R50 Clutch problems/issues/diagnosis

OK, I'm baffled and looking for suggestions from the hive.......

2004 MINI Cooper (base) with a little over 200k miles.  We bought it a few months back.  Son was driving  and noticed a slight clutch slip, but nothing critical yet.  He was at a light, let out the clutch and nothing.  Acted as if the clutch was still pushed in.  I got there and tried and had the same issues, so we tow strapped it home.  Pushing the clutch and watching the clutch fork, it was moving back and forth without issues.  I assumed the throwout bearing had gone, although there was no weird noises.

Car sat for a bit until I found time and pulled the transmission this weekend.  Throwout bearing looked fine and clutch fork moved freely.  Hmmmmm...  Pulled the pressure plate and the clutch disc was thin but not to the rivets.  Pressure plate and flywheel both looked clean.  Still had the machining lines in both.  Clutch was made in 2013, so fairly "new".  There was some oil in the bellhousing, but the clutch and surfaces all seemed dry. 

Now I'm baffled.  If the clutch slave went bad, then the problem would be not disengaging, correct?  The guide tube for the throwout bearing wasn't totally smooth, but the bearing didn't seem to hang up on it when moving the lever.  Turning the main shaft in the transmission was spinning the axle carriers, so I don't think the transmission went bad. 

I'll probably end up replacing the clutch, bearing and pressure plate (it's only $100) since I have it all apart now.  Thinking about replacing the throwout bearing guide tube to be safe.  Since it's apart and there was oil in the bellhousing, I'll do the main seal while I'm there.

PO was a mechanic and said he rebuilt the transmission (it's the dreaded Midlands) so I assume he did the clutch around the same time.  Axles look new-ish, too, so they were probably replaced then, too.

Just concerned that I'll button it back up without fixing it.

What am I missing that I can look for while it's all apart?

-Rob

Make sure the fork is not bent. 

Didn't seem bent at all.  I can push it all the way to where it hits the bellhousing, but doubt it goes that far when all hooked up.

-Rob

I have to assume the throwout bearing or fork was hanging up under load and not returning to the "released" position. I can't fathom that the pressure plate ceased to apply pressure, and it sounds like it was a total absence of clutch engagement.

If it were still together, I'd have wanted to try tapping the external arm of the fork to try to unstick it and see if it returns all the way.

This video seems like it *might* have pertinent info. They actually had the opposite issue, mostly; the clutch wouldn't disengage completely. But he did note that part of what was going on was a failing fork causing a cocking effect on the throwout bearing, which in turn wasn't moving smoothly on the guide tube, and that it appeared to not be returning smoothly as well (he only showed early wear on pressure plate fingers rather than actual failure to engage like you've got, but... pertinent).

In any case, I think if I were you I would indeed replace the guide tube, and I'd be taking a very hard look at the fork to be sure it's not tweaked or cracked.

I had something similar happen with my TR6.  The throwout bearing carrier was hanging up on the slide, but only when hot (driven more than 20 miles).  I assume the slide was expanding at a higher rate than the bearing carrier.  No amount of honing or grease seemed to help much.  If it's not too expensive, I'd replace those pieces as any scoring indicates they may be binding up.

I've done quite a few MINI clutches but never ran across the same symptom without an obvious cause.

I ALWAYS replace the guide tube and fork when replacing the clutches.  Have seen enough of them fail to preemptively replace while in there.

Here's what's left of a throwout bearing from an R53 I did last week.

  I can only assume they ignored horrible noises for a while before this happened.