ZR51 Performance - 6-speed Manual Transmission Specialists 

Frequently Asked Questions on the ZF S6-40

Q) After installing one of the new ZF S6-40 transmissions, during operation the transmission now whines while in neutral with the clutch out.

A)  These transmissions are very prone to alignment issues and will whine if not installed perfectly flush. 1991-'96 L98/LT1/LT4 Aluminum/Magnesium Bell Housings have insulator plates affixed to both mating surfaces of the bell housing. 1990-'95 ZR-1 and  1989-'90 L98 Corvette Bell Housings are all aluminum and DO NOT have insulator plates affixed to either mating surfaces. With regards to engine/bell housing/transmission stack, with "Aluminum" Bell Housing configuration there are four (4) mating surfaces requiring inspection/detailing. With "Aluminum/Magnesium" Bell Housing, there are eight (8) mating surfaces requiring inspection/detailing.  Scotch-Brite all surfaces checked for high spots due to dings/dents or possible mishandling during removal. Any high spots are to be filed flush, insulator plates carefully reinstalled and the transmission is not to be forced back into position. It should mate perfectly together when put  back into place if all surfaces are detailed and the clutch disc has been aligned correctly. With all mating surfaces detailed and all fasteners cleaned, optimal stack parallelism will be achieved with engine/transmission driveline assembly.



NOTE (he he): Oxidization Conundrum (OC)   A common problem with Aluminum/Magnesium bell housings that have the insulator plates are that the plates tend to form oxidation buildup during operation which can easily be missed if the insulator plates are not removed to inspect and detail undersurfaces.. Oxidation formations tend to puff up after de-stacking engine/bell housing/transmission case/mating surfaces. A 0.0005" thick formation developed while the stack is tightened to spec. once unloaded can displace (puff up) as much as 0.003"-0.004". This condition will result in 0.002"-0.003" interference of stack alignment regarding mating surface parallelism per insulation oxidization conundrum occurrence (2 plates per bell housing)..

Q) After installing one of the new ZF S6-40 transmissions, during operation with light-weight flywheel, there seems to be more gear-clatter or gear-rattle than one would expect to hear.

Q) What is the most common mistake made when replacing a C4 Clutch Kit?


For questions to Bill Boudreau, please post following information:

Year Corvette? How many miles are on the vehicle? How many owners before you? How many miles have you owned it for? What history do you know of regarding the driveline and service been done to it? Is the transmission the original or has it been rebuilt? Note - Rebuilds have a blue and silver rebuild tag below the ID tag on the passenger side of the transmission.

Q) My ZF6 was replaced back when the car was under an extended warranty, but now this one has gone bad! I was hearing a nasty sound that I thought was the throw out bearing, but after replacing it, the noise still exists. It is definitely coming from inside the transmission, and the best way to describe it is to say it sounds like it is running dry and grinding (but obviously the fluid has already been checked). The sound can be heard with the engine idling and foot off the clutch, as well as in any gear from about 1000 to 1800 rpm and then again from about 2500 to 3500 rpm or any time it is under a good load, say putting it in 6th and giving it some throttle to accelerate. The hydraulics are brand new and I do not have any problems shifting through the gears at all.

A) This type of transmission problem (Gear Buzz) can be corrected by removing the transmission from the vehicle and performing a more stringent main bearing shim selection process than the factory did when the unit was built. We recently had a brand new transmission (540Nm) develop the same symptoms. Instead of fumbling with trying to return it under warranty, we elected to go into the unit to investigate the cause. We found the front countershaft bearing was able to accept a 0.005 thicker spacer shim between the bearing and bearing retainer clip. The front and rear main bearings were found to have correct shim thickness installed. By installing the thickest spacer possible, the bearing and countershaft are not allowed to move along the longitudinal axis which disallows the occurrence of gear buzz. The transmission was reinstalled into the vehicle and road tested. All of the previously reported symptoms were gone and the transmission operated smoothly as if the problem had never existed. This repair procedure is provided in all 1990 through 1996 Corvette Shop Repair Manuals.


