with C4 MN6 CHS operational problems.
As a shop that mostly services C4 MN6 Corvettes, we have documented time and material loses exceeding $5,000.00 during the past two years directly caused by problems incurred when installing new GM CHS Master Cylinders (P/N 19158668) and new GM CHS Slave Cylinders (P/N 12509313) into our customer's vehicles.
C4 Clutch Hydraulic System (CHS) component fault diagnostic and corrective action report to GM SPO Quality Group regarding latest field observations of GM Replacement CHS Master Cylinder (P/N 19158668) and Actuator Cylinder (P/N 12509313) Quality Assurance.
We have noticed during the last 45 days only the first two (2) out of the last twelve (12) CHS Actuator Cylinders (P/N 12509313) had their seals installed backwards. Unfortunately with this (seal direction) quality assurance correction we have noticed another quality assurance issue has developed.

Failure Analysis (P/N 12509313):

Out of the last ten CHS Actuator Cylinders (P/N 12509313) disassembled for QA inspection, eight (8) out of ten (10) of these units were found to have the seal compromised. Upon further inspection of these failed units, it was determined that the damage to the seals had occurred during manufacturing when the piston assemblies were installed into the cylinder assemblies. Trace evidence indicates that Assembly personnel were/are not using a mandrel device to ease-load the piston assemblies into the Actuator Cylinder assemblies causing the seal to in some cases fold over backwards and tear or sheer off large portions of the seal lip while being inserted into the bore. We observed that on all units (P/N 12509313) there is a sharp burr edge present at the bore opening where a 45 degree chamfer relief is machined into the leading edge of the bore. This sharp edge happens to be at the major diameter edge of the 45 degree chamfer relief which only poses a threat to the seal integrity during insertion when a mandrel device is not utilized.
Operation of the MN6 CHS while having a compromised Actuator Cylinder seal will cause wide range of problems ranging from reduced longevity and notchy shifting to a state where operation of the CHS is not even possible.

Corrective Action (P/N 12509313):

It is an industry standard practice to employ the use of a mandrel device whenever inserting a piston/seal assembly into a cylinder bore assembly especially when the bore entry has sharp edges present which pose a hazard to the integrity of the piston seal(s) during the insertion process.
To fabricate a mandrel device for this particular assembly process cut a section of tubing from an existing Actuator Cylinder, machine the end faces flat and break (remove) all sharp edges.
With ample lubricant, load piston/seal assembly into mandrel feeding in direction where seal is gathered in gently by ramping smallest to largest diameter of seal.
Place the mandrel with piston protruding slightly for ease of alignment up to the cylinder bore opening and load into the bore by pushing with finger.

Failure Analysis (P/N 19158668):

The C4 MN6 CHS Master Cylinder (P/N 19158668) Hydraulic Fluid Reservoir (last 15 out of 20 samples) have been found to have defects in the bulkhead (seal) fitting portion of the plastic molded component. From an operational standpoint, this defect reduces the CHS operational level of effectiveness by allowing air to be drawn into the system during each actuation stroke at the bulkhead seal but does not allow hydraulic fluid to leak out. With the reservoir lid and moisture barrier removed, bubbles can be seen rising from the Master cylinder chamber just about every clutch pedal stroke even after the CHS has been reverse bled with all air removed from CHS. This causes stiff or notchy shifting during operation of ZF S6-40 manual transmission.

Corrective Action (P/N 19158668):

Check the Extrusion Mold surfaces for imperfections at the barb portion bulkhead fitting, reworking if necessary. Review the production process requirements regarding necessary component cooling before removing component from the molding fixture.
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Author Bill Boudreau (ZFDoc)
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