20 replies to this topic

[Wazzy]

Greetings,

 

I'm a new member hoping to find some expert advice on a mini pre-verto clutch. Mk 3, 998 mini, 1974 vintage.

 

The car is coming to the end of a 4 year rebuild and looking good - but the clutch don't work! Very frustrating.

 

Basically, the clutch disengages but only when you get to the last 1/2" of travel of the pedal. I've tried every which way of adjusting the stop bolt and the throw out stop nuts (nearly typed 'Back Stop' nuts then!). The system is bled with a Eezibleed pressure bleeder and I'm confident there is no air in the system. There are new clevis pins everywhere - the operating arm is new - new master and slave cylinders - new throw out stop plunger - new braided flexihose - new clutch plate, diaphragm and release bearing. The flywheel and pressure plate have been skimmed and (not trusting my own judgement) I asked the chap at Guessworks to look at the height and set up of the drive straps and he agreed they were fine. I opted for DOT 5.0 (Silicone) hydraulic fluid which, I don't think should be a problem - it's working fine in the brake system. The pedal and pedal box and the clevis pin are all in good condition - no oval holes - and there is nothing restricting the movement of the pedal down to the floor.

 

Someone suggested bending the operating lever towards the slave, which I know I shouldn't need to do given that the linkages are all fine and mostly new, but I tried it - no difference.

 

So - I started checking things again and noted that, even with the throw out stop nuts backed right off, the slave cylinder push rod was only moving about 3/8". This level of movement did not change with the clutch return spring disconnected or with the clutch operating arm completely disconnected either. The suggestions on the interweb say it should normally be 1/2" to 5/8". So I took the (new) slave and master cylinders apart and found a big chunk missing from the lower seal on the master cylinder, Ah ha! I thought - here is the problem. The bore looked fine so a new seal kit was fitted with high expectations that it would solve everything but no - it's still only moving 3/8". The slave cylinder looked fine. The bore of the slave is 7/8" and the master is 3/4".

 

I can't see any leaks in the system anywhere. In the (very) short distances I have been able to drive it there is no evident slip but there is drag sometimes depending on how I set the stop bolt etc but given that the clutch is only disengaging when the pedal is just a few millimetres from the floor I suppose this isn't surprising. I can't drive it far, partly due to the clutch and fear that I will not be able to get into gear when at a junction, but also because the engine is running rough due to a stage one kit being fitted and (I suspect) a need for a different needle in the carb. I'll get this sorted on a rolling road, but, I cant do that till the clutch is sorted!

 

I've spent about 6 weeks on this so far. Am I missing something obvious? I don't really understand hydraulics but is it possible, given the lack of travel in the slave pushrod, that it's still a hydraulic problem and should I try a replacement slave or master cylinder (or both)? Any ideas much appreciated as I've run out of them.

 

Tim

[Spider]

Welcome to the Forum Tim.

 

Sorry your are having troubles, but you have done some good diagnostic work.

 

With everything new, the Clutch Arm will travel 5/8" at the Push Rod and that will give 0.060" movement on the Diaphragm.

 

There are two length Push Rods, the very early ones being shorter, sorry, I don't have the lengths to hand, but will check.

 

If everything is in good order, it shouldn't be necessary to bend or otherwise manipulate anything.

 

I must say, at first glance, I am wondering if it is your Silicone Brake Fluid. It is good and usually doesn't give trouble, but is more fussy to use than ordinary Glycol Fluids. It tends to be more viscous and therefore holds air bubbles (some microscopic in size) longer and easier than Glycol based fluids. This is what some complain about with a spongy pedal after going over to it on their brakes. It wouldn't take much air at all in the Clutch System to give you the shortness of travel that you seem to be having.

[Magneto]

I have seen where the push rod has to be lengthened to push the cup in the slave cylinder further down to allow enough stroke to release the clutch. It's rare, but even with new parts correctly installed, this seems to be the only way to get it to work properly. 

[unburntfuelinthemorning]

I have been struggling with my clutch in a similar way - only ³/₈" slave cylinder travel when I should be seeing ¹/₂" - it only drags very slightly when the engine is stone cold but if it got any worse it would be a serious problem.

