Clutch - Throwout Bearing Problem (I think?)

[James OLC]

I am going to be the first to admit that clutches (et al) are not my strong suit to begin with and the T56 and hydraulic TOB in the '67 is the first one that I have ever done (besides servicing in my old WS6).

That said, as a lot of you know I was having issues downshifting the car car at the optima challenge - the car would upsift fine but when I tried to do higher speed/rpm downshifts the transmission wouldn't slip into gear. Heck, it wouldn't be forced into gear either... some times it would go with a little more aggressive blip but not always.

Opinions at the time ranged from transmission (doubtful - new by Rockland), to fluid (it was full), to clutch/TOB engaugement (possible), and hardware issues (but after inspection nothing is loose, backed out or missing).

So, out comes the transmission. At first look everything appears to be ok - nothing is loose or appears unusual. So, following some instructions I found on the RAM site I take some measurements to see if maybe the TOB needs a shim (which was one of the more common suggestions up front).

I measured the distance from the clutch fingers to the bellhousing mounting surface - 2.25".

I measured the distance from the compressed TOB bearing surface to the plane of the transmission mounting surface - 2.4"

It would seem to be (and as admitted, I am a far cry from an expert here) that the TOB isn't compressing enough and that based on these measurements there is pressure on the clutch fingers all the time.

so...

Does anyone have any suggestions, ideas, or recommendations on what these measurement mean and what remedy there may be?

My first thought was maybe bad TOB?



Any help would be appreciated.

Thanks,
James

[CarlC]

James,

You may want to check out the thread in the Transmission section started by a member with the same concerns. There is feedback by both sponsors and members that may be helpful.

Tell us more about the type/brand/model of the bellhousing, clutch, TOB, input shaft, etc. 0.250" is a ton of freeplay.

[James OLC]

James,

Tell us more about the type/brand/model of the bellhousing, clutch, TOB, input shaft, etc. 0.250" is a ton of freeplay.

I was going to do that Carl but ran out of time...

For the record:

McLeod SFI LS bellhousing
McLeod RST dual disc clutch
McLeod aluminim LS flywheel
Stock GM hydraulic throwout bearing (new but maybe refurbished)
Rockland Standard T-56 - Son of Tranzilla

[vp23271]

James,

I think this is the post CarlC was talking about?
https://www.pro-touring.com/forum/sh...ad.php?t=51232

My 68 is still in the build process, but I am very curious to what you find out because I have already purchased the same components as you and will most likely have the same problem. Keep us posted and hopefully it's a quick cheap fix.

[James OLC]

I read through that one after Carl mentioned it... It's got a ton of good information there but I'm not sure that it is the answer.

From what I gathered from the linked post, it would appear that the clutch fingers are (unusually?) close to the mounting flange of the bellhousing and that I would need a particularly "shallow" (for lack of a better word) TOB to work properly. I can see that but am having a hard time understanding why this combo would be any different.

Maybe the GM TOB (which I bought as "new" at NAPA but was later told was probably refurbished) isn't compressing enough and is bad. Or it just won't work.

The answer is out there... I just need to find it.

I've called a couple of local guys here but both times the first question that they had for me was "you have a what with a what in a what?"

[gmorris]

I've called a couple of local guys here but both times the first question that they had for me was "you have a what with a what in a what?"

That's funny because it's so true. Nobody around here is useful for anything but bone stock repalcement parts...and even then they blow it half the time.

[hotrdblder]

i have heard horror stories about napa/etc tb's, you need a gm as new part, not worth the hassle. well worth the 150 bucks
jake

[James OLC]

i have heard horror stories about napa/etc tb's, you need a gm as new part, not worth the hassle. well worth the 150 bucks
jake

Yeah Jake, I hear you, unfortunately, life in the great white north is rarely as simple as that. Somehow (and don't get me started on this scam) GM wants $320 bones plus for one... usually though, they tell you they don't have any in stock and that they will have to order one... from the States... it's freaking unbelievable.

Anyways, I have heard the same thing about the refurbished ones.

[parsonsj]

James,

I think you're not getting a full release. Whether or not the the TOB is your problem is something you can measure.

(this is an Andrew Borodin trick): with the TOB out of the car and hooked up to the master, see how much the bearing moves with a normal clutch depression. It will be somewhere between .375 and .750 (dunno the GM bearing). With that information, call McLeod. Find out what that clutch needs.

Another way (with the car together): put the tranny in 3rd gear. Have a helper push the clutch in. See if you easily rotate the drive shaft by hand.

