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'70 (pull) clutch woes -- help!

I had a problem with the clutch not releasing in my 1970 911 race car last time I took it out. Something changed/broke since its previous outing. I am using the '70 vintage 901/911 gear box -- the one with the funny pull type clutch, and screwy push type clutch cable.

When I went to reset the adjustment, I found that I couldn't get the specified throw of the release lever. The manual says that the lever should move .6" to disengage the clutch. That presumably means that the throw should be even more, including the movement past the disengagement point. I tried following the procedure to adjust both the front clevis and rear release lever, but still couldn't get the specified throw and it still does not release correctly. If I crank down on the rear adjustment so that I get zero pedal play, then I can get down to a total movement of .5", but still the clutch does not work right.

I have replaced the clutch cable, so that would appear not to be the problem. The pedal also appears not to be the problem. So my questions are: How much throw should I see at the release lever? Are these typical failure symptoms for the '70/71 gearbox? Are these symptoms indicative of a problem with the pivot point unique to those gearboxes? Is this what happens when the clutch is used up? Thanks in advance for any help.

-Juan

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Old 10-06-2005, 09:34 AM
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Crusty Conservative
 
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Juan,
i'm sorry I can't offer a lot of help here. The main difficulty I have seen with this design specific to the mechanism is excessive wear on the eyelet mount on the side of the transmission end plate.
Hopefully one of the real experts here can assist you soon.
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Bill

69 911 T Targa, 2.4E w/carbs (1985-2001)
70 911 S Coupe, 2nd owner (1989- 2015)
73 911 T Targa, 3.2 Motronic (2001- )
Old 10-06-2005, 09:53 AM
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Warren Hall Student
 
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How long has the car sat?

The clutch can get stuck to the pressure plate. I had this happen recently and the car only sat for about a month.

I just started in first and then just put in the clutch and while the car was still in gear I revved the motor and it broke loose.
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Old 10-06-2005, 10:06 AM
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The clutch is not completely stuck. I was able to drive it onto the track for the first session, but soon found that I couldn't shift. When I came off track, the clutch would not disengage fully, so I could no longer shift it into gear. I then adjusted the clutch tighter so that I could shift into gear, went back out, but the exact same thing happened again.

-Juan
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Old 10-06-2005, 10:11 AM
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Sounds like your clutch fork might be on it's last leg. It's very similar in symptom to a stretching cable that's about to break.

The fork gets a hair line crack in one of the arms and it starts bending back. You re-adjust and then it bends more until it eventually breaks. This is my guess.
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Old 10-06-2005, 10:19 AM
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Juan,

I would pull the engine and trans and fix all the bits & pieces. This clutch setup is a Rube Goldberg but will work just fine if you keep in like new or better condition. There are several updates and mods. That is a good combination for a lightweight 911 – 2.7/2.8 and smaller, Type 911 trans and 225 mm clutch.

Here are some links:
’70-’71 clutch”
clutch cable for 70-71 901
and
'70-71 T.O. arm play?
and
http://forums.pelicanparts.com/porsche-911-technical-forum/166598-clutch-cable-70-71-901-a-post1520882.html#post1520882
“How to ID a Type 911 transmission”
How to ID a 911 ty[e transmission tranny
“Moving the clutch lever pivot ball”
Moving the clutch lever pivot ball


This “Dam Seal” will fit your clutch release bearing guide tube. And is an easy and worthwhile mod.
“’72 915 Dam Seal”
Input shaft seal Problem with 915 Mag case
and
Shuddering Clutch 915
and
Replacing 901 Tranny Shift Shaft Seal
and
Early 915 tranny seal Questions

If the clutch hasn’t been releasing, you should look in the transmission. It is VERY easy to damage the gear dogs and sliding sleeve. You NEVER want it to pop out of gear under load.


I have to go run errands now but I’ll post more later. Study these links and ask questions. Lots of others can help.

Best,
Grady
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Old 10-06-2005, 10:24 AM
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Juan,

I had a "911" tranny in my 911. As Booby stated, your problem may be the clutch fork.

