Clutch Capers and Flywheel Follies
02-13-2016, 12:18 AM
#1
Melting Slicks
Thread Starter
Clutch Capers and Flywheel Follies
Ok... With the thought that I've had my '65 for about 48 years and have changed the combination of engine/trans/clutch/rear end every few years, I recall doing a parade with our corvette club with the 327/365 L76 and a GM L88 flywheel and clutch. My left leg was shaking after that episode - 3000 lb of clutch was way too heavy for my tastes in my senior years.
New combination: I had a new Gen 1 SBC (358 cu in) that I built recently with with a Comp Cams Nostalgia 30-30H using Rhoads V-Pro lifters. Compression ratio is 10.2:1 with old school 492 angle plug heads, heavily ported, and an Edelbrock Performer RPM intake with Holley carburetor. To complete the change of configuration, I added Borgeson power steering and a new PRW 20 lb steel flywheel (SFI approved), with a street performance Valeo clutch for lighter pedal forces.
Being meticulous, I made sure everything was right - however, the new flywheel appeared closer to the back of the block than previous. I measured; the L88 ductile iron flywheel and the previous L76 30 lb iron flywheel both measured 1 5/8" from the back of the block to the surface of the flywheel. However, the new PRW flywheel was only 1 3/8" away.
Measuring the new clutch... The old L88 clutch, with a slightly worn clutch disk clamped between, measured 2 1/2" from the flywheel face to the tips of the clutch fingers. Unfortunately, the new clutch measured only 2 1/8" from flywheel face to the clutch finger tips.
Adding the two dimensions together produced 3 1/2" versus 4 1/8" of the stock GM clutch and flywheel. In other words, I would need to lengthen the clutch fork pivot stud by 5/8" to make up for the dimension changes in order to make the clutch linkages work correctly!
Taking a look at my engine bay, I had pulled the engine by separating it at the bellhousing, leaving the trans and bellhousing in the car. And, as might be expected, the clutch pivot ball stud can only be removed by unbolting the trans from the bell housing! And I didn't want to crawl underneath, remove the shifter linkages, etc. to get at the 4 trans bolts.
Ok, more thought over a cool one (or two)... Yes, the new clutch has bent fingers and would normally take the short throwout bearing, BUT, the difference in thickness of the flywheel and pressure plate is exactly 5/8", and so is the difference between the short and long throwout bearings! Bingo! Just order a new long TO bearing and the measurement will be the same. This places the outside end of the fork in the same position as used previously! BTW, the corvette short bearing is 1 1/4" long, the early "long" bearing is 1 7/8" long - and there is the 5/8" difference.
When I received the new "long" bearing, it had a raised ring on its contact surface instead of the flat surface of the shorter bearing. I compared the contact pattern of the fingers to this ring and found that the wide flat spot of the fingers matched the ring and would not cause any problems.
Testing has gone well - the clutch pedal force is lighter and very reasonable; yet the clutch holds the torque of this strong small block very well. I'm happy!
If any of you change flywheel or clutches, measure these dimensions and adjust accordingly. The fact that dimensional changes can be solved by a different TO bearing length instead of a different clutch fork pivot ball stud can save someone considerable frustration!
New combination: I had a new Gen 1 SBC (358 cu in) that I built recently with with a Comp Cams Nostalgia 30-30H using Rhoads V-Pro lifters. Compression ratio is 10.2:1 with old school 492 angle plug heads, heavily ported, and an Edelbrock Performer RPM intake with Holley carburetor. To complete the change of configuration, I added Borgeson power steering and a new PRW 20 lb steel flywheel (SFI approved), with a street performance Valeo clutch for lighter pedal forces.
Being meticulous, I made sure everything was right - however, the new flywheel appeared closer to the back of the block than previous. I measured; the L88 ductile iron flywheel and the previous L76 30 lb iron flywheel both measured 1 5/8" from the back of the block to the surface of the flywheel. However, the new PRW flywheel was only 1 3/8" away.
Measuring the new clutch... The old L88 clutch, with a slightly worn clutch disk clamped between, measured 2 1/2" from the flywheel face to the tips of the clutch fingers. Unfortunately, the new clutch measured only 2 1/8" from flywheel face to the clutch finger tips.
Adding the two dimensions together produced 3 1/2" versus 4 1/8" of the stock GM clutch and flywheel. In other words, I would need to lengthen the clutch fork pivot stud by 5/8" to make up for the dimension changes in order to make the clutch linkages work correctly!
Taking a look at my engine bay, I had pulled the engine by separating it at the bellhousing, leaving the trans and bellhousing in the car. And, as might be expected, the clutch pivot ball stud can only be removed by unbolting the trans from the bell housing! And I didn't want to crawl underneath, remove the shifter linkages, etc. to get at the 4 trans bolts.
Ok, more thought over a cool one (or two)... Yes, the new clutch has bent fingers and would normally take the short throwout bearing, BUT, the difference in thickness of the flywheel and pressure plate is exactly 5/8", and so is the difference between the short and long throwout bearings! Bingo! Just order a new long TO bearing and the measurement will be the same. This places the outside end of the fork in the same position as used previously! BTW, the corvette short bearing is 1 1/4" long, the early "long" bearing is 1 7/8" long - and there is the 5/8" difference.
When I received the new "long" bearing, it had a raised ring on its contact surface instead of the flat surface of the shorter bearing. I compared the contact pattern of the fingers to this ring and found that the wide flat spot of the fingers matched the ring and would not cause any problems.
Testing has gone well - the clutch pedal force is lighter and very reasonable; yet the clutch holds the torque of this strong small block very well. I'm happy!
If any of you change flywheel or clutches, measure these dimensions and adjust accordingly. The fact that dimensional changes can be solved by a different TO bearing length instead of a different clutch fork pivot ball stud can save someone considerable frustration!
Last edited by larrywalk; 02-14-2016 at 01:20 PM. Reason: Corrected spelling of Rhoads
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65BlkVrt (02-13-2016)
02-13-2016, 07:47 AM
#2
Race Director
Well done - good example of researching and solving a problem rather than just throwing parts and more parts at it.
02-13-2016, 09:49 AM
#3
Team Owner
And love the title of the thread...pays to keep a sense of humor about these cars....
02-13-2016, 10:11 AM
#4
Team Owner
Member Since: Nov 2005
Location: Beach & High Desert Southern California
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I just went through this on my 63.
The same surprise when the PRW flywheel & Centerforce clutch assembly ended up shorter. Comparing the old and new assembly revealed a rough difference between 1/2 & 3/4".
The quick solution, cutting and lengthening the clutch rod, with a stiffener sleeve. We mocked it up on the car with the sleeve and vice grips before deciding on the final length, and welding it solid. We adjusted for the best pedal feel. The leverage point at the clutch fork may be different, but so far it works good.
The same surprise when the PRW flywheel & Centerforce clutch assembly ended up shorter. Comparing the old and new assembly revealed a rough difference between 1/2 & 3/4".
The quick solution, cutting and lengthening the clutch rod, with a stiffener sleeve. We mocked it up on the car with the sleeve and vice grips before deciding on the final length, and welding it solid. We adjusted for the best pedal feel. The leverage point at the clutch fork may be different, but so far it works good.
Norm