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Corvette Extra QC Audits

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  • Corvette Extra QC Audits

    During the assembly of a new Corvette, it undergoes a standard quality control (QC) review. Some Corvettes however, are subject to an additional audit called a Global Customer Audit (GCA). This thread describes the GCA process. What is a GCA?
    * It is a supplemental quality control audit that a very small percentage of Corvettes produced at BGA experience. While I do not know the exact particulars of how much extra auditing is currently being done, a few years ago it was a team of QC technicians spending many additional hours extra-closely-examining the car’s assembly. They focus on any issues that heretofore had been previously identified as assembly line issues, plus provide additional scrutiny of all exterior and interior panel gaps, and similar QC potential issues. The GCA also includes a typical 8 mile additional road test.

    * At every one of GM's 140 vehicle assembly plants throughout the world, after the day's shift is over, the computer randomly selects 2 cars to go through a supplemental quality control audit. This is not something one can buy, but again is computer selected.
    The current rate of assembly at BGA is 11.5 vehicles per hour, 115 per day, and a minimum of two Corvettes are chosen every production day for a GCA. Sometimes more than that are selected.
    * Results of GCA are sent to the Plant Manager (perhaps also to the GM corporate). Most Plant Managers have an every work day, a daily meeting with top staff, going over those GCA results and much more. Represented at that table at BGA at 7:00 AM , is Plant Manager Kai Spande, often Assistant Plant Manager Nora Roper (who is always present if Kai is not available), their Exec. Assistant, and at least the following: top members of the Plant quality control team, managers from the different major plant assembly "lines" (e.g., General Assembly, Paint, Body Shop, Trim 1, Trim 2 and more), the Plant's liaison with all of its suppliers and, at least one representative of the UAW. Often they are around, or even more than 15 present at the meeting.

    *The purpose of these meetings is to discuss and problem solve most if not all of the following:

    The day before's production schedule; number of cars produced; what percentage which passed the required, end of the assembly line, 800-test, test vehicle booth results; how many cars did not pass and what were the specific amounts for every failure; how many are now in the "re-work" or fix it shop; each/every "line stop" and what caused it; the computer analysis of yesterday's issues; result of the global customer audit teams (more on that below and what those teams did/learned); input by the line managers and the UAW reps as to yesterday's and potentially still current issues; what earlier implemented changes went well vs. what still needs additional attention now; what is the latest data from warranty claims as to failed parts, parts needing adjustment as identified by the dealerships’ service departments; and much more.

    *If a car is computer chosen for a Global Customer Audit, it is pulled into a special the "GCA examination room" (my terminology), where a team of up to typical 5 QC engineers/staff spent a total of up to 25 hours on it; it is further subject to an additional 8 mile specialized-course road test and additional QC Reviews. Fit-n-finish is closely checked. Everything not per assembly standards is adjusted and when rarely needed, a part(s) is replaced. That car is "loved and pampered and messaged."

    Also checked is what issues were previously found in previous QCA's noted in previous daily plant QC morning meetings, and especially and specifically, all parts which have been recently modified as a result of previous learning, i.e., if it was determined that if the angle of the rear flux capacitor flange should be moved 1/4 of a degree rearward to more easily install the nuclear power plant, once those modified flux capacitor flanges arrive at the Plant, some of those cars automatically go to the GCA area, and that modified part's fix vis-a-vis all parts attached to it are reviewed for hopefully improved fit. AND, critically important, will be the input of the various line supervisors within the plant as to how the new part's install went, answering the critical question of "does installation now go easier,” the purpose of this part change? Not just management's input is considered, but equally considered is the perspective of UAW rep, having been part of previous meetings and knowing this new part is now going in while the line is moving, the rep will have talked with his/her employee about how it went yesterday installing that new part. Was it the improvement that was expected, would a further 1/4 of a degree angle bend help more, etc.

    * How can I get my car to be chosen for a GCA? ; Conversely, how do I avoid my car being chosen for the GCA as I do not want to have it arrive with the minimum eight extra miles? However, the latter is a very rare request AND it is very rare that the additional road test exceeds 8 miles, that is unless something needs to be further fixed and tested to insure production standards.

    Answer: You can request GM to not have your car be chosen for the GCA. Well, I guess you can tell them, but it is not going to make a difference, for if the computer were to select your VIN out of those two VINS it picks out daily, your car will mandatorily receive a GCA. Plant Managers can not even de-select a car chosen by the computer for a GCA, e.g., it will receive one.