"While your in there", also check the clutch pivot bolt. They tend to fracture causing the fork to drop down grom the throwout bearing slot and the clutch pedal to go to the floor
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Old 10-06-2005, 10:27 AM
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If you are getting too little movement at the lever and have replace the cable, it may be the pedal assembly. I had similar problems which turned out to be the pin for the pedal half broken acting like a spring.
ED
Old 10-06-2005, 10:48 AM
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"like new or _better_ condition"

- hahahaha

sad tho...
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Old 10-06-2005, 04:33 PM
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OK, so the engine is out, and it looks like the problem is related to the fork pivot. The spring was rotated so it wasn't contacting the fork directly. Also the TOB appeared to be rotated so that fork did not contact it perfectly. Finally, the fork showed some signs of scraping against the pressure plate. Don't know if the scraping is new or old.

A few questions:

1) See pictures of the pivot bolt and arm. I noticed that the pivot shows a shoulder, presumably due to wear. Is this typical, or is the wear beyond usable, and possibly the reason for the failure? Looking at the picture of the fork, the bolt looks a little recessed within the fork. What is normal? Is this recessed too far?

2) The spring was placed between the washer and the pivot bolt. Should it have been placed on the other side of the washer between the transmission and the washer? Does it matter?

3) There was a lot of transmission oil in the bell housing. I believe it was coming from the threaded hole for the pivot bolt. Apparently, the hole goes through to the inside of the transmssion where there is oil. Is it common for the pivot bolt to leak? How do you prevent it? Or is this an indication of the bolt backing out? Should the pivot bolt be installed with loctite?

Thanks in advance for the help.

-Juan

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Last edited by logician; 10-09-2005 at 09:38 AM..
Old 10-08-2005, 10:03 PM
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Yes, the wear is common, if left unlubricated with grease, however, will cause galling and eventual failure. I've had it happen a few times. The conical seat in the fork gets worn and you need to replace the fork, not cheap. I also have put a piece of plastic sheet, like jet sleds are made our of in the transmission side receiver hole to reduce the wear (if that makes sense). I'll try to get a picture and send sometime. It has worked well for a couple of years.
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Old 10-09-2005, 09:31 AM
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On some of Porsche's tranny's there is a little ball cup made of hard plastic that is suppoded to go into the reciever hole.

What does the ball end of your pivot look like?

Fluid should not be coming thru the pivot ball mount hole - the threads, boss on the ball, & threadlocker should prevent that. Could it have been loose?
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Old 10-09-2005, 11:41 AM
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The pivot bolt has the metal ball version. See picture above. Before I removed the pivot bolt, I tried screwing it in to see if it might be loose. But it felt tight, at least to the force I applied with a 3/8" ratchet. Should there be a sealing washer against the case? The Factory manual shows a sealing washer in the earlier 901 configuration, but has no information about the 911 configuration.

-Juan
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Old 10-09-2005, 12:05 PM
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Thanks for all the replies. Regarding the oil leak at the pivot bolt, I found something that looks odd. Look at picture of the seat of the pivot bolt, at about 11 o'clock. There is a hole in the magnesium. I was able to stick a pin in there to about 3/8" depth. Is that normal, or is it some kind of flaw? What is behind there. Can oil from inside the case get out through there? I'm thinking that may be the source of the leak.

Also, an someone tell me if the pivot bolt looks too recessed within the fork to be usable? I'm thinking that even if I replace the pivot, that the fork may be too worn to use.

-Juan






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Old 10-10-2005, 08:05 AM
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Juan,

Please post some images of the ring on the side of the differential
side cover and the casting where the end of the cable anchors to
the transmission. Your images are great and I will include them
in my file – thank you.

While you are doing this, could you make some measurements for me?



I assume you have found a leak at the green arrow. I would pull
the differential side cover and inspect from the inside. The only
parts you will need are a side cover O-ring and possibly an axle
flange seal. This also gives you the opportunity to look at the
ring & pinion contact surfaces.

If the oil is leaking through the casting, enlarge the hole for
about ½ the thickness of the casting and fill with epoxy. Try to
not disturb the surface where the finger spring, washer and
pivot seat. You might consider removing the side cover stud
that closely abuts the leak (behind the green arrow) and see
if that drilling is involved.

