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MG MGB GT V8 Factory Originals Technical - Clutch problem

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Philip Shingler

Philip,
When my B was a four cylinder car (it is now half way throught getting its V8) it had a similar trouble as what you speak of. Because my car was a four cylinder car I don't know about the range of travel that is required for your car but the problem does sound farmilar. My problems were associated with the linkage between the pedal and master cylinder.

To be more exact the connection in the engine bay where the top of the clutch pedal drives the end of the clutch master cylinder piston. The forked connection had become ovaled and loose which reduced the effect of the clutch.

To go on, a year later I snaped the clutch return spring inside the clutch master cylinder and this gave a similar effect. The pedal is returned to its normal position (by the pedal spring) but the piston in the master cylinder was not returned all the way so there was not sufficent fluid volume to acitvate the clutch. This problem was difficult to find as when the spring broke not alot changed, but gradually the two halfs of the springs wound inside one another reducing the spring length. This made the problem get progressivly worse and hard to detect.

Don't rip your carpet out I think your problem is definetly in the master cylinder. Whether it is one of the solutions presented here or another I don't know, but the pedal should not be hitting the bulkhead. Sorry if my message is a bit confusing, give me an email if you don't understand anything and I'll try to make more sense, I hate trying to describe a picture with less that a thousand words.

Spriggsie
Clem Spriggs

The pedal always hits the carpet/toeboard at full travel. Do you mean another part of the pedal arm is hitting another part of the structure? What problem are you troubleshooting? Having recently replaced the clutch hydraulics on my factory V8 I checked the slave piston travel both before and after - it was 1/2" to 5/8". What is your travel?

I think it's more likely that the V8 has slightly less travel than the 4-cyl to partially compensate for the heavier clutch, not more travel, and this is achieved by the combination of m/c and slave, one or both of which are different to the 4-cyl.

If you have insufficient travel i.e. less than 1/2" travel and the clutch engages very near the bottom of pedal travel then play in the clevis pin and holes in the m/c piston and pedal is the first thing to look at as Clem says. If you have the correct amount of travel at the slave but still have difficulties then the problem is more likely to be inside the bellhousing.

PaulH.
Paul Hunt

Thanks Paul,

Iím getting just over ĹĒ of travel so that sounds OK. The clutch problem I have is an intermittent one, which is why I canít work it out. The symptoms are that sometimes, maybe one time in ten, itís hard to get into or out of gear. My gut reaction was that the clutch hydraulics werenít working 100%. So Iíve fitted a new slave cylinder, rebuilt the master cylinder with new seals and fitted a S/S braided flexible pipe and this has not fixed it. It always seems to happen first thing in the morning; not the first time you hit the clutch but the second time. Coming off the pedal waiting a few seconds and then hitting it again usually frees it. When the clutch works it bites right at the top of itís travel with no judder or grabbing, although the shift does feel a little firm, as if the synchro rings are having to work hard to overcome some clutch drag. Researching the archive on this BB I read about something called ďclutch spinĒ where the clutch plate sticks to the flywheel. Similar to the problem of the clutch plate sticking to the flywheel after a long lay up period. Has anyone met this before? What causes it? I can only think of some sort of fluid contamination, but an oil leak from the rear of the engine or the gearbox would cause slip not stick. Someone mentioned heíd had a problem with the clutch plate sticking on rusted splines on a TR4 could this be causing the problem surely that drag wouldnít be sufficient to keep he car in gear? In which case, if I got the car on a down hill slope maybe I could use an oil can with a flexible nozzle to get a little oil onto the gearbox input shaft which would then run down onto the splines and free them up. This sounds a bit desperate but compared to pulling the engine and gearbox out to replace the clutch this would be a really easy fix.

Any thoughts?

