lars

I started a multi-part post on this subject earlier, but the post got a bit buried and "fuzzy" due to the time lag between my progress posts, and it lost interest. Here is the current entire continuous technical portion of the post (so far) for those interested in the C1 clutch issue (as also applicable to C2/C3 cars):

In a post by “hderr” almost a year ago, Hank was having trouble getting his ’62 clutch to fully disengage. A lot of people offered a lot of advice, but the problem was never resolved:
https://www.corvetteforum.com/forums...ease-62-a.html

On a similar note, I have found that the problem with the clutch not fully disengaging is a very common issue on Corvettes. I posted the following article here on the Forum dealing with this subject:
https://www.corvetteforum.com/forums...vel-range.html

So Hank trailered his ’62 in from out-of-State this afternoon and dropped it off with me. Here's Hank dropping off his super-nice '62 after the road trip:


As with any major work, the first order of business is to disconnect the battery... the starter will need to be unbolted in order to remove the bellhousing (the starter was attached to the bellhousing in 1962 - not bolted to the engine block), and we don't want to make any sparks doing that:


The clutch rod is then disconnected from the clutch fork:


...and the big, heavy return & over-center spring is then disconnected from the clutch bellcrank (aka, "Z-Bar"). The C1 Vette uses a super-heavy return spring that would go over-center to add force to the clutch pedal effort, making the clutch seem lighter than it really was. Rather than attaching the Z-Bar pivot ball to the engine block, as was done on the C2/C3 cars, the C1 attached the pivot ball to a bracket bolted to the bellhousing. So the return spring must be disconnected prior to removing the Z-Bar attach ball bracket on the bellhousing. Many people struggle with this spring, but it can easily be removed: Put a Crescent wrench at the top of the bellcrank lever to neutralize the spring load while removing the bolt that attaches the clutch pedal rod to the bellcrank. Once the rod is removed, use the leverage of the Crescent wrench to allow the bellcrank to move all the way forward, past over-center, and down to the frame. At this forward, lowest position, the spring will be unloaded, and can be disconnected from the bellcrank. If you need more slack, you can remove the top bolt in the spring attach bracket and loosen the lower bolt. This will allow the bracket to "fall over" to further eliminate all spring tension. For re-assembly, use the Crescent wrench to re-assemble and to pull the bellcrank back up into position for attachment to the pedal rod:


The C1's have a standard-type driveshaft, and the yoke must be removed out of the transmission, unlike C2/C3 where the driveshaft can be unbolted from the tranny yoke. So the driveshaft is unbolted at the differential, and the driveshaft is slid out of the tranny:


Driveshaft coming out:


This leaves a big open hole in the back of the tranny, which will spill gear oil all over your floor....


To prevent this, I use a spare yoke like this to stick into the tranny prior to removal:


Yoke installed, tranny plugged, and ready for removal. If you don't have a spare yoke, you can put a sandwich baggie over the tailshaft and duct-tape the heck out of it to seal it up. But the way my luck goes, I always tear the baggie as soon as it's full of oil, so I use the yoke...


The C1 Vettes have a small, removable tranny crossmember, which makes transmission removal much easier than the C2/C3 cars with the non-removable crossmember. The bolts are simply removed from the crossmember:


...the transmission is supported and raised up off the crossmember:


...and the crossmember is removed:


A note about engine support after transmission crossmember removal: The C1 Vettes use “front” engine mounts. Unlike the C2/C3 Corvettes, you cannot let the engine hang completely unsupported from the front mounts. Most C1 Corvettes have the exhaust headpipes running through holes in the “X” frame right at the transmission crossmember. If the headpipes are secure and tight at the exhaust manifolds, the engine will be firmly supported by the headpipes in the frame holes:


If you are not comfortable with the headpipes taking the engine tilt load, you will need to put a support under the engine’s oil pan to hold the engine level.

With the crossmember out of the way, the shift linkage can be removed. I like to unbolt the levers from the transmission to get it all out of the way. Note that the 1-2 and 3-4 levers go "up," while the reverse lever goes "down":


Shift linkage hanging free, and speedo cable removed from the transmission:


The 2 bolts holding the Hurst shifter to the transmission can then be easily loosened, and with the shifter ball removed from the shifter, the entire shifter with linkage attached drops right out the bottom of the car:


The 4 bolts holding the tranny to the bellhousing can now be removed:


The tranny then slides back until the input shaft clears the bellhousing:


...and drops clear out of the car. The C2/C3 transmissions cannot be removed this easily, because they cannot be slid back far enough for the input shaft to come clear of the bellhousing. In C2/C3 cars with non-removable crossmembers, the tranny must be unbolted from the bellhousing, and then the bellhousing must be unbolted from the engine. The tranny and bellhousing must then be "jack-knifed" out of the car together. But the C1 will drop clean - tranny is out and on the tranny jack:


With access to the throwout bearing on the clutch fork, a quick inspection shows that the geometry of the clutch fork is way off: In the "pedal up" position, and the throwout bearing just touching the pressure plate, the "face" of the clutch fork holding the bearing should be flat and perpendicular to the centerline of the vehicle.