    Actually the opposite is what much more often occurs, that folks request a GCA for their car, as they want additional sets of eyes-n-ears, more brains and skilled QC hands to look at their car. That is my strong preference, i.e., to have BGA skilled personnel adjust and/or fix any issues at the plant — where there is the best skilled talent, the best tools, and "spare/replacement parts" immediately on hand to fix your car.

    So, you can request your car to be GCA's, but your chances are small, e.g. 2 mandatory and sometimes one or two more out of the 115 Corvettes per day now being produced. But, there is, IMO, one thing you can do that tends to increase your chances of getting a GCA for your baby is to purchase a "Buyers Tour," and at the end of the tour, at the very last station when you are given the opportunity to start your car, given its "Birth Certificate" (it is really named that), ask the end of the line Supervisor for a GCA. Does this always work to getting your GCA? NO. But IMO, it can result in an increased chance of your car being GCA'd.
    Should you choose a Buyers Tour for that reason? Again, NO, but that increased GCA change is IMO an added side benefit. I requested that at the end of my C6 Buyers Tour and during my C7 Buyers Tours, both received one. However, I know someone who purchased the Buyers Tour and at its end, asked for a GCA, and his car arrived at his dealer with 4 miles on it, e.g. no GCA happened. BGA simply does not have enough staff with enough time, nor enough testing areas to do anywhere near the percentage of GCA’s requested.

    * That there is one additional testing component which every car that is GCA'd receives, an 8-minute water mist test. [Separately, every Corvette’s receives, and must pass a 3 minute, 800 gallon per minute, water deluge test.]

    I am fervently hopeful the Buyers Tour option continues, and if it does, I will contact NCM’s Museum Delivery staff and purchase it for my ME.

    A separate thread on what is a Buyers’ Tour is under construction this weekend and will be linked inserted here when finished.
    Last edited by John; 10-06-2018, 11:57 AM.
    2023 Z06: Elkhart Lake Blue, 2-tone blue interior, HTC, Standard Z06 with Z07 aero pkg, CCB’s, 3LZ, E60, every visible carbon fiber option including wheels, Jake Design Pkg. Lifetime, annual contributors, and 25 year NCM members. Home is the beautiful Pacific Northwest.

  • #2
    Great description of the extra QC audit program. As I mentioned in another thread, my 2014 C7 was chosen for this extra QC.

    A question folks might have is "How would I know if my new Corvette was selected and went through the increased QC". There is no notice given to the owner or even the dealer that the car went thru the review.

    In my case I was able to monitor the production status codes of my car as it was being assembled at BG thru info Chevy put on line as well as Chevy Customer Care information. When my C7 was completed and should have been moving for transport, the status code was changed to 4D00 and there was a coupla day delay at BG before shipment. When I noticed this, I was a little concerned and sought to find out what was going on with the car. I thought maybe a problem had been found after manufacture and it had been held back for correction.

    While nobody could/would confirm what was going on, thats when I learned about the GCA program described by John above. I learned if the car was subject to the GCA it would have a status 4D00 although thats not the only scenario when the 4D00 code is used. When the car arrived at the dealer it had 24 miles on it which was the expected mileage C7s at the time would have if they had experienced the QC review due to track testing at BG.

    Those clues plus info from a few other C7 buyers who experienced the same scenario allowed me to conclude my C7 had been subject to the GCA. Another hint is that I've had no issues with my C7.

    There was a somewhat unique factor that existed when my car was being assembled. While demand for the C7 was extremely high at that time, there were severe constraints on the roofs. The only roof that was not on constraint was the transparent. This resulted in a week or so of limited production at BG and thats when my car with its transparent roof was assembled. Maybe because of the limited production, a higher percentage of cars actually produced that week were selected for the QC. But thats just speculation on my part.


    • #3
      I would love to have that happen to my ME! While I have a great Corvette Technician to work on my car, would prefer that extra eyes gave it the final once over, minor adjustments that might be necessary before it leaves Bowling Green.
      Enjoying my Shadow gray C8. What a car it is.


      • #4
        Interesting, Is there a way to only be there when the crank and piston rod bearings are being miked out to tolerance and torque specs? Does anyone know what the bearing clearance and tolerances are?
        After all - What better way to start than the internals? The Heartbeat Of America !
        Last edited by Frenzy36; 10-07-2018, 12:11 AM.
        Rocket City Florida - 2013 427 Vert - 2015 ZO6 - 2020 Stingray


        • #5
          We know that the3 torque on every bolt on the car is set by pre-determined parameters, and that to do that both the torque setting is measured and the number of turns to achieve it — so a burred up bolt does not even up giving the specified torque setting, yet not be fully seated/tightened.