Thanks again

Phil
Philip Shingler

Philip,
you say "itís hard to get into or out of gear" - can you amplify a little ? When trying to go into first, does it graunch and grind, like the clutch has not disengaged, or is it just that the gearlever is solid and won't move ? And once you are in first, if it won't disengage, what happens if you hit the accelerator while your other foot is still hard on the clutch ? Does the car leap forward, or do the revs rise without transmitting the drive ? I'm trying to work out from what you say whether it's clutch or gearbox that has the problem....
David Smith

Hi David,

It doesnt graunch trying to go into first it just won't let you in, as if the clutch is dragging and the synchro rings won't overcome the drag (I'm not a gearbox expert, the only gearbox ive ever rebuilt is a Hewland on an F2 car and they dont have synchros, but I understand them to be small conical clutches in bronze?)The same happens when trying to get out of gear. The cars pretty unused (only 35k miles) and the gearbox has always felt tight to me (I've only had it for two months) Could it just be tight clearances when cold? It happens most frequently first thing in the morning and seems to clear as the car warms up. I'm planning to go down to Beaulieu tomorrow to get some miles on it to see if it's just an underuse problem, but I'm going to check the Gearbox oil level first.

Philip
Philip Shingler

When it won't go into first, have you tried using 2nd instead ? What happens then?
I don't know gearbox internals, just trying to use layman's logic to isolate the problem to a particular assembly.
"And once you are in first, when it won't disengage, what happens if you hit the accelerator while your other foot is still hard on the clutch ? Does the car leap forward, or do the revs rise without transmitting the drive ?" - did you try this to see what happens too?
David Smith

Philip,
With your more in depth description I think your problem is well beyond me. It does not sound like a clutch problem given the amount of work you have done on that. I will leave you in hands of someone who knows gearboxes, they scare me to much to anything but add oil and change gears. Good luck!

Spriggsie
Clem Spriggs

The larger bore cylinder is used to help keep pedal effort in line with 4 cylinder cars due to the larger and heavier clutch.

With the engine not running I expect that you will be able to quite easily select gears. There may be a little baulkiness to get in one or more to start with, but once all the gears line up there should be easy passage up and down the ratios with or without clutch pedal operation. If the clutch is the issue then as soon as the engine is started then the problem should rear it's head.

I suspect that you have either distorted plate linings or contamination of the same. The overnight problem is likely to be one where once heat from a few applications is introduced it eases the problem. I can suggest a crude, but often effective, semi solution which is often effective for between 3 months and 2 years on Maestro and Montego 2 litre cars that have such a series problem and which gives much the same symptoms.

This si to be cruel to the clutch and 'burn' it. This involves being stationary, handbrake on. Engage 4th gear and bring revs to about 2500 and by 'heel and toe' operation hold the footbrake on. Ease in the clutch to a point where the engine is being dragged down in rpm, increase the throttle opening and let the clucth out some more so bringing the revs down sopme more. run like this for about 4 to 5 seconds and then dip the clutch and let the engine idle for about 30 seconds before repeating the operation. You should by the end of the second time be able to smell the distinct aroma of a hot clutch. Take the car for a drive and use the clutch frequently for about 5 minutes drive. Note any difference which should be quite apparent.

Rog
Roger Parker

Rog - that sounds very painful. However I have exactly the same symptoms as you, Philip. It was being masked somewhat by the m/c leaking back but having replaced all the clutch hydraulics and got 5/8" travel I was a bit depressed to discover it still baulking at forward gears (sometimes I could get in second with a push, sometimes only third) and severely grinding reverse gear (so much so that I have had to switch off, engage gear, then start up again). It is intermittent, and seems to be occuring a lot less now than it was in the winter. At one point I wondered if it could be the linings swelling up overnight in the cold and damp (it lives outdoors), but then sometimes it would engage perfectly from cold and only graunch when backing onto the drive after a run. It got to the point where I felt I had to start the car in gear with the clutch down. As I say, it is happening much less frequently now, and on the rare occasions it does drag it is only from starting and 'clucth up, clutch down' has always fixed it. In all this, the clutch biting point has remainded constant, even when it is obviously dragging.

I wonder if the release bearing is seizing in the arm so that the plate is being pushed at an angle and still maintaining some pressure on the friction plate at one point in the circumference - that would explain the drag but normal biting point.

PaulH.
Paul Hunt

Hi Phil,
I don't have a factory V8 to look at , but I have worked on many hyd clutches and many gearboxes. This sounds like a hydraulic or pushrod/pin wear problem. The travel sounds a little small. Spriggsie mentioned ovaling of the push rod hole or maybe the pins are worn. Any loss in lenght can bottom you out on pedal travel. On the hydraulics, I know you have rebuilt yours but. Is it the right stuff on there!
What is the bore of the clutch master? It's cast on the side of the cylinder. These cylinders look alike on the outside but can vary in ID from 1/2" to 3/4" I'm not sure which one you should have. Any help from you factory guys out there?
I chased a problem like this before. The problem was a !/2" clyinder where a 5/8 should have been. This reduced slave travel by 25%!