Q) On my car I can power shift 2nd and 4th, but I'm having problems with 3rd. During normal driving going to 3rd does not feel as smooth as going to 4th or 5th. Sometimes I don't even feel like it went into gear. The shifter grabs third gear, but it feels like it is half way there, like it has an additional 1/8" of travel left to go, not like all the others gears where you feel the shifter securely engaging the gear. Third grabs, but you feel like the shifter did not go in all the way.

A) There may be an issue with linkage that is slowing down the actuation of the 3rd gear shift event. How is the "return to center" spring action of the shifter feel? This is how well the shifter springs back to the 3/4 neutral position when released from the 1/2 neutral position or the 5/6 neutral position. The return to center action should be quick and crisp. This return to center action is initiated by the mountain detent of the shift shaft (internal) and the ball detent (roller) and two springs located just forward of the CAGS bung. If the ball detent gets crud in it, the return to center action becomes diminished. This condition is of the easiest to correct. It requires a new Ball Roller($15), 2 Compression Springs($5) and less than 5 minutes to install once access under the vehicle is gained.

When installing a Hurst Billet/Plus gear shift handle, there is the possibility that the shifter setup may be binding somewhat at the axis points. I have found this to be the case when installing the Hurst shifters unless removing all of the powder coat from the contact areas at the pivot points prior to installation into the shifter.  There should be no hang-up in movement in the x and y axis and no play in the axis joints. The condition of the shifter frame (Remote Control Arm) is also important. If you can rock the frame (Remote Control Arm) and it rolls more than just the slightest little bit, it has a delaminated rubber eyelet at the front mount point. This condition will also work against you when transitioning from gate to gate (1-2 gate, 3-4 gate, 5-6 gate).

Once you have cleared these items as possible issues and you are certain that the clutch hydraulic system is good, you can be certain that the problem lies internally at the specific problematic synchronizer.


(Q.) I had my transmission rebuilt (1995 corvette ZF6)  and a clutch installed about a year ago.  If I am at a stoplight I sometimes have trouble shifting from neutral into first gear.  I also have trouble getting into third gear sometimes.  Sometimes the shifts are very smooth and other times it seems like it goes halfway into gear but I have to pull back and try again or shift into another gear first before it will go.  Does this sound like an internal transmission problem or a clutch problem? Recently I noticed my clutch fluid was low so I refilled it but the problem still remained.  I plan on changing the clutch master and slave because of the low fluid. The car also shutters a little bit when I let out the clutch going into first gear.  I have put about 15k miles on the clutch and transmission since the work was done and I don't drive the car to the extreme.  Kevin Redmond

(A.) With out a doubt, this sounds like you are experiencing clutch hydraulic problems. Continued use of the transmission while in this condition will cause excessive wear to the synchronizers and eventually the need to have your transmission rebuilt again. The clutch hydraulic master and slave cylinders should be replaced as a set. Regarding the clutch shudder during engagement, if it has shuddered from day one after the clutch replacement, I'd say that the pressure plate was not tightened down evenly with a uniform star pattern sequence. If the shudder developed over time, you can clean it up by changing slightly your clutch engagement technique. Try a few good launches when you are in a safe place or occasionally have a higher engine speed when you feather the clutch pedal. The idea is to alter the trend that has causing the clutch to glaze up in certain spots. Run it out a bit while feathering the clutch from time to time and the shuddering should be minimized for the times that you do a soft/more gentile feather. I had the same condition and was able to make it go away in about 1 weeks worth of driving.