 

It was worse (and undriveable) than this until I read on here somewhere that reducing the free play clearance slightly from the correct spec on the top adjustment could help.  It did and the vehicle became driveable with the bite point a sensible distance from the floor.  I'm not sure why this is so and I know it isn't fixed properly but it has allowed me to drive the vehicle.

 

At some point I intend to correct the problem but at the moment it isn't the most pressing concern.

 

(Standard clutch release lever clearance is 0.020".  I set mine to somewhere between 0.010" and 0.015")

[Wazzy]

Thanks folks for the replies so far.

To respond to the thoughts above;

1. Thanks for confirming that the movement should be 5/8". I have not measured the push rod but it was the same length as the old one and I would have thought that if that was the issue then bending the lever towards the slave would have improved matters but it hasn't. I agree with what you are saying about silicone fluid, and I know it has it's detractors for that reason, but I bench bled the system first and was careful not to agitate the fluid - I then left it to settle in the system so that any micro bubbles would escape and then bled again. No difference. I think the clutch actuation would firm up as well after pumping the peddle if there was air in the system but it doesn't. 
2. I have heard about lengthening the push rod, and the other trick of putting a nut in the cup of the slave cylinder, but again this would only achieve the same effect as bending the clutch lever would it not? I did try measuring the displacement of the push rod, with the clutch lever disconnected, at 3 different start points for the piston in the slave cylinder and the travel was 3/8" for all positions. So I'm not convinced that manipulation of the push rod, or even fitting an adjustable one, will change this.
3. I've tried various settings for the top adjustment, 0.02", 0.01" and also the tight up then back off one flat and I still  have the same problem. The only thing that does improve it is to screw the top adjustment out against the pressure of the diaphragm but, of course, this would put pressure on the clutch release bearing all the time the engine was running.

I think you can see why this is doing my head in!

 

Tim

[nicklouse]

I see two issues. Eezibleed (not as bad as a vacuum bleeder) and silicone brake fluid.

 

any pressure bleed can cause issues. as can old rubber seals.

 

if I had to use silicone fluid I would try to disturb it as little as possible. it holds air easily and for  long time. not a fan at all.

[Spider]

The issue here does appear to be in the Hydraulic side of it as you are only seeing short Travel of the Arm. Altering the Arm or Push Rod will only lkikely result in the same Travel, only at a different part of the Arc of Travel (though read on).

 

Do you have a shim or extra thick Gasket fitted under the Master Cylinder? Is the Plate under the Masters bolted down hard and tight?

 

I do still come back, at least in part, to the fluid. Don't discount it. It takes weeks for the air to come out of it.

 

Having said all that, and just coming back to the Push Rod, the (factory) short one can give these troubles and I should have explained myself here.

 

The Shorter Push Rod was used with the very early Coil Spring Clutches. When used on a Diaphragm Clutch, and all else being in good order, the Piston in the Slave Cylinder will travel all the way to the end and hit the Circlip - if it is an OEM style that has one - before you get to full travel.

 

Occasionally - and I stress, it's fairly rare - an unfortunate combination of factory parts, all in spec, but at the 'wrong end' of their tolerances can give a lack of travel. The factory method of overcoming this was to fit a 24 Gauge (roughly 0.020") x 7/8" ID Flat Washer between the Thrust Bearing and the Shoulder of the Plunger.

 

It's fairly quick and simple to drop a nut in the slave just to see if that makes a difference, before doing something better if that sorts it out.

 

One last question, did you check that the hole for the Clevis Pin in the Clutch Cover (for the 3/8" Pin) wasn't worn?

 

I only ever set the Stop Adjustment (the little 1/4" UNF one) so that the Bearing isn't running all the time, maybe at about 0.007 to 0.010", just over Crank End Float.

[unburntfuelinthemorning]

I see two issues. Eezibleed (not as bad as a vacuum bleeder) and silicone brake fluid.

 

any pressure bleed can cause issues. as can old rubber seals.

How does pressure bleeding cause problems?  Is that only when using silicone fluid?

[nicklouse]

 

I see two issues. Eezibleed (not as bad as a vacuum bleeder) and silicone brake fluid.

 

any pressure bleed can cause issues. as can old rubber seals.

How does pressure bleeding cause problems?  Is that only when using silicone fluid?