          Second, every single motor is tested with literally 800 performance standards, and to pass every motor must meet the standard on every single one of the 800 measurements. However, as to the crank and piston rod bearings, or any other measurement that comprises those 800 measured items, we do not know.

          However, if the “Build Your Own Motor” option is again offered (as it was earlier this year though the last week of August, last year and the year previous, Frenzy36 you could literally build your own motor if you were to select that option. You would have a your own Powertrain Build Center Specialist there with you the entire day, to answer all your questions, and to perform or assist you in performing any/all things you want. Or conversely, if you are comfortable and capable, that Specialist will not be lifting one tool all day long.

          We all know LOL, why it is they kicked every customer out of the Plant prior to September 1st.
          2023 Z06: Elkhart Lake Blue, 2-tone blue interior, HTC, Standard Z06 with Z07 aero pkg, CCB’s, 3LZ, E60, every visible carbon fiber option including wheels, Jake Design Pkg. Lifetime, annual contributors, and 25 year NCM members. Home is the beautiful Pacific Northwest.


          • #6
            That would be awesome ! I would be most interested in the clearances on the bearings, torque on the heads etc. I would be happy to sign up for that program and see the facility as well as anything else I could do.

            Here is some info for Gearheads


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            SubsTech’s sister website Smooth Sliding provides independent engineering consulting services that help you to solve engine bearing related issues: failures, material selection, geometry design and optimization of hydrodynamic conditions.

            Smooth Sliding is an engineering consulting company run by Dr. Dmitri Kopeliovich:
            • Director of Research & Development of King Engine Bearings.
            • World leading expert (25 years of experience) in design, technology and materials for Engine bearings in applications such as automotive, renewable energy, aviation, racing and others.
            • Founder and owner of SubsTech (Substances & Technologies) – a leading professional website on Materials Science and Engineering.
            • Author of numerous scientific and engineering publications and patents.
            • Founder and owner of Smooth Sliding.

            For further information and for requesting consulting services please visit our sister website Smooth Sliding.

            to Metals
            to Engine bearings Oil Clearance and Engine Bearings

            Dr. Dmitri Kopeliovich

            Hydrodynamic Lubrication

            Among all geometric parameters of an engine bearing, oil clearance is the most important. It determines one of the key functions of the bearings – support the creation of hydrodynamic lubrication. In a hydrodynamic bearing, a rotating journal produces a hydrodynamic force that squeezes lubricant through the gap between the journal and bearing surfaces [1]. The hydrodynamic force counteracts the external force of the crankshaft. This prevents direct contact between the bearing and journal surfaces, which are separated by an oil film.

            In a hydrodynamic bearing, the loaded rotating journal is always displaced from the concentric position. As a result, a converging gap (wedge) is formed between the bearing and journal surfaces (Fig.1). The wedge parameters are determined by the difference between the diameters. This difference is called oil clearance (or bearing clearance). The presence of an oil wedge is indispensable to the normal operation of a hydrodynamic bearing [2].

            Fig. 1 Journal bearing

            The rotating journal generates hydrodynamic force. The greater the force, the more the journal center shifts away from the bearing center.
            to top Effect of Oil Clearance on the Parameters of Bearing Lubrication

            The hydrodynamic conditions of bearing operation can be calculated theoretically. Software developed by King Engine Bearings (called ENSIM™), is used for simulation of engine bearing performance under various conditions. ENSIM™ is capable of calculating the thermodynamic, dynamic, hydrodynamic and mechanical parameters of bearing operation. When a bearing for a new or existing engine is being designed, theoretical simulation of hydrodynamic lubrication is performed to optimize bearing operation. For the purposes of this article, effects of oil clearance on the parameters of hydrodynamic friction were determined by means of calculations made for high performance bearings working under the following conditions:
            • Bearing diameter: 2”
            • Bearing length: 1”
            • Oil type: 5W50 synthetic
            • Oil inlet temperature: 175 °F
            • Bearing loads: 5,100 psi and 12,000 psi
            • Rotation speeds: 2000 RPM, 4500 RPM and 8000 RPM

            The main objective of the calculations was to determine the optimal values of clearance for various operational conditions.

            The most important hydrodynamic parameter affected by oil clearance is the value of minimum oil film thickness. It should be as great as possible in order to prevent metal-to-metal contact between the crankshaft journal and the bearings. The results of the calculations of the effect of clearance on min. oil film thickness are presented in Fig.2 and 3.