Correct clutch fork alignment:



Incorrect clutch fork alignment, as is the case in this car:


In this case, a very long clutch fork rod has been installed, adjusted to the end of its adjustment, putting the clutch fork at a significant angle before the clutch is ever depressed, as shown in the “incorrect” sketch above. Putting the throwout bearing past centerline in this position causes all mechanical leverage to the clutch to be lost. This can be caused by any combination of the following: Wrong throwout bearing length; wrong clutch fork pivot ball height; or wrong/defective pressure plate:


The bellhousing-mounted starter can now be unbolted from the bellhousing. The starter is not removed from the vehicle - it just rests in place after being unbolted (this car has headers, but on stock cars, the starter can still be slid forward and secured in place without the need to un-do wiring and drop the starter all the way out):


The bellhousing can now be easily unbolted using a swivel socket:


Bellhousing coming out:


Ready for pressure plate and clutch removal, and things aren't looking right....


Not related to clutch disengagement, but notice that the engine block is missing one bellhousing alignment pin. The alignment pins actually take the rotational shear load induced by engine torque, so it's important to have them installed:


Flywheel is badly heat scored, but only on 1/3 of the swept area... Hmmmm...


Pressure plate is also burned:


With the flywheel removed from the engine, the flywheel, clutch and pressure plate are re-assembled on the bench to check things out. Confirming that "something didn't look right" in the photo above, notice that one of the 3 clutch fingers (the one at the top in this photo) is depressed far lower than the other 2. This explains why the heat burning/discoloration is only on 1/3 of the pressure plate disc circumference:


It's now the finger on the right - it's almost 1/4" lower than the other 2:


The clutch "fingers" have actually been touching the clutch disc at the spring "bulges" during full depression of the clutch - notice the shiny wear marks:


The throwout bearing should sit flat and level across all 3 fingers:


Typical "exact reproduction" pressure plate and clutch being sold by many of the "restoration" parts houses - watch out for this stuff...


With all the parts out of the car, the clutch linkage is then re-installed with the Z-Bar, return spring, bellhousing, clutch fork rod, and the clutch fork. It is imperative to check and verify that the car itself has adequate clutch linkage travel to fully disengage a correctly-spec'ed pressure plate. A correctly-operating pressure plate will achieve full clutch disc release (defined as .050" of clearance between the clutch disc and the pressure plate surface) at .480" of throwout bearing movement. So the linkage in the vehicle must be capable of moving the throwout bearing through at least 1/2" of travel. If the throwout bearing can't move a full 1/2", no clutch system is ever going to work correctly.

So with a helper in the car to depress the clutch pedal, a bubble-level was clamped to the forward surface of the clutch fork (to assure that it was measured in the same vertical position in both "pedal up" and "pedal down" positions). A steel scale was clamped across the tranny hole in the bellhousing to provide a datum surface for the measurements. In the "pedal up" position, the throwout bearing mounting surface on the fork was 1.10" from the surface of the scale. With the clutch pedal fully depressed, the measurement was 1.70". This means that the clutch linkage in the car has a travel range capability of .600", which is more than enough to make a good clutch work correctly.


So what does all this mean? We have discovered several facts:

• The clutch linkage in the car is operating correctly, and will function right with a good clutch.
• The clutch fork pivot ball is incorrect (too short), and is causing the wrong geometry for the throwout bearing fork.
• The pressure plate is defective.
• All friction surfaces are burned, and the flywheel will need to be turned.

To test and verify that the existing pressure plate is faulty, the entire flywheel/clutch/pressure plate assembly is set up in a hydraulic press with the throwout bearing. A dial indicator is placed on a lever arm to show total bearing movement. If the pressure plate is good, there should be a .050” gap between the pressure plate and the clutch disc (and the disc should be free to slide around) at less than ½” of bearing movement.

At .500” of compression, the clutch disc has still not disengaged, and the disc cannot be moved:


At .600” of bearing travel, the pressure plate fingers are smashed hard up against the clutch plate spring enclosures:


At this point, the clutch is starting to release, but has still not achieved the required .050” gap. The disc can be moved with some effort:


This verifies that the pressure plate is no good – it does not disengage fully from the clutch before the plate fingers hit the clutch disc. This pressure plate must be replaced.