Food for thought.

Kelly
Kelly

I should correct the first line of my last comment which indicated that the V8 slave cylinder has a larger bore than the 4 cyl cars - wrong, it's smaller. For the thread the dimensions are as follows...

MGB 4 Cyl cars
master 19mm bore
slave 32mm bore

MGBV8
master 17.8mm bore
slave 25.4mm bore

Rog
Roger Parker

Hi all,

The car ran mostly OK this weekend. The clutch had itís sense of humour failure on Saturday morning so I bled the clutch again and it then ran OK for the rest of the weekend (400 miles, admittedly mostly motorway) I talked to a brake specialist at Beaulieu who looked up the V8 cylinder sizes in his Lockheed reference book for me; 1Ē: 0.7Ē Slave : Master. Giving a slave cylinder travel ratio of 0.49 (actually a HIGHER ratio than for a 4 cyl car. Thanks for the numbers Roger) So Iím going to measure the master cylinder piston travel this evening and compare that to the slave travel which will tell me if the hydraulics are at 100%. If the problem recurs, Iím going to buy and fit a complete new m/c on the assumption that the cylinder bore is invisibly scored and that this is causing an intermittent seal failure.

Iíd actually already thought of Rogerís the idea of ďBurningĒ the clutch to try and get rid any contamination which might be causing sticktion rather than just friction, but I think the fact that the clutch doesnít grab tells me that this isnít my problem.

The fact that the bore ratio is higher that the 4cyl car, and that clutch is stronger both make for higher pedal pressure. The natural reaction of the designers would have been to try to reduce the ratio so as to reduce the necessary pedal pressure. If they actually increased the ratio that suggests to me that the full designed travel is critical for the operation of the clutch.

Paul, it sounds like youíve got the same problem as me but are that much further down the line with it. Do you have any symptoms that I havenít described that might point us in another direction?

Aaaarghh.....I just love intermittent faults donít you?

Philip
Philip Shingler

Philip,
my maths is rusty - I guess like most of us - but comparing the 32mm MGB master cyl with the 19mm B slave cyl gives a multiplication factor of 1.68-something. Now comparing the 25.4mm V8 Master with the 17.8mm V8 slave gives a factor 1.42-something. Surely the smaller factor means the travel on the V8 system will be LESS than the B (for the same input movement) but the closer relationship in sizes means the effort will be less too.
It struck me that having juggled cylinder sizes to give acceptable pedal effort for the stronger clutch, a need for longer throw could be achieved by the factory designing in a greater amount of pedal movement, or even altering the pivot point of the pedal (but then that affects the effort again). As a matter of interest, is the V8 pedalbox assembly the same as the B's of the same years ?
David Smith

Roger's figures definitely indicate a lower pedal pressure and shorter travel for the V8 than for the 4-cyl. I am not aware of the pedal being different, but there doesn't seem much point in changing the pedal as well unless the combination of 'off the shelf' V8 m/c and slave gave too little travel and they had to get some back by altering the pedal as well.

The only thing I didn't mention was that the car was getting very little use for about a year - literally a few miles a week with the very occasional 80-200 mile trips - whereas I have been using it daily since February albeit for only 16 miles a day plus the longer trips as before.

PaulH.
Paul Hunt

Archemedies would have you lot shot!

Think about the volumes, if the MGB slave has a greater area than the V8 slave and the masters are the same size then the V8 slave will travel further than the MGB for any given master cylinder travel. The MGB slave will however apply more force than the V8 for the same hydraulic pressure.

I can't be certain without seeing the bits but I suspect that the V8 clutch cover has a different internal leverage ratio so less force is needed on the ends of the diaphram fingers even if the driven plate is clamped harded.

If you really want to get into the maths then the clutch is just like a brake with pads all the way round the disk.

Back at the clutch problem, are you sure its the clutch? you haven't said which gearbox it is but if the shift is tight it might be an interlock problem. Could be something fouling the gearlevel, I started having trouble engaging fifth on the SD1 gearbox which turned out to be a loose front engine mount allowing the engine and gearbox to turn in its mountings so the gearlevel fouled the side of its aperture.