            Fig.2 Effect of oil clearance on min. oil film thickness at load 5,100 psi

            Fig.3 Effect of oil clearance on min. oil film thickness at load 12,000 psi

            The calculations prove that optimal oil clearance (clearance providing the greatest value of min. oil film thickness) depends on the rotation speed and load applied to the bearing. An increase of oil clearance provides two opposite effects on the value of oil film thickness. On the one hand, oil film thickness increases due to the formation of a larger supporting oil wedge. On the other hand, non-uniformity (localization) of oil pressure, caused by excessive clearance, results in a decrease of oil film thickness. Oil pressure distribution has a peak close to the location of minimum oil film thickness. The value of the peak is dependent on the average load applied to the bearing, the journal rotation speed, lubricant viscosity and the value of journal eccentricity “e” (Fig.1) relative to the bearing. The non-homogeneity of oil pressure distribution is characterized by the ratio Pmax/Pav.

            Where: Pav – the average load applied to the bearing surface (5,100 psi or 12,000 psi in our calculations)

            Pmax – the peak value of oil pressure

            An example of oil pressure distribution is shown in Fig.4.

            Fig.4 Distribution of oil pressure over bearing surface

            Excessive clearance can produce a decrease in min. oil film thickness, resulting in undesirable metal-to-metal contact. It may also cause fatigue of the bearing material in the area of peak pressure. This phenomenon is known as knocking. The effect of clearance on oil pressure distribution is presented in Fig.5 and 6.

            Fig.5 Effect of oil clearance on distribution of oil pressure at load 5,100 psi

            Fig.6 Effect of oil clearance on distribution of oil pressure at load 12,000 psi

            These graphs depict that clearance exceeding 0.003” (in our 2” bearing diameter study) results in substantial non-homogeneity of oil pressure distribution. One of the most wide spread causes of engine bearing failure is oil starvation. Insufficient oil supply may be caused by problems with the oil pump, clogging of oil passages or by excessive leakage of oil out of the bearing due to excessive clearance. Oil leakage has an important positive role – it removes the heat absorbed by the bearing, thus preventing overheating of the oil. However, too large a flow of oil leakage may cause a drop of oil pressure and even oil starvation, which terminates hydrodynamic lubrication. Besides oil clearance, oil viscosity affects the oil leakage rate. Oils having greater viscosity better support oil pressure and reduce oil leakage from the bearings. The influence of clearance on oil flow is seen in Fig.7.

            Fig.7 Effect of oil clearance on oil flow

            This graph shows that low oil clearance provides low oil leakage and more uniform distribution of oil pressure. However, oil clearance should not be too low due to the following adverse effects: - Tight clearances generate lower hydrodynamic force, which causes a decrease of min. oil film thickness (see Fig.2 and 3). - Bearing housings and crankshaft may distort/deflect at high loads and high rotation speeds. These distortions are much more dangerous in bearings with small clearances. - Excessive heating of the oil caused by friction energy generated at the bearing by high journal rotation speeds. The effect of clearance on oil temperature rise (delta T) is depicted in Fig.8.

            Fig.8 Effect of oil clearance on oil temperature rise

            to top Effect of Oil Viscosity

            The effect of oil clearance on minimum oil film thickness for oils with different viscosity and at various rotation speeds is demonstrated in Fig.9.

            Fig.9 Effect of oil viscosity on minimum oil film thickness at 2000, 4000 and 8000 RPM

            The positive effect of oil viscosity on min. oil film thickness for any rotation speed and any oil clearance is evident from the graphs. However, the effect becomes more considerable at greater clearance. The calculations prove that the most viscose oil, 10W60, provides the most stable hydrodynamic regime of lubrication (the greatest value of minimum oil film thickness) with an oil clearance of 0.004” (1/500 of the bearing diameter). Only at the rotation speed of 2000 RPM does minimum oil film thickness slightly decrease (from 41.6 to 35.0 µinch) when the clearance is increased from 0.002 to 0.004”.

            The behavior of the most fluid oil (0W5) is quite different. At relatively large clearances its viscosity is too low to create hydrodynamic force sufficient to maintain the required level of min. oil film thickness. This effect is most prominent at low rotation speed 2,000 RPM. The greatest value of oil film thickness is achieved at the lowest clearance of 0.0004” (1/5000 of the bearing diameter). The oil grade with a moderate viscosity index, 10W30, demonstrated the most stable hydrodynamic regime at the clearance value of 0.002”, which equals 1/1000 of the bearing diameter.

