Chevelles.com banner

Status
Not open for further replies.
1 - 18 of 18 Posts

·
Registered
Joined
·
1,525 Posts
Discussion Starter #1
I had my new M20 rebuilt locally by a shop that came highly recommended and does mostly roundy round work ( Old Man Enginerring, Buffalo NY and in NC too). Here's a tip they gave me to help improve the longevity of my Muncie, or any manual box for that matter.

Apparently at high RPM shift the throwout doesn;t always disengage the clutch fully ( I think that's what they said !?), so they recommended I drill a second hole in the Zbar for the clutch rod ( the rod from the clutch pedal ) located about 1/2" below the original. This will effectively give the clutch more travel and prevent the no disengage condition. CAUTION: The Zbar is hardened so you either have to grind the surface before you drill or use carbide. The hole can;t be too sloppy and some drills have slightly wider dia tips, so measure everything. I haven't done this yet, but it's a mod that makes sense.

------------------
 

·
Registered
Joined
·
32 Posts
Gean, very interesting,,to say the least,,ummm I admit that I dont know that much about cars really (am tying like hell though). I was just kinda thinking, could the same effect be acomplished by leagthning (modifing) the cluch adjustment rod?.... it was just a thought.. Thanks Again ... Tintop55
 

·
Premium Member
Joined
·
3,438 Posts
I think the problem they are talking about is specific to diaphragm clutches, and is not related to clutch geometry. If the clutch is adjsted wrong, then the pedal can actually over center the diapragm pressure plate, and if this happens at high rpms the centrifigal force will hold the diaphragm fingers inward and they will not release fully until the rpms come down. The right way to avoid this with this style pressure plate is to have the pedal adjusted correctly, which sometimes feels too loose at the pedal so it gets over adjusted. Check a service manual for the correct adjustment procedure. 3 finger pressure plates do not have this problem, and I believe the weights on the centerforce pressure plates also fight this problem. Think about it..if you modify your linkage to release the clutch further, it may not engage the clutch fully after the work, so you adjust it, and you are back where you started..just my opinion.

------------------
70 chevelle ss396 conv
66 chevelle ss396 hdp/conv
55 chevy prostreet
 

·
Registered
Joined
·
12,841 Posts
I initially read this wrong. Then I reread it. Gene, yes, if you drill a second hole BELOW the existing hole, for the pushrod between the pedal and Z-bar, it WILL increase the throwout bearing travel. But--------------------- as drptop states (and definitely it will cause a problem with the flat diaphragm vs the cone diaphragm pressure plate) you may end up with an overcenter problem. Maybe. I would say, drill the hole, try it, if it works, great, if not, put it back in the original hole (or maybe get another Z-bar and drill a hole only 1/4in rather than 1/2in apart). You be the testbed for the rest of us.

------------------
Tom Parsons
 

·
Registered
Joined
·
1,525 Posts
Discussion Starter #6
Geez Tom, a guinea pig for my TC bros. SURE. I'll see how difficult the job is once my car is back and get underneath her. Again, gentlemen, I don;t pretend to know why exactly this works, but I'll certainly print this post out and share it with them. They owe TC one becuase I printed out the speedo gear chart and they were much appreciative of it.

They also have a special way of setting up the shifter which I kindly asked them to let me see on the bench when they do it. Also they told me to ditch the overtravel bolts. "Teats on a bullhog" was the owners quote.

Anyway, DON'T BLAME ME. I'm just the messenger.
 

·
Registered
Joined
·
49 Posts
I wonder if my problem is related. Driving a little harder than normal and when shifting into third I let out the clutch and it sounds like the syncho teeth grind for a second. Hasn't done it yet when really getting on it.
Only does it once and a while. Trans is a m21 and has low miles since being completely redone. When this happens the shifter is definately all the way in third. Its a Hurst Comp Plus shifter. Maybe the stop bolt isn't adjusted enough? Don't mean to get off the original subject which I am very interested in but I figured now was a good time to bring it up because it may be related. Could the diaphragm be stuck for a second when this happens? The linkage is all new for the shifter and clutch
 

·
Registered
Joined
·
335 Posts
Another tip I've used over the years, seems stupid, but it works. Put the seat forward a notch or two. In the excitement of racing and trying to do everything just right, not unusual to not actually push the clutch all the way down, at least I've done that. With the seat up it helps. Free anyway.
 

