broken clutch, guess what broke

[68 TT]

The clutch in my 78 K20 quit working yesterday on the way home from work.

It started a week ago with the clutch pedal not returning back to the top all the way unless you popped it up with the toe of your foot. Still worked just fine. I just thought the bell crank linkage was sticking and needed grease so I put it off since the weather sucked and I don't have a garage to work in.

Yesterday morning I noticed that the clutch was engaging and moving the truck with only about 1/2" of pedal movement off the floor. As I headed home it got worse and worse until it was nearly impossible to shift into another gear and it started creeping forward with the clutch pushed in all the way.

By the time I got home I could not get it into gear with the engine running.

Crawled under the truck and took a look. Everything looked normal and all the parts were in the right place. Nothing obviously broken.

The only problem was that I could move the bell crank half way through its motion toward the clutch fork (moving the fork too) by hand before I felt it increase resistance to what the pressure plate should be giving. I set the adjustment rod all the way to the end of the threads and still could not get enough clutch function to shift into gear with it running.

I will start tearing into it tonight and try to figure out what broke.

Any guesses what failed?

I'm thinking the clutch fork cracked at the pivot ball in the bell housing.

Pictures to follow after disassembly begins.

[USSkoval]

I'll go along with your theory.

[DirtyLarry]

I agree, sounds like the clutch fork is forked up

[Stepsideride]

Either a cracked fork or a broken pivot ball.

[James McClure]

I had the same difficulty. Driving one day, push clutch and got a loud bang and pedal got loose. Very hard to shift. Adjusted it and all was OK for a while. When I did the restore I replaced it. Come to find out one of the three fingers on the pressure plate had worn completely off and was sliding up around the release bearing. I have it here someplace, I'll try to get some pictures. It was the origional clutch, honest to God & it had 400 some odd thousand miles on it. I don't use the clutch except for start and stop so thats going to make a difference. Any one else? Jim

[old Rusty C10]

ive seen the clutch fork snap on a 76 3 on the tree i had years ago... ive also seen the throw out bearing break apart

[68 TT]

Originally Posted by old Rusty C10

ive seen the clutch fork snap on a 76 3 on the tree i had years ago... ive also seen the throw out bearing break apart

It is a diaphragm type clutch and new throw out bearing from NAPA that I put in last summer so the clutch parts are not old or high mileage.

Hopefully none of the new parts failed and it is something original to the truck.

It is a pretty high effort clutch so I can believe a fork or pivot ball failure.

I didn't get to tearing into the truck last night since I had to pick up some parts for the 67 from my brother before he gave them away with his 64 C-10 truck that he is selling.

[wedgemon]

Happened to me in my '80. Pushed the T/O bearing right thru the spring arms. Managed to get it home by dry shifting. And using the starter to get going at lights.
(disconnected the NSS.)

- Dave

[Desert1957]

Pressure Plate...

Desert

[CSGAS]

I'm leaning towards the throwout bearing (sort of) myself, even though it is new...I'd bet it slid out of a worn out clutch fork.

If so, it probably did some other damage. Check the pressure plate; you may want to run emery cloth over the bearing retainer before reinstalling everything; and repair or replace the clutch fork IF I'm right.

[Desert1957]

Originally Posted by Desert1957

Pressure Plate...

Desert

Pressure Plate!...

Desert

[powerdriver 1958]

Only two clutches have every failed me, Both were Pressure Plate failures. Both of them would not release.

Made driving home an adventure.

[68 TT]

I got the boot off the clutch fork and took a few pictures since I couldn't get my head anywhere near there to actually look in there.

Pivot ball and clutch fork look fine and you can see the throw out bearing still connected.

It looks like pressure plate may have failed as desert suggested.

It is possible that it may not have been the correct pressure plate that I installed last summer as I can see where the fork had been digging into the side of the bell housing from over travel.

I will start tearing it all out after I drop my girls off at grandma's house so I can get some work done.

[68 TT]

As it turns out the clutch fork was broken right at the end. Pressure plate shows no signs of damage.