(Q.) Do you have any detailed instructions relative to the "PROPER" way to R&R the Clutch hydraulic fluid for a 1984 Vet? A local shop replaced the Clutch Fluid (DOT 3) - before they did the R&R, the clutch worked great (30K original miles). But when I drove away from the shop, the clutch seemed different, and produced gear-clash when I tried to get into reverse. I let the car sit for the weekend, and a few days later the clutch had lost all of it's pressure and shifting the manual transmission was very difficult. I took it back to them and suggested that there was air in slave, and gave them instructions to tilt the slave Cylinder at 90* when bleeding because of the bleeder valve location -they did it, and the clutch pressure came back, could shift into the 4 gears and occasionally ground when going into reverse. I drove the car for 32 miles, and I lost 85% of the clutch pressure, and was unable to shift into any gear.     Mike C4 Corvette owner  

(A.) Service (R&R) of the clutch hydraulic fluid for the '84-'88 Corvette is generally the same as the '89 - '91 C4 procedure.  The procedure can be viewed at http://www.zfdoc.com/clutch_hydraulic.htm. Remember that DOT3 is a classification of fluid type and does not necessarily contain the additives found in GM P/N 12345347 "Hydraulic Clutch Fluid". For Clutch hydraulic Systems, we actually recommend using Valvoline full-synthetic Dot3/Dot4 Brake Fluid over any other fluid.

(Q.) Bill, I own a 1991 Corvette with the 6 speed ZF. I have a transmission noise which many other Corvette & Camaro owners report as "normal". This noise is most prominent in neutral & 1st gear and is commonly described as the "rocks in a can" sound. Two local companies that rebuild (only) Muncie & Borg-Warner 4 speeds have heard this description and believe it to be the front bearing and possibly one of the ball bearings rolling around inside the trans case. From your experience is this "really" the problem which many people report hearing only in THIS model trans?                Dean of north Chicago.

A.) Dean, it would be a nearly impossible for a ball bearing to fall out of the main bearing case. If the noise goes away when you push the clutch in at idle,... that could be 1 of  3 things, either a bad dual-mass flywheel, or the flywheel is not an original dual-mass type but instead is a single mass type. The ladder situation will produce a sound known as gear clatter while in neutral which is harmless to the transmission. The springs in the clutch disc, which are not present in the disc of a dual-mass set up, will oftentimes produce a rattle like resonance in the 1K - 2K RPM range when under moderate to heavy power load settings. These are the drawbacks of converting over to a single-mass flywheel.
 A bad input bearing in the transmission will normally produce a whirling or whining sound proportional in pitch to engine revs. In 1st gear the whine is most prominent decreasing in intensity in each subsequent gear to the quietest level being 4th gear (direct drive energy flow).
 A bad dual-mass flywheel with a failing damper will sound like a diesel truck engine at idle or a clacking noise. Under certain load settings, this sound may be heard occasionally in 1st gear too.
The main bearings in a ZF S6-40 are very different from those used in Muncie & Borg-Warner 4 speeds. The ZF main bearings are self-contained non-preload non-tapered double-row type bearings. One row is comprised of cylindrical type bearings and the other row is comprised of ball type bearings. This type of bearing would be more similar to the type of bearings used in turbine engines.
If it sounds like a diesel engine at idle, the flywheel dual mass damper may be failing. What I've seen happen before is that when a flywheel is determined to be bad, some people choose to replace it with a less expensive single mass type flywheel where they can buy the flywheel, pressure plate with throw-out bearing and clutch disc for $800 instead of $750 for OEM flywheel and $300 for the clutch kit.
If it is more of a whining or whirling sound, the input bearing has been compromised. This results from the alignment of the transmission during reinstallation is off slightly where the trans, bell housing butt up to the block. I've never heard of a ZF S6-40 main bearing going bad on it's own. FYI, I've rebuilt 125 of these units and have at least 7K hours of experience with the ZF S6-40 design.