·
Premium Member
Joined
·
3,438 Posts
I think I read this wrong too, sounded like they were talking about the "clutch pedal stuck to the floor at high rpms" problem, which is what I was talking about..so they are talking about changing the geometry for more pedal travel then?? Guess I was confused! And I read it twice!

------------------
70 chevelle ss396 conv
66 chevelle ss396 hdp/conv
55 chevy prostreet
 

·
Registered
Joined
·
826 Posts
I am having a problem slightly similar to 68SS396Camaro. In my case however it seems that maybe I DON'T have it all the way in third, but man is it ever hard to get it there fast enough! Should I adjust the shift rod a little to bring in the third gear engagement with less shift handle travel? Am I making sense? I really wanna be able to wing off those shifts at full throttle!
And, yeah, any tips on getting a Muncie to shift faster---or perhaps the Hurst Comp Plus, then?

Dang, sorry, I'm gettin' off the subject.

------------------
Tom H.
Member #259
Indianapolis,IN www.iquest.net/~bharold/chevelle/
 

·
Registered
Joined
·
1,525 Posts
Discussion Starter #12
Man, did I open a Pandora's Box or what???Tom3, I definitely know what you're referring to. My daily driver has the seat in kick back and cruise mode while the 67 is up close. Simply due to shift PRECISION!

Hey, I figure us 4 speed guys need every little advantage we can get.

Guys before you freak out and start drilling your Zbars, I noticed that the clutch adjustment is EXTREMELY sensitive. I didn;t come close using the factory manual as a guide, but that's prob due to my cneterforce multi fingered clutch v the stock 3 fingers.

Again, I was a complete newby at this when I tackled it so I learned quick how precise that free play adjustment has to be. A couple of turns makes it or breaks it. A far cry from the cable clutch I adjusted on my old Geo Scrotum, er Spectrum. That adj was quick n dirty.

Secondly, I've never got my shifter arms adjusted right using the guage/neutral method. And I did it as precisely as I could. Cause: my beat up M21? Maybe, but I real curious for the folks at OME to show me the right way.

Even with all the hassle, manual is the ONLY WAY TO FLY!!!

I'll keep you all posted on any other nuggets of wisdom I get from the old pros.
 

·
Registered
Joined
·
167 Posts
I have the same centerforce dual friction clutch, and a Hurst Comp plus shifter on my M20. I adjusted the shifter with the neutral alignment pin set-up when I put it in new, and never had a problem with it since, nice throw length, close but not too close gates, works great!

As for adjusting the clutch, I don't remember exactly how I determined when I thought I had it right, it's probably been 7 years or so since I installed it, but it works perfect for me, however there is about 1" of pedal travel that isn't being used. What I mean to say, is that when the pedal is just hanging there minding it's own business, waiting to be side stepped, you can actually pull it back toward you an 1" further. I have no problems with shifting, or the clutch. I do know that the engaged to disengaged distance on that clutch is shorter than the factory HP clutch I had in the car previously. Even at 6500 rpm shifts with the old LT1, no problems.

------------------
Steve

72 Chevelle SS402/4sp
 

·
Registered
Joined
·
67 Posts
I am with Steve. I have the centerforce clutch with a Lakewood scatter shield in my car. If you set the pivot point for the fork on the throwout bearing correctly you should not have to drill any additional holes for overtravel on the clutch. I too have about 1 inch of freeplay with the clutch all the way out. Adjust the hurst shifter with a pin in the neutral position and no problems shifting at 6000 rpm.
 