The next thing to check is the pilot bushing clearance to the transmission pilot shaft. If the clearance is too sloppy, the pilot shaft with “rattle around” and “chatter” in the bushing. This chatter will cause the rotating crankshaft to “grab” the tranny input shaft, and it will continue to rotate even when the clutch is depressed. This will make the transmission grind going into reverese, and it will be difficult to shift into 1st gear at a standing stop, I pilot bushing that is too tight will cause the same issue. The clearance between the pilot shaft OD and the bushing ID should be .001” - .004”.

To accurately measure this, you need accurate measuring equipment. You cannot accurately measure an ID within one one-thousands of an inch with dial calipers. I use a digital ID micrometer with 3 measuring prongs like this:




Inserting the ID mic into the pilot bushing provides a very accurate measurement:


After several measurement and repeats to assure a correct measurement, the ID of the bushing is determined to be .5910”


The tranny pilot shaft is measured with a micrometer. Its diameter is determined to be .5895:


That gives a clearance of .0015, which is right in the tolerance range. There is no need to replace this pilot bushing – it will be cleaned up, polished, and lubricated prior to re-assembly.

Now it’s time to have the flywheel surfaced to remove the damage caused by the bad pressure plate. Here is the flywheel being wire brushed and cleaned up on the back side to assure that it will lay completely flat on the grinding machine bed:


Grinding operations in process. It’s easy to see that the center area where the clutch disc has been slipping is dished down below the height of the outer perimeter of the flywheel:


Completed surfacing operations has the flywheel absolutely flat and uniform to assure correct clutch engagement. The flywheel is ready to be bolted back onto the crankshaft, after it is used to test and verify the new pressure plate and clutch…


Correct, original GM bellhousing alignment dowel installed in the engine block to replace the missing pin:


Once again, the flywheel is set up with the pressure plate and clutch. This time, the new clutch and pressure plate is centered up and bolted down. The assembly is loaded up into the hydraulic press, and the dial indicator is set up to measure total travel of the throwout bearing:


The new pressure plate and clutch system is verified to produce the full .050” of clutch disengagement at just .400” of throwout bearing movement. This will assure nice, clean clutch disengagement, with room to spare for pedal free travel, in the Corvette:




Since we have verified that the Corvette has linkage travel capability of .600" and that the clutch will now disengage with only .400” of travel, we know this will work. Now, we need to install and set up the diaphragm-style clutch system in the car designed for the 3-finger “Borg & Beck” style system. The 3-finger system used the “long” 1-7/8” throwout bearing, whereas the later-model “bent-finger” diaphragm systems use the short 1-1/4” bearing. Our objective is to obtain a clutch fork geometry as shown in the sketch above: The “leg”, or “face”, of the clutch fork which supports the throwout bearing needs to be flat and perpendicular to the fore-aft centerline of the vehicle. We have 2 tools to make this happen: We can work with the height of the fork pivot ball, and we can work with the length of the throwout bearing. The stock pivot ball height is 1.48”. We have an adjustable pivot ball which will allow us to raise this height by up to .200”. We have the stock 1.25” tall throwout bearing for use with the diaphragm pressure plate, and we have the tall 1-7/8” bearing for use with the flat-finger pressure plate. We will use these “tools” as needed to obtain the proper geometry of the fork, and this will assure proper mechanical advantage and “travel arc” of the fork and bearing. Here are the various components we have available for use:


Fresh-surfaced flywheel installed and ready for a good acetone wipe-down prior to installing the new clutch and pressure plate:


Clutch disc is then hung off the pilot bearing alignment tool:


…and the pressure plate is then bolted in place:


I like to hang the pressure plate very loosely initially, and then “wiggle” the alignment tool around until I find the “dead center” for any radial movement of the clutch disc. The tool, being plastic, allows the disc to “droop” a little under its own weight, and if the pressure plate is tightened down in this position, it will be difficult to get the tranny to push into the pilot bearing.

Next step is to set up and adjust (if necessary) the clutch release fork angle, as shown in the sketches above. Just based on my experience, I like to start out using a clutch fork pivot ball that’s set up about .150” taller than the stock 1.48” ball stud. First step is to measure the existing ball height. With the measurement recorded, the ball is removed. These are usually REALLY tight in the bellhousing, and will require either a very good ½” drive impact gun or a very long breaker bar. I use the long handle off my floor jack to slip over my ½” drive breaker bar to loosen and remove the ball stud:


The Lakewood Adjustable Ball is given just a dab of LocTite so it won’t back out:


It is then installed and adjusted to a height .150" taller than the one we just removed. The lock nut on the adjustable stud is also given a little dab of LocTite:


Grease it up….