Let us know what you find. All of us are searching for the right combination.

From a recent post of my current solution:

I was set to use an LS7 clutch in II Much, and so I talked to Tom over at Carolina Clutch. He walked me through my setup (600 hp, TKO-600 road race version, 3.25 rear gear, rpm ranges of my cam, etc.), intended usage (mostly street, but optimized for big road courses, some autoX, couldn't care less about drag racing), and size of my wallet.

He told me that the LS7 clutch would hold my power, have a light pedal, and, in general, be pleasant to drive. But... he also said that it made shifting at high rpm harder due to its 60 lb. (with flywheel) weight. That's exactly how my LS6 clutch felt to me: nice street clutch, but it made shifting difficult at high rpm on the track and autoX.

So I bought an RPS Street Twin. It weighs 40 lbs with flywheel, and has carbon surfaces on the mid-plate and the corresponding disk surfaces. The pressure plate and flywheel are steel surfaces with organic material on the corresponding clutch disk surface.

Tom told me it would be nearly as easy to shift as the LS7 clutch, and would have much better high rpm manners, and would hold my power levels without problem. So far, that's been exactly right. I've only got about 50 miles on the clutch, but it's easy to drive, and high rpm shifts are a snap. I haven't pounded on it yet, but so far I think I made the right decision.

The downside: at $1800, it costs 3x the cost of an LS7 clutch. :(

jp

[Vegas69]

John...I was thinking the same thing about rotating the driveshaft in neutral. To late now...the tranny is out from my understanding.

[James OLC]

Thanks John - I'm going to do some bench testing on the TOB today. More than a few people I spoke to yesterday mentioned that the stock GM unit is sensitive to the MC and throw. Given the tilton pedal assembly (read adjustable cluch position) and MC there might be insufficient hydraulic force(s) there.

[fabhammer]

http://www.thedrivenman.com/ call this guy

[James OLC]

So... couple of things already this morning. First is that I am at a loss as to how to properly gauge the throw of the TOB in it's "dissassembled" state - at least by myself. I'll get some help this afternoon because it would seem that (with the stock GM TOB) you need one person to "compress" the TOB to it's in situ depth while the other person presses the clutch pedal. Without the TOB compressed, pumping the clutch pedal has no effect other than to (eventually - with multiple pumps) pressurize the complete slave to it's full extended (dissassembled at rest) depth.

Second, I pulled the clutch out (leaving the bellhousing in place for simplicity at this point. I didn't see or find anything that stood out to me but I took some pictures of the individual components for your input:

The Flywheel and base ring:


The Pilot Bearing:

[James OLC]

Flywheel Disk (back or Flywheel side):


Flywheel Disk (front or pressure plate side):


Pressure Place (back or flywheel side):



Pressure Plate (front or clutch side):

[James OLC]

Clutch Disk (back or pressure plate side):


Clutch Disk (front or clutch side):


Clutch Back (disk side):


Clutch Front (transmission side):

[James OLC]

If anyone sees anything or wants more detail on anything let me know. Like I said, nothing obvious to my (inexperienced) eye.

There is probably 100 miles total on the clutch - some polite street miles, some sort-of inpolite track miles...

And for the record - checked the transmission fuild level - it was completely full and clean with no indication of contamination.

[andrewb70]

There is nothing obvious that stands out from the pictures. Looks like a clutch with about 100 miles on it.

The stock GM TOB is hard to diagnose because as you have found it is spring loaded in such a way that it is always in the extended position. When the transmission is installed, the bearing is in contact with the fingers of the clutch spring and is compressed to installed height.

What size MC are you using? Approximately how much stroke are you getting at the MC?

Andrew

[James OLC]

I just called my local GM dealer to get a new slave/thowout bearing... $377.50 - not in stock. They asked if I wanted to have one brought in... lol

Anyways, new one is on it's way from Summit ($125 plus shipping) just in case.

Found an interesting article regarding hydraulics in the master-to-slave line: Chevy High Performance

I am going to redo my line so that it is -4an "thoughout" (although the master cylinder output is -3an thread) and will check the stock portion of the line that remains for the restriction mentioned in the CHP article.

I have the option of changing out the MC in the future if need be but I'll go down that road only if I have to.

Other than that, I am out of ideas...

[Vegas69]

Why not a McLeod instead of GM?

[James OLC]

Right now I think that McLeod would be one of my last choices for anything - based solely on customer service.

I did look at the McLeod TOB and it has a couple of negatives - cost and ongoing required adjustments - which made it less appealing.