Regards,

Dave
Dave Brooke

David,

Your logic is perfect but youíve got the numbers the wrong way round. (Slave bore = 1Ē, m/c bore = ĺĒ) The gearing ratio between the two cylinders is actually the ratio of the piston areas.
Piston area = PI * radius of cylinder squared gives

4 cyl
Slave piston area = 1.23 in2
M/c piston area = 0.44 in2
Thus ratio = 0.36Ē of slave travel for one inch of m/c travel

V8
Slave piston area = 0.79 in2
M/c piston area = 0.38 in2
Thus ratio = 0.49Ē of slave travel for one inch of m/c travel

The V8 slave therefore actually travels 36% further than a 4cyl slave for the same m/c travel (which I think is the same)

Philip

P.S. The clutch was a real pig last night so Iíve ordered a complete new m/c wish me luck
Philip Shingler

Ah yes, read the question properly, get the figures the right way round ! thanks...and await the next instalment with interest..PS - have you checked all the pins and pivots for lost movement (wear) as someone else suggested ? I remember finding a pedal on a Midget once with the hole so oval it was almost off the edge..
David Smith

Philip - I had the same problem with mine after it had been laid up for a few months. The clutch did come back to life but intermittently gave problems. The answer was a new clutch to replace a very polished plate and a new seal for the front end of the box - worked fine after that.

If you have replaced your factory box with a LT77S
you may well need a spacer to increase the length of the slave lever. A small socket is the normal solution.

Suggest you try Roger P's trick before you take it all to bits.

Good luck - Roger
Roger

Since Philips problem only occurs 1 in ten I think we can discount mechanical wear in pivots as that would be evident every time. In my case, which I suspect is the same as Philips, the clutch biting point is constant, what I would call 'normal', and yet the clutch still drags causing difficulty engaging forward gears and crunching into reverse. In my mind that definitely rules out hydraulics and pivot wear. I agree with trying Roger's advice, if you can't get out and drive it more, since my problem has practically disappeared since my car started getting regular use again.

Dave Brookes and Philip are having a little joke, I suspect. Philip criticises Daves rationale but comes to the same conclusion. They both seem to have missed that the V8 m/c has a *smaller* bore than the 4-cyl, not the same, in their statements, but then Philip uses the different sizes in his calculation.

Unless your biting point is low sometimes, Philip, be prepared to be disappointed with changing the m/c. I would also recommend that if you *do* change the m/c that you change the slave and hose as well unless you know they are fairly new - no point in having to drain the system down again in a few months time. Also pump the old fluid out through the slave nipple to flush the pipe (assuming the fluid in the m/c is clean) and refill the whole system through the slave nipple using an Eezi-bleed on very low pressure - avoids the need to bleed the system.

PaulH.
Paul Hunt

I have to agree with PaulH and Roger.

If the 'bite' of the clutch consistantly occurs at the same point in the pedal travel then the pedal and linkages and cylinders are all OK.

Therefore I'd suggest the clutch is contaminated, if Roger's suggestion to burn it off works all well and good but I'd still worry about how it got contaminated - rust from under use or a leaking gearbox input shaft?:-(

Regards,

Dave
Dave Brooke

Thanks Roger,

I finally bit the bullet and tried your "burning the clutch" technique, although I was pretty gentle with it and didn't get it warm enough to smell hot. The car has just come out of an extended period of light use, it had done just 3k miles since 1981 up until 4 months ago. Do clutches pressure plates age harden or soften in the same way tyres go hard with age? Anyway it's been OK for a week or so now, I didn't even have to fit my new m/c and it has a "cleaner" feel to the release point. I had to replace the clutch on my old V8 and knowing the gearbox is one of the weaker points in the factory car I've always been pretty gentle on the gearshifts. Maybe I ought to use it a litte harder to bed it in more.

Next job is to take one of the heads off to fix a pair of stripped exhaust port threads. My wife can't understand why I enjoy this kind of thing!

Thanks again

Philip

Philip Shingler

This thread was discussed between 04/05/2000 and 17/05/2000

MG MGB GT V8 Factory Originals Technical index

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