(Q.) I just bought, at the local BMW dealership, 3 liters of the Castrol TWS 10W60 oil. I was surprised to see that it is motor oil. Not being a lubrication engineer myself, who did the investigation and determination that this product is compatible with and good for our ZF transmissions?             Jim Grand Sport Registry

(A.) Jim, the C4 Corvette ZF S6-40 6-speed transmission uses engine oil for lubrication. I was told by Jeff Henning, Warranty Administrator of ZF Industries North America, that Engineering of ZF Industries in Germany determined that the BMW imported Castrol (RS superseded by TWS) 10W-60 oil was the recommended alternative to the (GM P/N 1052931) factory-fill oil for use in the ZF S6-40 transmission. In effort to verify ZF Industries alternative lubricant recommendation, we ran our own test series on the BMW imported Castrol TWS 10W-60 oil. Independent testing of the transmission oil samples was sub-contracted out to CTC Analytical Services. The test series went as follows:
<1> Spectrographic analysis indicated that it is has full synthetic composition.
<2> After 2 hours of operation, approximately 100 miles, oil sample analysis tests indicated that the viscosity rating was reduced from 60 down to a 43 level. No need to worry, this is a normal occurrence for this heavier type of oil. I attribute this to microscopic-level lubricant-strand trimming through operational loading where all of the oil contents has been passed through gear pressure-loading regions at least a few times.
<3> At 200 miles, the viscosity level stabilized at a 42 level viscosity since the 100 mile oil analysis test results.
<4> At 5000 miles, the oil analysis test results indicated a 40 level viscosity.
The test-transmission was completely disassembled and checked for wear. There were no signs of carbon film like experienced with the factory-fill oil. The phosphor-bronze lined synchronizers had no glazing and experienced an average mass loss of approximately 4% based on reserve-wear-range mass equivalency between 0.062"(new) and 0.048"(spent)
gap wear/mass measurements.
<5> At 10,000 miles, the oil analysis test results indicated a 39 level viscosity.
<6> At 12,500 miles, the oil analysis test results indicated a 37 level viscosity.
<7> At 15,000 miles, the oil analysis test results indicated a 34 level viscosity.
The test-transmission was again completely disassembled and checked for wear. There were no signs of carbon film like experienced with the factory-fill 30 oil. The phosphor-bronze lined synchronizers had no glazing and experienced an average mass loss of approximately 17% based on reserve-wear-range mass equivalency between 0.062"(new) and 0.048"(spent) gap wear/mass measurements.
<8> At 15,000 miles the oil had enough phosphor-bronze particles suspended
in it that deposits began building up inside of the synchronizer sliding
sleeves from the normal centrifuge-like rotational occurrence.
In Conclusion, until someone invents a copper magnet, we recommend that the ZF S6-40 6-speed transmission oil be changed at 10,000 - 12,000 mile intervals so as to minimize the amount of deposits of the suspended spent synchronizer material from collecting in critical component contact surface areas.

Q.) Bill, I don't know if this is in your ballpark but I have a "growl" from the release bearing ( a least I think its the bearing) every time I push the clutch  and/ or let it out. The trans still shifts smooth but the "growl" is getting louder. In your opinion would this noise be the release bearing? If so, would you recommend changing the clutch plate at the same time? The clutch feels good and does not slip. Also, could you / would you recommend a capable mechanic to do this work in our area (I live in Tucson). Any suggestions on the best place to buy the kit? Thanks for any help you can give me.

 Floyd Hoffman
'90 ZR1 #923

A.) If you can make the growl sound fluctuate by wavering the clutch

pedal right around the first inch of travel, that tells you that the throw-out bearing
is getting worn. It's like a coast-to-load transition grumble.
The entire clutch kit is less than $300. You might as well replace it all while the
transmission's is out. The clutch kit consists of a pressure plate with through out bearing, clutch disc and an alignment tool. Carolina Clutch carries Valeo O.E. clutch products and can be reached at (888)462-2739. Tom and the rest of the guys from Carolina Clutch will take good care of you.

