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Old 08-16-2005, 10:13 AM
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Throwout bearing for BB Chevy.

Does anyone know how to determine if a "long" or a "short" throwout bearing is needed for a Chevy clutch setup? I swapped a BB into my 70 Z-28 using the SB bell housing, throwout fork, and 4-speed. I installed a new Ram flywheel, clutch disk, roller type pilot bearing, and pressure plate. I'm having trouble (grinding) going into reverse but only after a few miles of driving. When first starting the car it can easily go into reverse but again, after a few miles of driving, it grinds (rather severely) going into reverse. I've adjusted the clutch to the point where there's little play left in the pedal. I'm wondering if I was sent the incorrect bearing. Also, when the clutch pedal is depressed there's a noticeable and audible vibration.

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Old 08-16-2005, 11:18 AM
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If I remember correctly the only difference for the long and short throwout bearing is the fork arm...? One uses the short and the other the long bearing...? As for your problem it sounds more like heat and release problem...? Some of the high performance materials in clutchs tend to expand more when hot also this is a new setup so it may take a few mile before it settles in...? It helps to put it in a forward gear to stop the trans and then move it quickly into reverse so it will not grind the gear...?

Also if it works good cold then the length of the throwout bearing is correct... But be sure to have the 3/4-1" of free play setup...
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Old 08-16-2005, 11:35 AM
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There's about 7/16 or so differance is lenght between the bearing types. the fork is the same. Usaully the longer pivot ball is used to correct geomtry's with the long bearing.

Are you saying the trans will engage reverse after starting but not after moving it? If it will engage first smoothly, I blame the trans not the clutch.

Try engaging 1st or second, then reverse. If it goes, I blame the trans. If it doesn't I would blame the clutch. I would start looking at your fork geometry's really close. It sounds as if you are using up a lot of bearing movement to take up for wrong angles.
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Old 08-16-2005, 12:39 PM
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I think my freeplay at the pedal is about an inch so I guess I'll take it down to the 3/4 inch and see if that works. I did have more play and I removed some and the grinding did improve. Prior to that there was no way it would go into reverse without shutting the engine off and now it can be forced in with some grinding. Maybe it's headed in the right direction.

I don't think it is a tranny problem because it didn't exist until I did the engine swap. I had the tranny rebuilt about 5 years ago and there hasn't been too many miles on it since. It's only driven in summer and only a day or two a week. That was the first rebuild since the car was new (1970).
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Old 08-16-2005, 02:26 PM
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I just went out and measured the freeplay at the pedal and it is exactly 3/4 of an inch. Any suggestions of what I can do next short of pulling the tranny and clutch?
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Old 08-16-2005, 03:24 PM
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I did.nt really mean internal trans problem. I was thinking more along the lines of the distance from the crank mounting flange to the clutch fingers has changed. This is called stack height. Maybe the stack height has changed with the (remanned?) clutch. Then you would be using some of the pedel/slave just to reach the return point for the bearing as measured from the front bearing support.

You only need 1/4 or so of play.

I would adjust the slave or bearing to return 1/16 to 1/8 from the diaphram fingers.
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Old 08-16-2005, 05:58 PM
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I'll toss this in just incase. If I remember my old clutch days, there were also to bearing "faces" used too. The flat face bearing was used on a 3 finger pressure plate. There was also a moderately rounded face used on the diaphram type pressure plate. Using the wrong one caused engagement and dis-engagement problems.

Mark
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Old 08-17-2005, 06:14 AM
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I have the three-fingered pressure plate and since I bought the entire setup at the same time I assume they gave me the correct bearing. I'd try reducing the freeplay to 1/4 of an inch but I'm still concerned about the vibration.
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Old 08-17-2005, 07:37 AM
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Quote:
Originally Posted by Jmark
I'll toss this in just incase. If I remember my old clutch days, there were also to bearing "faces" used too. The flat face bearing was used on a 3 finger pressure plate. There was also a moderately rounded face used on the diaphram type pressure plate. Using the wrong one caused engagement and dis-engagement problems.

Mark
And a really stiff pedal. I blew up a clutch one race night out west and all I had was a flat faced bearing to replace it a rounded one with on a 5.5" triple disk clutch. Before the days of Bert and Brinn's. It about killed my leg, sometimes needing both legs.
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Old 08-17-2005, 03:15 PM
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the "vibration" statement got me thinking again, (maybe not a good thing!).

Back in my early days, I had a Vette come in the shop with a horrible vibration when you pushed on the clutch pedal. Everything was brand new. Come to find out the T/O bearing had a larger inside diameter than the nose diameter on the tranny. Did it slide on nicely or did it fall on and wiggle around? Just another thought.
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Old 08-18-2005, 10:43 AM
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I'm not sure how the bearing fit over the tranny; my brother installed it while I was doing something else. However, I did talk to Summit last night and although the Ram specs indicate that the bearing I have is the "most commonly used" one the McCleod literature seems to say that it is not the right bearing. The guy from Summit is going to check with Ram to see if they can determine wether it is correct for the clutch and pressure plate or wether I need a different one. I suspect I'm going to be pulling the tranny in the next few days. Seems I've spent so much time under the car lately that the back of my head is getting flat.
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Old 08-18-2005, 11:41 AM
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It seems to me that if you have to adjust the pedal freeplay to 1/4" it would suggest that there is a lack of travel on the clutch pedal. Perhaps the longer bearing would eliminate this possibility. The throwout bearing also acts like a brake on the input shaft at the opposite end of it's stroke. If you have experienced improvement with less free play and more travel, I think it warrants a little more back time and practice at pulling your drivetrain yet one more time....getting pretty good at my vehicles unfortunately! The vibration also concerns me, but I cannot think why that would be. Did you change the bellhousing to a scattershield? if so you may require offfset dowels to align it...the problems are most likely unrelated. Also a bunch of crud ( or pinched wire?) trapped in between the bellhousing and engine mounting flange it could upset the driveline geometry. Food for thought anyway.
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Old 08-18-2005, 12:00 PM
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Quote:
The throwout bearing also acts like a brake on the input shaft at the opposite end of it's stroke.

The bearing does not actually contact the input shaft, mearly slides on the nose of the input bearing retainer. For it to act as a break, it would have to have a physical link to the shaft.


The vibration could be that the re-man clutch is out of balance, not unheard of. Also, chech the finger height for atual distance, and uniformity. It is also not unheard of to ge a re-man with machining issues. Did you have the flywheel machined? Significant machining of the flywheel can cause release bearing travel issues.
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Old 08-18-2005, 12:34 PM
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I meant via the clutch disc via the fingers or diaphragm of course. it puts a squeeze on the disengaged clutch disc that stops it from spinning. It's something you can live with if you stab it from 2nd to rev. to stop the spinning.
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Old 08-19-2005, 05:56 AM
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The bell housing is the factory unit from my small block as is the fork. There's nothing caught between it and the block and I checked the bolts (to block and tranny to bell housing) and they're all tight.

The clutch, pressure plate, flywheel, and bearing are all new from Ram, so I don't think it's a remanufacture problem. It's sounding more and more like an incorrect length to the throwout bearing.

Someone told me I should have "dialed in" the bellhousing (centerline of flywheel to centerline of tranny hole in bellhousing). He said there can be no more that .010" runout. But I talked to a guy who works with me and races circle track. He didn't think that would be an issue. He said all the circle track guys cut the bottom off the bellhousing and hang the tranny on without a rear mount (only two front engine mounts). That would certainly throw the alignment off by more than .010".

I expect Summit to calll back today to tell me what they learned from the people at Ram.
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