·
Registered
Joined
·
57 Posts
Hey Gene...I also had Old Man Engineering rebuild the 12 bolt posi in my '69 a few years ago...excellent work! I also used to work with Kenny Bainbridge's brother, Larry (he has since retired). One more thing...if you ever get a chance to hear their band, The Bainbridge Boys, it will be time well spent!

------------------
Bruce M.
Buffalo, NY
Team Chevelle #197
69 SS Chevelle
80 Malibu
members.aol.com/amhpd17/index.html
 

·
Registered
Joined
·
1,525 Posts
Discussion Starter #16
Bruce, these guys are wizzes for sure. And I'm probably just going to do everything they recommend to keep this Muncie alive, which Kenny says is a friggin hope and prayer deal with that nasty L88. He really wanted me to consider an ST10.

Moreover, they also rebuild shifters and they'll get that work too in lieu of Hurst.

I've found some real good workmanship here in Buffalo, OME, the MagShop, Denny's. Hope all the guys are as lucky as us to have such good local talent.

Where the hell is that ImpalaSS bud? Oh and I assume the other issue you brought up is now moot with your new ride? Willing to help any way I can Mr AMHPD. Door's always open for ya. 633-4372

[This message has been edited by Gene Chas (edited 12-14-99).]
 

·
Registered
Joined
·
2 Posts
For starters, if it works, don’t mess with it, right? I’ll let you decide. Like in Gene’s case and mine, I too had a slipping clutch any time I drove my car really hard.

Before you even mess around with adjusting the linkage, You need to look inside of the bellhousing. From the back of the engine block to the top of the clutch fork pivot ball should be 4.75 inches. If you have a stock setup, you could assume that this dimension is correct. However, several things that will affect this measurement are other than stock weight flywheel, after market bell housings, block plate for a blow bell, and wrong or improperly adjusted (adjustable type) pivot ball.

As far as flywheels go, manufactures will vary the thickness to vary the weight. A stock GM mechanical linkage flywheel is .960 inches thick. Variations of .200 inches can be expected with different weight flywheels which will affect linkage geometry. A thicker flywheel will be in your favor if your problem is similar to mine. Flywheel weight however, is a totally different subject.

My flywheel is the stock thickness so this is not a variable. My Lakewood blowbell measures 4.85 inches. My block plate is .150 inches thick. That’s a difference of .225 inches from 4.75 inches. Less than quarter of an inch??? Whoopdee doo right? Well, actually when viewing the clutch pivot fork, it pivots at about a 2:1 ratio. So we’re really off by 2 times .225 inches which is almost have an inch. I’m losing have an inch of throw due to an improper height pivot ball. To correct this problem, Lakewood makes an adjustable pivot ball that can be shortened or lengthened to make up for .100 inches. GM has a truck one that is not adjustable but is .300 longer than stock.

Trying to make up this difference by adjusting the linkage further out will cause the lower Z-bar arm to travel more than it was intended to. Basically if this lower Z-bar arm passes the vertical plane towards the rear of the car (this is when your clutch should be fully disengaged), this will force the clutch pivot fork upwards which in turn will cause premature throwout bearing failure.

There's one more thing but I'm not sure about it. I've seen different length throwout bearings. I can see how having the wrong one could affect how your clutch works. Use what you have? I'm not even sure what mine came stock with. Comments???
 

·
Registered
Joined
·
1,525 Posts
Discussion Starter #18
Steward, I don't seem to have any slippage problems but I will admit that a full powershift into 2nd gear ( at 6500 ) causes some slipping somewhere and I can't yet determine if it's the clutch or the tires. If it's the clutch it's adisengaging problem becuase the Centerforce seems really tight all other times. So I'm tempted to relocate the clutch rod lower as recommended.

I actuially posted this as a matter of info to members is all.
 
1 - 18 of 18 Posts
Status
Not open for further replies.
Top