I have never found the threads to be a source of leaks. If you
want, coat both sides of the finger spring, washer and under
side of the pivot with a thin coat of aerosol sealant.


I recommend you use a new pivot and fork. Use molly grease
between the two. I have considered lapping the two surfaces
together but I think the production surfaces retain the grease
and tolerate contamination better.

Take the clutch release bearing guide tube off and inspect it for
wear where the bearing rides. Remove the input shaft seal
and inspect the sealing surfaces (in the casting and on the
input shaft.) Replace the seal with new and install with sealant.
Clean the splines and inspect for wear. Test the sliding fit of
the clutch disc on the splines. Lube the splines and hub of the
disc minimally with moly grease and slide fit again wiping off
ANY excess grease. Inspect the pilot bearing surface of
the input shaft. Measure the runout of the input shaft (spec is
0.10 mm max.)

When you reinstall the guide tube, it is a good idea to install
the “dam seal” and grind a drain path here. The inside of the
bearing and outside of the tube need a THIN coat of grease.
Test fit and wipe off any excess.


It is important to measure the thickness of the flywheel. As it
wears and is machined, this moves the clutch assembly away
from the transmission. This must be compensated for with
something additional under the pivot but you run the risk of not
being able to have “free-play” without the arm hitting the casting
(yellow arrow). A new flywheel is best.

The other issue is the arm hitting the edge of the casting opening
on actuation. You can safely remove some metal where the arm
could contact (red arrow). There is also contact between the
arm and the pressure plate. You can see where there has been
contact on your parts. You can safely remove some metal from
the arm (up to about 10% of the thickness) so long as you
carefully radius and polish your work. It is also possible to
remove a little metal (1 mm) from the corner of the pressure
plate housing. This should be done by a competent machine
shop and the clutch/flywheel assembly rebalanced afterward.
Usually not necessary with all new parts.

If you are careful, this can work better than new and as well as
any system. The critical issue is regular maintenance which
includes dropping the engine and trans periodically. Nothing
new to a 911 owner.


Randy, yes "like new or _better_ condition" is necessary here
with 20-20 hindsight. This arrangement didn’t leave much room
for wear – Graf Von Rube Goldberg only considered it to stay
new forever.

Best,
Grady
Old 10-11-2005, 05:12 PM
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Grady - did you see that hole he pointed out? He started a new thread on it also.

Maybe the 911 electrical designer designed the pull clutches...
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Old 10-11-2005, 06:40 PM
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Grady, thanks for the detailed reply. I'll take the pictures you requested when I visit my shop next. You asked for a few measurements. Other than the thickness of the flywheel, are there any other measurements that you want?

Regarding that odd hole at the pivot seat, I found the same hole in another '70 911 case that I inspected. So it seems that it is not a defect, but rather a feature of all cases. It seems an odd place for such a hole. I am not certain that the oil was coming from the hole. But it didn't seem to be coming from the guide tube, and it certainly looked like the oil was centered around the pivot. I am tempted to seal it up, but I'm thinking it may have some obscure purpose, perhaps similar to that guide tube drain path that you recommend. So I'm worried that sealing it up will cause a problem. Perhaps oil coming out of it is an indication of a problem elsewhere?

One other thing I noticed. Some oil was leaking out of the overflow vent at the top. After some study, I realized that the vent hole was not oriented exactly right. It is supposed to be at a 45 degree angle. However it was actually at a 60 degree angle relative to the centerline of the car. I wonder if that 15 degree difference in the vent orientation would make a difference, and that perhaps the gearbox was not venting correctly? One caveat: this gearbox is in a racecar, so it's definitely experiencing more G force than a street application. On the other hand, the gearbox was presumably designed for racing.