Q.) I have a noise that I can only describe as gear lash or slop. It is most noticeable when the car is coasting and when I gently touch and release the throttle I can hear a clunk. Also, from time when I shift from first to second I can hear the same type of sound. The transmission shifts fine, no gear clash. When I had the car on a lift in gear and moved the drive shaft back and forth the noise seemed to emit from the transmission. I don't feel the drive shaft movement was excessive. Also, there is a different pitch whine when the transmission is in gear with the clutch in verses when the transmission is in neutral and the clutch out.

A.) Similar to the setup of a differential gear set, to much backlash between opposing gears will cause this same type of slapping sound upon let-up and/or load-down of the drive gears. If this sound has been present during all of the time that you have had the car, it is very possible that the transmission was incorrectly assembled. The countershaft nut is responsible for taking up the backlash when the transmission is being assembled. This nut has a torque specification of 162 lbs-ft. The nut is then locked into place by peening/staking a part of it's outer edge down onto a machined flat spot of the threaded end of the countershaft. A seal cap is then inserted in the front face of the transmission over the countershaft bearing journal.
If you are sure that the slapping noise is coming from the transmission and would like to verify that the countershaft nut is correctly tightened to 162 lb.ft., you will need to remove the transmission in order to access this area. 
Based on your sound emission reporting, the countershaft nut may have not been tightened to the specified torque rating. If this is the case and the transmission has seen 60K miles of service, it is likely that the countershaft bearings have experienced a wear trend that is not repairable by tightening the countershaft nut to specs. The correct method to remedy such a predicament such as this would be to disassemble the transmission and replace all of the main bearings (5 sets per Bearing Kit). This will bring back the ideal shaft parrellism between the input, counter and output (main) shaft. While the transmission is partially disassembled, it would be most logical to replace the synchronizers if they are significantly worn.
"Do it once and do it right" is my philosophy on this type of issue. 
Most of the transmissions that I balance and blueprint (rebuild) are from out of state. They get shipped to me, I do my thing and they get shipped back to the customer or installer. I have rebuilt 90 of these transmissions over the last eight years. I have over 6000 hours of experience with these transmissions and can say that they require a considerable amount
more of attention to detail than most other manual transmissions. If you decide to have it serviced eventually, be very picky on who you choose to do it. I have repaired many units that were serviced by other facilities that did not do as good of a job that this transmission deserves.

Q.) I was hoping you could answer this question I've asked many times and have never gotten a real answer:  What is the cause of the noise from a ZF with the clutch out when it is changed to a single mass flywheel?  So far everyone just tells me it does it without telling me why.  If you could tell me or direct me somewhere I can find out, I'd very much appreciate it.

A.) The sound that you are referring to is called "gear rattle". At all times during engine firing, rotational-force impulses are present at the flywheel. When your engine is at idle, fluctuation of these forces occurs at a slower rate. When at idle and in neutral, the countershaft gears clatter back and forth in the teeth of the main shaft coaster (non-engaged) gears. This rattle is in no way detrimental to the transmission. This phenomenon is more pronounce with transmissions that have larger shaft center distances. Shim selection during the build process is very important so as to ensure that the bearing is unable to move in it's journal longitudinally.
At 95 mm shaft center distance, the ZF S6-40 transmission can produce higher levels of gear rattle than most. During development of this transmission it was determined that the conventional dampening springs in the clutch disc were not sufficient to eliminate this problem. In avoidance of customer satisfaction problems, GM developed the dual-mass flywheel. The dual mass system consists of a secondary flywheel mounted within a primary flywheel. A set of ball bearings and dampening springs cushion the two masses to minimize Idle-speed torque-fluctuation induced gear rattle. When switching back to a single-mass flywheel and sprung disc setup, some level of gear rattle will be present. With that in mind, gear rattle becomes more pronounced with engines that have a rougher idle.

Q.) I would also like to know what your typical turn-around time is for rebuilding a zf6.

A.) Typical turnaround time for rebuilding (balance and blueprint) a ZF S6-40 is 7-14 days.

Send email to billb@zfdoc.com or call Bill Boudreau at (602) 319-6575.
Last modified: April 22, 2012