 

yes as silicon fluid catches and holes air very easily and for a long time. I am not a fan of the stuff one bit.

[unburntfuelinthemorning]

The issue here does appear to be in the Hydraulic side of it as you are only seeing short Travel of the Arm. Altering the Arm or Push Rod will only lkikely result in the same Travel, only at a different part of the Arc of Travel (though read on).

 

Do you have a shim or extra thick Gasket fitted under the Master Cylinder? Is the Plate under the Masters bolted down hard and tight?

 

I do still come back, at least in part, to the fluid. Don't discount it. It takes weeks for the air to come out of it.

 

Having said all that, and just coming back to the Push Rod, the (factory) short one can give these troubles and I should have explained myself here.

 

The Shorter Push Rod was used with the very early Coil Spring Clutches. When used on a Diaphragm Clutch, and all else being in good order, the Piston in the Slave Cylinder will travel all the way to the end and hit the Circlip - if it is an OEM style that has one - before you get to full travel.

 

Occasionally - and I stress, it's fairly rare - an unfortunate combination of factory parts, all in spec, but at the 'wrong end' of their tolerances can give a lack of travel. The factory method of overcoming this was to fit a 24 Gauge (roughly 0.020") x 7/8" ID Flat Washer between the Thrust Bearing and the Shoulder of the Plunger.

 

It's fairly quick and simple to drop a nut in the slave just to see if that makes a difference, before doing something better if that sorts it out.

 

One last question, did you check that the hole for the Clevis Pin in the Clutch Cover (for the 3/8" Pin) wasn't worn?

 

I only ever set the Stop Adjustment (the little 1/4" UNF one) so that the Bearing isn't running all the time, maybe at about 0.007 to 0.010", just over Crank End Float.

Thanks Moke Spider, there's a lot  ideas there. 

 

If Cooperman was here he'd say it's a hydraulic issue if you're not getting a minimum of ¹/₂" of travel at the slave cylinder.

[Wazzy]

Thanks again for the replies and ideas.

 

In answer to you Moke Spider;

 

1. I see your point that changing the push rod will not change the length of travel - only the arc of movement.
2. I have the normal gasket under the master - its quite a thick card one - and everything is bolted down tight.
3. The push rod was renewed when I renewed the slave cylinder - the push rod was the same length as the original - and the slave is the type without a circlip.
4. The clevis pin is in good condition and a good fit in the clutch cover.
5. I think the suggestion you make about the nut in the master cylinder will not change anything due to what we said above in (1).
6. Setting the stop adjustment back 1 'flat' gives about 0.004' clearance but that didn't help.

Your other suggestion, about putting a washer between the thrust bearing and the shoulder of the plunger, I can see would change the position of the operating arm relative to the stop adjustment nut. Again though this would not change the travel of the push rod. Also, I would have to dismantle the clutch to do this and again to undo it if it didn't work. I wouldn't mind doing this if I knew it was the issue but if the hydraulics don't seem to be working as they should my gut feeling is sort that out first. I hear what you say about silicone fluids. As I've said I was careful and in terms of time for micro bubbles to settle - leaving it a couple of weeks at times. 

 

It seems to me that my next steps should be to;

• swap the master and/or slave cylinder (even though they are new) and see if doing that gives better travel

Or

• Change to DOT 5.1 hydraulic fluid

Unless there are any other suggestions which one first do you think? And any tips on cleaning the old DOT5.0 out of the system before putting the 5.1 in?

 

Thanks agin for your help everyone.

 

Tim

[nicklouse]

3: do you have a photo of the slave or a link to the exact one you bought.

[cal844]

Use DOT4 fluid, it will bleed better. You'll need to change it every couple years but that's no big deal

[absx2]

​I have had issues with the length of the clutch master cylinder rod being about 3/16" shorter than the original which gave the problem you describe.

Does the clutch pedal sit quite low ?

[Rorf]

How's your primary gear end float and crankshaft end float? When you set up the clutch pressure plate on the bench were the diaphram plate and the three straps parallel when bolted up. Ear height should just be just below 25mm from the pressure plate.

 

Hydraulics should give you 1/2 inch travel on the push rod. Do you have a new clutch arm, if a poor quality one then the ball end could be bent.

 

Silicon fluid not good.