Regarding interference issues with the arm, I am realizing that is a complicated subject for the 911 gearbox. I did notice that the arm on the fork showed signs of contact, as did the the pressure plate. I can't tell for sure, but the contact did not look recent. After comparing my fork to another that was donated to my cause by a friend, I found that the fingers on my fork had been altered, and that the washer under the pivot was cut down about 1mm. Further my pressure plate turns out to be a light weight aluminum one, which is apparently a common performance modification. I gather the washer and fork modifications were needed to add clearance to the pressure plate. I think the washer was cut to move the arm further from the pressure plate, and I'm guessing that the fork was cut so the ends of the tabs on the side of the TOB did not hit the fingers of the fork, which would be at a greater angle. Do you know anything about this type of modification? Perhaps this modification was not done correctly for my gearbox, and perhaps the cause of my problems.

So now, I'm not sure if it's all going to work when I put the new fork in my car. If I use the factory washer, I bet I will find an interference problem with the pressure plate. If I use the cut down washer, I bet I'll have a problem with the fingers contacting the tabs on the TOB. I won't know for sure until I test drive it. It's always something!

-Juan
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Old 10-11-2005, 06:45 PM
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Juan,

Don’t put it back together until we get this sorted out.

The linkage (particularly the arm) has a very limited range of effective motion before it hits something. I don’t see anything wrong moving the whole assembly one direction or another, it just must be compensated for with the location of the pivot and clearance to the transmission casting and pressure plate.

My normal advice is to retreat to absolutely stock everything. In your case, you have the skills to make this work better than original. The key is the angle of the arm. Using old, worn out parts you can manufacture an arm that works better than original. (Always make a spare.) I have seen some with a welded “dog-leg” to clear the pressure plate.


The two measurements you can provide are:

1) The distance between the center of the pivot and the center of the input shaft. You can screw a M8 bolt in the pivot insert and measure the distance, compensating for the diameters of the bolt and shaft.
The reason this is important is to be able to take a 914 casting and adapt it to the 225 mm clutch. Note the casting where the pivot seats.

2) The distance between the engine-to-transmission seating surface to the seat for the pivot. I have seen the pivot surface re-machined and fought the battle to get the linkage to work. This number is necessary when someone wants to convert to a 225 mm clutch and machines that surface.


Does anyone have a NOS side cover? It would be worthwhile to post the contour of the clutch cable ring on the side cover. It is not a straight through drilling; it has a specific contour that allows the clutch cable Bowden tube to change angles. Very important – actually critical for proper operation.



I think the type 911 was sort of a rush to fit the 225 mm clutch common to all 915s. It may be the casting design allowed too little remaining metal between the side cover stud drilling and the internal transmission and to the outside you picture. Given 30+ years, that wasn’t sufficient to prevent a corrosion problem.


I agree about the overflow vent. Most racers add a connection to an overflow tank required by rules. An additional advantage is you can use it to fill the transmission with lube.

Unless you are preparing a 911 for the Manhattan, grinding clearance for the arm is perfectly acceptable in my opinion. Yes, I prefer virgin stock OE but I compromise when improvements can be made.

Yup; It's always something! The art is making it right.

Best,
Grady
Old 10-11-2005, 07:38 PM
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Hi Grady,

Here are the measurements that you asked for. The distance between the center of the pivot and the center of the input shaft is 63.00mm (on the nose.) The distance between the bell housing mating surface and the seat of the pivot bolt is 96.51mm. Probably the measurements are accurate to only one decimal place. Here are the pictures that you requested also. The side cover is actually from my friends 911 transmission. It looks like the cable support hold has been elongated.

-Juan




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Old 10-12-2005, 10:56 PM
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Regarding the pinhole at the pivot bolt seat, I believe it's actually a design flaw, where the side cover stud hole is drilled a little too deep, and penetrates into the pivot bolt seat. I found that out by removing the stud and looking into the stud hole to see light coming through the pinhole. Actually, there is a backing plate inside the transmission for the pivot bolt threads. See picture. I think the stud hole also penetrates into that backing plate, as I could see the shine of that metal in the stud hole too. I am guessing that my leak is due to oil is leaking at the backing plate into the stud hole, and then out the pin hole at the pivot bolt. I ended up sealing the pin hole with JB weld. Hopefully that will do the trick.

-Juan


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Last edited by logician; 10-12-2005 at 11:23 PM..
Old 10-12-2005, 11:06 PM
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