…and then slip the fork onto the ball stud and install the bellhousing to the engine with 2 bolts. The “short” 1.25” throwout bearing is then located into the fork and the fork geometry is visually checked. Visual inspection shows that the fork is not perpendicular to the vehicle centerline, but angled forward as shown in the “incorrect” sketch above. This is no good. Switching over to the “long” 1-7/8 throwout bearing pushes the fork just a little too far back…


So the bellhousing is removed, and the adjustable, “tall” ball stud is removed. The old, original 1.48” ball is re-installed. Bellhousing is then re-mounted, and the fork angle is re-checked with the “tall” 1-7/8 throwout bearing. The alignment is now perfect, with the release fork at exactly a 90-degree angle to the centerline. This will produce excellent pedal feel and clutch disengagement. In this photo, the old “tall” bearing was used for checking. I now need to order a new “tall” bearing for actual installation into the system:


But things don’t always go smoothly… During the initial disassembly operations, I had not paid any attention to the condition of the clutch release fork. It was only when I was handling the fork and installing it for the above alignment checks that I noticed that the fork is bad. It has 2 problems… First, the one anti-rattle clip “ear” is broken off, and the other ear is severely worn. If this fork is used, the throwout bearing can “rattle around” fore-and-aft when the clutch is released, and I don’t like that:


The fork also has severe wear in the bearing contact areas. This is caused by the clutch being adjusted “tight” with no pedal free play in an attempt to get the clutch to release:


Moral of this: Carefully and thoroughly inspect all the parts when you disassemble them…

The new clutch release fork showed up today, and it revealed yet another problem... Here is the "original" (black) fork next to the new fork:


The new fork is the correct Corvette fork.... in various attempts to "fix" Hank's car, somebody had installed a full-sized car clutch release fork. This completely screws up the clutch geometry in the Corvette. Here are the differences:
The 1955 - 1962 Corvette fork is part number 3739043. The 1957 - 1960 passenger car fork is part number 3737454. The Corvette fork is about 9-1/2" long from the throwout bearing end to the center of the pushrod hole, whereas the full-size Chevy fork measures 10-1/8". This length difference dramatically changes clutch geometry and throwout bearing travel. The "arch height" of the Vette fork is about 1-1/2", where the "arch height" of the "454" fork is only 1-1/4":


The new, correct fork was installed into the bellhousing, which had been configured for the "454" fork with the short ball stud and long throwout bearing. A quick test fit showed that the long bearing could not be used, and the short bearing was too short with the short, stock ball stud:


So the assembly was pulled back out of the car and reconfigured: The adjustable ball stud was re-installed at a height .200" taller than the stock ball. The short bearing was used. This brought the geometry with the new fork right back into configuration:


With the clutch linkage hooked up, the fork can be seen perpendicular and flat to the vehicle centerline with the throwout bearing just touching the pressure plate diaphragm fingers. This is a perfect setup and geometry:


With the pedal linkage hooked up and the return spring re-attached, the clutch pedal has 3/4" of freeplay at the top with good return spring feel and action. A good starting point for final setup.

Time to get the tranny back in. I use a tranny jack to get the transmission into rough position, and then just lift the transmission into place:


If you did a good job centering the clutch alignment tool and aligning the clutch disc with the pilot bushing, the transmission will simply slip into position with just a little wiggling. If the transmission will not go all the way in, but starts to engage into the pilot bushing, do not use the tranny bolts to "draw" the transmission into place. If the transmission pilot shaft is not quite aligning with the bushing, "drawing" the tranny into position using the bolts can actually push the pilot bushing deeper into the crankshaft, and cause inadequate engagement of the shaft into the bushing. If the transmission is "slightly" engaged in the bushing, but won't go all the way in, you can actually depress the clutch pedal with the tranny partially engaged, and this will re-center the clutch disc, allowing the transmission to be slid all the way into position. Here the transmission has fully seated onto the bellhousing, and the bolts are now ready to be snugged down:


Bolts tightened down, and transmission installed:


Shifter and shift linkage installed:




Transmission mounting plate re-installed. ONce the plate is installed, the tranny jack is used to lift and tilt the transmission upwards so the crossmember can be re-installed:


Crossmember mounted, and tranny jack is removed to drop everything back into position. All hardware installed and tightened:


Driveshaft re-installed:


And, finally, the clutch rod anti-rattle spring is installed to wrap up the job (pin retaining clip still needs to be installed in this photo):


Clutch pedal has a very nice "feel" to it, and firing the car up verifies that the entire system is working as it should: Clutch can by depressed, and the tranny can be immediately put into reverse with no gear grinding. Clutch engagement is smooth and nice in every gear, with very modest pedal effort. Job is successfully complete!