            to top Bull’s Eye Tolerance™

            This study proves that oil clearance determines the |hydrodynamic parameters of lubrication. The bearing parameter that directly influences the value of oil clearance is wall thickness. Scattering the bearing wall thickness in a wide range causes inconsistent hydrodynamic parameters. Consistency of wall thickness produces more consistent hydrodynamic characteristics, and a more stable regime of hydrodynamic lubrication [3]. King Engine Bearings has developed and implemented a production technology ensuring the production of very accurate and consistent wall thickness from shell to shell. The technology is called Bull’s Eye Tolerance™. It includes fully automatic processes of precise boring and thickness measurement performed with machines developed and designed by King engineers. The process is integrated into the quality assurance system of the factory, monitoring the production process and the machines’ performance. The level of accuracy of Bull’s Eye as compared to that of other bearing manufacturers was evaluated by SGS S.A. – a multinational company headquartered in Geneva, Switzerland. SGS S.A. provides inspection, verification, testing and certification services. Identical bearings produced by 6 different leading bearing manufacturers were measured in the investigation. 32 bearings (4 sets with 8 pieces each) from each manufacturer were compared. The wall thickness of each bearing was measured in two points of the bearing crown (384 measurements total). The statistical variances of the measurement results [4] are shown graphically in Fig.10.

            Fig.10 Statistical variances of bearing wall thickness

            Among all evaluated products, King bearings exhibited the least statistical variance of 173 µinch. The variances of the bearing thickness of other manufacturers were between 217 to 1114 µinch. The values of the standard deviations of bearing wall thickness of each manufacturer are presented in Fig.11.

            Fig.11 Standard deviations of bearing wall thickness measurements

            Standard deviation characterizes consistency of the measurement results. King bearings showed the best wall thickness consistency of all bearings tested in this investigation. The standard deviation of King bearing thickness is 37 µinch, whereas that of other manufacturers varied between 54 to 193 µinch. The values of standard deviations were used for building curves of Gaussian (normal) distribution of bearing thickness (Fig.12).

            Fig.12 Gaussian distribution of wall thickness of bearings made by different manufacturers

            Fig.12 depicts the clear advantage of King technology Bull’s Eye Tolerance™ over the competitors by providing consistent shell-to-shell bearing wall thickness.

            to top Conclusions

            • There is a value of clearance at which minimum oil film thickness reaches the maximum value.
            • Loose clearance results in lower temperature rise, but in greater oil leakage (risk of oil starvation), and in higher oil pressure peak (risk of material fatigue).
            • Tight clearance results in less oil leakage and lower peak oil pressure, but in greater oil temperature rise and greater sensitivity to geometric distortions of the housing and crankshaft.
            • The optimal range of clearance is 0.0015-0.003” (for a bearing with 2” diameter). Looser clearances are more suitable for highly loaded engines working at high rotation speeds and with thicker oils. Tighter clearances provide a better combination of hydrodynamic parameters in less loaded engines working at lower rotation speeds and using thinner oils.
            • Higher oil viscosity produces greater minimum oil film thickness, more power loss due to friction and more uniform oil film pressure.
            • King Engine Bearings developed a production technology - Bull’s Eye Tolerance™ - ensuring very accurate wall thicknesses.
            • The consistent shell-to-shell thickness of King bearings results in a minor difference of oil flow rate and a more stable hydrodynamic lubrication.

            to top References

            • [1] Dudley D. Fuller (1958), A Survey of Journal Bearing Literature, Amer. Soc. of Lubricating Engineers.
            • [2] Dmitri Kopeliovich (2011), Geometry and Dimensional Tolerances of Engine Bearings, Engine professional, AERA., p.70-76.
            • [3] Dmitri Kopeliovich (2015), Optimization of Clearance Design for High Performance Engine Bearings, SubsTech (Substances&Technologies), Available from erformance_engine_bearings
            • [4] Dmitri Kopeliovich (2015), Consistency in Bearing Wall Thickness, SubsTech (Substances&Technologies), Available from

            to top
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            oil_clearance_and_engine_bearings.txt · Last modified: 2018/02/03 by dmitri_kopeliovich
            Promote in SubsTech Creative Commons License Except where otherwise noted, this work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 License

            Last edited by Frenzy36; 10-06-2018, 11:44 PM.
            Rocket City Florida - 2013 427 Vert - 2015 ZO6 - 2020 Stingray


            • #7
              Thats some complicated stuff Frenzy36. Well over my head but glad to know there are folks that speak that language. Amazing what goes into making that engine hum after I press the start button.

              Here is a thought that I will contemplate for the remainder of the day:

              "In a hydrodynamic bearing, the loaded rotating journal is always displaced from the concentric position."
              Last edited by Boomer; 10-07-2018, 09:18 AM.