Opelotus

I installed a new clutch on my '84 about a year ago and have put ~4500 miles on it since. I had issues bleeding it then, and ever since it will act abnormally once in a while. First of all, there is the pedal "feel". With the car off, the pedal is always nice and smooth and does not require much effort. However, sometimes with the car running it feels like something is scraping as I apply and release it - it almost shudders (not any sort of clutch chatter though). Does not matter whether I am sitting still in neutral applying/releasing clutch or in gear/upshifting. Other times it will be smooth as butter and then at some point during driving it will start scraping again. I noticed that it feels normal on cold mornings, can't tell whether or not it worsens with temp as it is somewhat intermittent. I also tried applying/releasing clutch while revving engine and I can feel the problem get worse with higher revs. Could there be a problem with the throwout bearing?

Second, my clutch will intermittently drag or fully release. I have tried bleeding it again using WSM procedure but it only lasts for a short time. Followed the WSM procedure for adjusting the pedal/master cyl. pushrod. Master and slave cylinders were replaced last year. How can I tell if the wrong master was installed?

Mrmerlin

first remove the aircleaner and inspect the plastic bushing on the top rear of the bell hosing see if the release arm is rattling around on the ball.
To check the MC for the correct piston size you need the old one to compare it to,
I didnt take any measurements of old and new parts, but do a search for MC piston

Opelotus

Checked the plastic bushing - all good. Would there be a measurable difference in pedal travel between old and new master cylinders? That might be one way to tell. Should the pedal go completely to the floor/touch the padding? Mine stops short of touching the carpet, but there are black marks of where it used to touch a some point (before replacement?)

Mrmerlin

if the push rod is adjusted correctly,
then from the info provided it sounds like you have a new part with the shorter stroke,
the only way to fix is to remove the piston and cut about 4 to5 mm off the tip and then put a slit in the end of it so the fluid will still be able to pass.
Also the spring must be shortened (about 2 winds should do it) ,
NOTE careful measurements must be taken to assure that the piston rear seal wont travel past the feed port on the MC if it does then the MC will leak out the rear.
this is easy to do once the seals are removed ,
also measure the spring in full compression if its longer than the piston tip ,
then it will shatter then the shards will cut the seals

Opelotus

Can someone comfirm what the correct pedal travel is before I tear into it? The MC was replaced before I bought the car so I have not had the pleasure of replacing the MC.

Mrmerlin

to pull out the MC piston remove the driver side lower parcel tray then remove the clevis pin then remove the pushrod then the C clip.

NOTE clamp off the feed line to the reservoir so it wont fully drain on you while your working on the parts
.
if you post pictures things will be easier to diagnose,
remove the floor mats and then put down a towel

Opelotus

Wow, thats much easier than pulling out the entire MC just to check the piston. I should be able to do that sometime this week. How much time will I need to do this?

EDIT: I just remembered; last time I was under the car I saw that the blue hose feeding the MC is positioned odd, from the MC it curves up, back down, then up to the reservoir. Could this contribute to the back-bleeding issue because I have a high spot in the feed line?

Opelotus

Bump, any ideas about the rough clutch pedal? Or should I wait until the dragging is solved before I tackle this?

dprantl

So you are saying the clutch is smooth when the engine is off, but rough/scraping when the engine is running? If so it may be time for a torque tube rebuild. Or, it may be a pilot bearing going bad.

Dan
'91 928GT S/C 475hp/460lb.ft

Opelotus

Correct - smooth with engine off, rough when running e-brake on in neutral, rough when driving/changing gears. Some days it is perfect even with engine running though. Torque tube is possible, but I spun it by hand last time the clutch was apart, felt smooth and no excessive force was needed. If it makes a difference, I can hear the bearings in the TT with the engine running (just a whirring), push clutch in and it goes away. There is also a noticable "whine" while in gear (especially 3rd) and I let off the gas. It is road speed related but seems to be coming from TT (volume decreases when clutch is pushed in while still in gear). Pilot bearing is new, doesn't mean it couldn't be bad.

dprantl

It is not uncommon for the TT bearings themselves to be good, but the front bearing to migrate backwards causing driveshaft vibration. This causes problems with the clutch. I know because it happened on my car.

Dan
'91 928GT S/C 475hp/460lb.ft

Opelotus

Never would have thought of that. I am assuming this migration can be measured by removing the clutch housing cover and sticking something like a tape measure into the TT until it bottoms out on the bearing? What should this distance be?

The more I think about it, the more it seems that this could be the problem if not just a contributing factor. If the crankshaft and supporting rear bearing are too far apart (farther than a standard front engine RWD layout with trans coupled to engine) there may be some vibrations/unstable rotation of throwout bearing, etc which might cause a scraping/rough feeling clutch.

This is getting interesting! Looks like I have a good weekend's worth of work ahead of me

dprantl

What you describe was a problem in my car. It constantly backed out my guide tube bolts (they then got machined by the coupler) and eventually trashed my guide tube. The front bearing should be approx. 12 inches from the TT opening. Measuring is not 100% definitive since it moves around and could be in the right place when you measure, also happened to me. When I took my TT out, the front bearing was up against the middle one.

Once apart, it was obvious that the rubber surround of the bearing holders was no longer tight enough against the inside of the TT. I would expect this after 20 years. I'm confident that Constantine's bearing holders that are over 3x as long will last much much longer than that.

Dan
'91 928GT S/C 475hp/460lb.ft

Opelotus

Ah, so the intermittent clutch roughness might be attributed to engine vibration/TT spinning and moving the bearing around. As far as measuring, should I see if the bearing can be moved by lightly pushing on it? I will check this tomorrow if possible, as well as guide tube bolts.

How much does it cost to overhaul a TT?

dprantl

You can buy a rebuilt one for $495 with core charge, or you can buy Constantine's SuperBearings and rebuild yourself. SuperBearings cost more, but for me they were worth it.

Dan
'91 928GT S/C 475hp/460lb.ft