Lars

Hermn59

DansYellow66

This is a great service to forum members LARS. I appreciate your thorough discussion and demonstration on fork geometry which is so critical in C2s with their somewhat short clutch pedal travel.

USMC 0802

ChattanoogaJSB

CaptainEgg

I know you work on lots of Corvettes but that is just more faith than I want to put on possible 60+ year old front motor mounts!

lars

On engines with mid-mounted engine mounts (1963 and newer), the motor mounts see significantly more load from engine torque at wide open throttle than they ever see from hanging the tranny off them... If you chose to support the engine as part of your working practice, that's great! Doing tranny R&R demo's at the GM Training Center on 1976 model year cars, we never supported the engine, but we did tell the students to remove the distributor cap. Those cars, of course, had the mid-position engine mounts.

However, the C1 cars with the "X" frames use front-located engine mounts, so things cannot "hang" as they do on the later engines. On these cars, if you do not remove the exhaust head pipes, the pipes will support the engine tilt where they pass through the frame holes at the tranny mount... there is no problem... Using the head pipes for support, the engine only tilts a few degrees after the tranny mount is removed. If you're not comfortable with using the headpipes as the tilt support, you can put a support under the engine oil pan. This has been added to my original post above. Thanks!
Lars

lars

Thread updated with sketches, additional data, progress photos, test data, and more accurate descriptions in the original posting above. I have also added words as suggested by CaptainEgg.

Lars

lars

More photos and updates just added to the original post above. New clutch parts will be here in 1 day, and we'll then do a test of those new parts prior to installing the parts and re-assembling the car. On the home stretch now...!

Lars

lars

Original post above just updated with test results of the new clutch and pressure plate. The installation and fork geometry setup will be done tomorrow, so watch for the wrap-up of this job...

Lars

lars

More updates, progress, and photos, and a little bit of a setback... See the ongoing story in the original post above...

Lars

Donald #31176

Lars,
I have a question concerning the throwout bearing.. I have a 62 like the one your working on. In my case I was using an 11 inch diaphragm clutch which worked extremely well except for the fact that the throwout bearing was in constant motion with the engine running. After about a year the bearing would dry out and start to squeak which necessitated replacment. I went back to a three finger clutch with no problems but I would have preferred a diaphragm type. My question is have you ever run into this problem and is there a solution...

lars

If the bearing was in motion, it means that there was no free play in the linkage. Why was the linkage adjusted that tight? Did you have to have it adjusted that tight in order to get full disengagement? If so, the pressure plate was not a well-spec'ed plate, and did not disengage within the 1/2" travel distance as demonstrated above in the measurements, which would then not allow proper linkage freeplay to be set. Was there another reason the bearing was touching the pressure plate fingers..?

Lars

ghostrider20

Donald #31176

Thanks for the quick & detailed reply.. Its been a while but I remember the clutch engaged about 2 inches off the floor and there was some free pedal . Always bugged me why.. Again thanks for your time.

lars

If there was free play in the linkage, the throwout bearing should not have been touching the pressure plate fingers. Not sure I understand why the bearing would be spinning and wearing out if it was not touching the pressure plate. Odd things like this is what justifies the **** checkouts and measurements I'm doing in this write-up: These cars are old, and many people have done odd things to make the systems not work correctly. As you can see. it's a matter of being very meticulous in checking and measuring every aspect of the system operation. Only then can you be assured that everything is going to work as intended. Especially when using modern parts that were not intended to be used in these old cars (diaphragm pressure plate in a pre-'63 car).

Lars

lars

More problems discovered and solved with the arrival of the new clutch fork... photos and more narrative added to the original post above. No wonder people were having trouble getting Hank's clutch to work... Hoping to be on the home stretch with re-assembly now.

Lars

69ttop502

lars

The crossmember in the car appears to be a new reproduction replacement, and there were no shims anywhere. Any original shims that could have been there at one time have probably been missing for about half a century... This car is typical of most of these older Vettes, in that it's been messed with by a lot of people who should never be working on cars... Once the car is running, I can check it for driveline vibration and see if there could be a shim problem.

Lars

Kerrmudgeon

Thanks Professor Lars. Another great tutorial in the books. Is it time for beer yet?