Why does my clutch linkage keep breaking?
 

Over the 10 years I've owned this truck, I have had numerous problems with my manual clutch linkage.

I had recently upgraded to heim joint linkage I purchased from LMC.
First thing that broke was the little wedge nut that sits in the groove on the fork, snapped in two.

After that, the bottom push rod snapped where it pushes on the clutch fork.
So I put the old rod back in.

The stock clutch rod just snapped this morning, in the same place, where it meets the clutch fork.

What could be going on here? Has anyone had the same problem?
Take note, there's a pattern here, they both broke first thing in the morning, in cool weather.
Could the exhaust heat be weakening the metal? The header runs right under it.

It's a very worked 350, 4 speed with a Centerforce stage 2 clutch, so the clutch pedal is pretty stiff.

Re: Why does my clutch linkage keep breaking?
 

The only thing LMC has is cheap chinese crap. Low carbon steel work hardens then breaks when it's cold.

Re: Why does my clutch linkage keep breaking?
 

Probably the case, but the stock linkage broke too.

Re: Why does my clutch linkage keep breaking?
 

Sounds like it is over traveling against the pressure plate and breaking at the weakest link. My old 67 had a piece of threadall with a couple of jam nuts that never broke. Threadall is not known for being very tough. It does not work very well for shifter linkages in my experience.

Re: Why does my clutch linkage keep breaking?
 

Have you checked you motor mounts? The engine could have excessive movement causing a binding problem.

Re: Why does my clutch linkage keep breaking?
 

the z bar might be wallered out pretty bad too

Re: Why does my clutch linkage keep breaking?
 

Thanks for the advice, Z bar and motor mounts are still ok, sounds like burnin oil hit the nail on the head. But what causes it to over travel? The clutch and throw out bearing seem to be fine.

Re: Why does my clutch linkage keep breaking?
 

It's either one or the other. Something is moving and something is not which causes failure at this point. You might chock up both axles in front and behind and try to move forward in 3rd gear while someone (in law or other) is underneath watching of course in a position not to be run over. A lot of times something will look fine (perfect) sitting still, until a heavy load is placed on it. You should see what is allowing the movement.

Re: Why does my clutch linkage keep breaking?
 

Hahaha (in law or other) Good point, cooper, thanks. I will definitely try that.
I think I may have to make something stronger to replace it though.

Re: Why does my clutch linkage keep breaking?
 

Over travel or wrong throwout bearing can cause damage to the pressure plate. Where does the clutch grab? If it is really high on the pedal and you push the pedal to the floor I am betting this is the problem. Like someone else said funny things happen in operation. My trucks clutch pedal refuses to return all the way up and will let the throw drag if you don't tap the pedal up with your toe. I unhooked the linkage and everything is smooth in the pedal and has full returns. Zbar and linkage seems fine and not to worn considering age. Hook it all up and the bearing will drag. Sometimes you need to look real close.

Re: Why does my clutch linkage keep breaking?
 

Well, the linkage is adjustable, so where it is now it grabs in the middle, with about an inch of play in the pedal. I'm going to check out the movement of everything underneath.

Re: Why does my clutch linkage keep breaking?
 

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Also check the clutch fork and make sure it isn't hitting the bell housing when the clutch pedal is depressed. If it is, only a couple of things can cause this, either the ball stud inside the bell housing is broken off or missing, or the throwout bearing is too short. I have seen clutch forks worn bad enough that the ball stud had worn through the clutch fork socket also.

LockDoc

Re: Why does my clutch linkage keep breaking?
 

You are getting some really good advice on things to check. All of which should be checked. Keep checking until you find the culprit. As to installing a stronger/unbreakable rod, that the symptom, not the cause. Like a plug that keeps fouling. You can put a better/hotter plug in, but the cause of the fouled plug will continue to be there. I am thinking one of your rubber mounts is split. Sitting there not moving it looks fine, but chocked and under load it would allow the motor/trans to raise to a point that shears/breaks the rod.

Re: Why does my clutch linkage keep breaking?
 

All good points, thanks, I'll be looking over everything. However, the engine and trans have 5 mounts, all seem fine. 2 on the engine, 2 on the bell housing, and 1 on the trans.

Re: Why does my clutch linkage keep breaking?
 

I have two trucks with SM465s and Centerforce II cluches.
I've experienced the Z-Bar arms fracturing on both of them. No problems with the lower clutch rods, but both are OEMs.
Case 1] '71 GMC Jimmy 4x4 V8 350/SM465/NP205/12 Bolt 3.73 Posi. Because of throw-out bearing not disengaging Centerforce II clutch, after intital install of crate 350 [ca.1999], we had a shop weld a spliced-on '73 style threaded rod end to the original '71 lower clutch push rod. Weld is bolstered by a 3" section of 9mm gun barrel. This puts a lot of tension on the clutch fork, and any work requires a ratchet strap on the clutch fork to pull it back to ease tension. Also when Z-Bar arms were joggled to clear headers, the machinist used heat. It lasted 13 years before the Z-Bar failed. Z-Bar was rewelded and a Borg & Beck clutch replaced tho old one. Still have the Jimmy's old Centerforce II pressure plate -- resurfaced, it will go on a future project.
Case 2] '68 C/10 Stepside, L6 292/SM465/12 Bolt 3.37 Posi. Arm of Z-Bar separated [in traffic], original part @ 46 Y.O. Z-Bar had been heated to joggle to avoid Clifford headers. Got towed home. Rewelded Z-Bar. Also this truck has a Heim Joint style upper clutch rod, but one that was custom-made by my machinist [before LMC offered theirs]. No issues in 10 years. Still running the Centerforce II.

Re: Why does my clutch linkage keep breaking?
 

^^ Thanks for the important info, '68. So in your experience, the clutch linkage has been the issue, not incorrect throw out bearing, over travel, etc. ?

Re: Why does my clutch linkage keep breaking?
 

Well, throw-out bearing size is critical too. With the GMC, we may have used the wrong T/O bearing but were too lazy to drop the tranny and transfer cases and associated crossmembers, so we fabricated a superlong lower clutch rod to get around the problem. That put stresses on the z-bar.
Centerforce clutches take a lot of 'stomp'.

Re: Why does my clutch linkage keep breaking?
 

Agreed, Centerforce does take a lot of stomp. And I think I figured out why I've been breaking parts, I had the fork sandwiched too tight on the push rod. I'm going to try leaving a little play in it, so the rod can swivel inside the fork. Pretty sure that's all it needs.

Re: Why does my clutch linkage keep breaking?
 

Have you considered getting rid of the Centerforce pressure plate for something a little softer? Seems you are stressing the stock linkage beyond what it was designed for with that stiiff plate. You state that you have a very worked 350, so you shouldn't need all the grab that the Centerforce is providing. If you do keep the Centerforce, I would consider replacing the old linkage with a hydraullic slave cylinder and never look back. You would have a nice easy pedal with the stiffest pressure plate you could throw in there.

Re: Why does my clutch linkage keep breaking?
 

Yes, the hydraulic clutch is another good idea, I'm definitely considering that upgrade in the future. I really like how this clutch grabs though, I really don't want to tear it apart until it blows. I just want to drive the hell out of it, more than keep working on it.