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Still not a believer that it's the clutch. If I lug my truck down so low that it bucks, it does not make the noise and it has never made it at idle (no matter how rough the idle was). Mine almost completely went away when I changed to synthetic fluid but after 45,000 miles, it has slightly come back. Maybe as one of the other posters stated, the fluid will only quiet it down for about 30,000-40,000 miles. I am going to change the fluid again and see what it sounds like then.
Supposedly the South Bend has a better clamping/ holding force than the LUK and stock, how with stiffer springs/ fingers is this going to make it absorb shock better?
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Not talking about clamping force of the clutch and fingers. It is not clutch SLIPPAGE that is causing the issue. If you look at a clutch disk, you will notice that it has a center hub, surrounded by the friction material. The center hub is mounted so that it moves independantly from the outer hub & friction material for a small distance, until a mechanical link occurs. In this mechanism, there are little springs between the inner hub and outer, so that when you mash down on the throttle, the little springs give a little before they mechanical link occurs, allowing a somewhat more "gradual" transfer of power between the engine and transmission, more power transferring as the springs compress until full power is transfered by the mechanical link when the springs fully compress. It is not slippage, mind you, just allowing the center hub to move about 3/4" independantly from the outer hub. This function completes the same task as a dual-mass flywheel. It just provides a little 'give" between the engine and drivetrain to pick up power spikes, etc and dissapate them. Every clutch i have ever seen has them. It is these springs that would allow a vibration to transfer into the transmission, causing gear chatter. The better springs in an aftermarket clutch do not allow this transfer (or reduce it). In the stock clutch, these srpings are unable to dampen the vibration, so you get chatter. Take a look at a picture of a clutch disk, and you will see the springs I am talking about. It has nothing to do with clamping force or anything like that. All it is is a vibration and power spike dampener to keep the engine and tranny working together a little more smoothly.
Your better fluid probably just quiets this "chatter" down so you cannot hear it, because it lubricates better. I put good synth in mine, and it still does it...