[powerdriver 1958]

Cool a cheap (money wise ) fix.

[68 TT]

Originally Posted by powerdriver 1958

Cool a cheap (money wise ) fix.

It would have been fairly cheap had I not discovered a problem while looking at clutches at the parts store. The clutch fork over travel damage to my bell housing got me thinking that there was something else wrong so I had them pull both clutch types off the shelf so I could compare them.

It turns out that the two types of pressure plates are different in depth. The diaphragm type is much shallower than the lever type, about 1/4 inch different. The parts store all listed both to fit and list the same throw out bearing for both so the depth is not made up for if you go by the book and get the diaphragm type clutch. This is what came with my truck when I bought it and I installed it last summer not even noticing the depth difference when I did.

This is why my clutch engaged so close to the floor. It was barely engaging as assembled and when the fork broke & allowed the bearing to pivot just the slightest amount it no longer engaged enough to shift into gear from neutral with the engine running.

I ended up buying a new lever type clutch kit in addition to the new clutch fork plus some gear oil and output shaft seals needed to seal up my leaky NP205. Out the door for just under $165.

I will take some comparison pictures of the two pressure plates in a bit after I get the old one out.

For how greasy the NP205 was it was only half a quart low and the fluid looked pretty good. With so few miles on the truck (under 73k) the output seals got a lot of dry start up wear from all the time it spent sitting still.

[CSGAS]

How's the front bearing retainer (at least the surface the throwout bearing slides along)?

[Nick's81]

ok here i go the lever type and diiaphfram cover do interchange static heigh on a bench will be diffrent what you need to measure is installed heigh the fulcrum is diffrent on the 2 types of covers the only other change is gm offered 3 diffrent heighs of t/o bearings but from the parts in pics you have the right set up your fork was probably cracked and flexing causing your low pedal oh and the 3 finger usaly have higher pedal pressures

[Desert1957]

Originally Posted by Desert1957

Pressure Plate!...

Desert

It turns out that the two types of pressure plates are different in depth. The diaphragm type is much shallower than the lever type, about 1/4 inch different.

Ok so it was related to the Pressure Plate?

Just checking.....

[grinch4om]

check the pivot ball on the bell housing. they make different lengths. if you need to take up that quarter in diff in the PP, look for a ball that's longer or shim it if possible

[Stepsideride]

FYI though...The three finger pressure plate is called Borg and Beck.

[68 TT]

Still working on the clutch replacement job. I had planned on upgrading the oiling system on the engine later this summer but decided to use this broken clutch opportunity to save some time later on since the hard part of getting the engine out was already done.

I pulled the engine out Saturday and mic'd the crank to see how it was doing. No real wear to speak of and all the journals looked great so i just freshened it up with new bearings and a high volume oil pump. The plugs are all tan and it doesn't burn any oil so the rings are still fine. This thing has less than 73k miles on it so the only damage was a little bearing wear from all the long sitting period dry start ups it saw over the years of sitting around waiting to get used.

Got the engine, trans & transfer case put back into the truck on Sunday but haven't finished getting the accessories back onto it to make it drivable.

The borg & beck pressure plate definitely has a much higher pedal engagement position than the old diaphragm clutch did. I should get it back on the road by next weekend and see how the new clutch does.

[68 TT]

Finally got the time to finish getting the peripherals bolted back onto the K20 drive train and took the truck for a drive yesterday.

It works great and the clutch engages near the top of the pedal movement instead of at the bottom like with the diaphragm clutch.

It now shifts smoothly into 2nd and 4th with 3rd being a little tougher but still much easier than it ever was before. Reverse doesn't grind either now. The old clutch was definitely not fully functional before. I think the 3rd gear syncro is a little worn.

The oil pressure is much better and I should be able to run lighter weight oil now and still have decent oil pressure. I need to put in the 80 psi gauge & sender now with the high volume pump. It hits 60 psi at idle when cold with the 20W50 oil I was running in the engine to compensate for the high bearing tolerances it used to have. The engine is quieter now too with the fresh bearings.