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MG MGA - Slipping Clutch

My 1957 MGA is just back from a complete 2 year body restoration which was done after a 2 year mechanical restoration which included installing a newly re-conditioned engine and gearbox and installing new clutch, master cylinder, slave cylinder, lines, etc. The car was running perfectly before the body restoration but now the clutch is slipping when I apply power in 3rd or 4th gear (no noise, no shuddering). Additionally, to prove the slipping, the car will sometimes stall (and sometimes not) when I engage the clutch in 3rd or 4th gear when standing still with a little bit of power applied and the brakes applied.

A few extra comments, Im using Shell DOT 4 fluid and I have bled the lines several times and the clutch appears to be working normally apart from the slipping issue. The car only had a few hundred miles on it before the body restoration and the clutch looked as new when we re-installed everything after the body restoration.

Ive just ordered a new slave cylinder and a new hose and will fit them when they arrive in a week or two. Also, Ill also flush out the lines and replace all the fluid when I install the new slave cylinder and the new hose. I've also ordered a gunson Eezi-Bleed kit just to make sure I bleed the system as best as possible. Im not convinced that any of this new stuff will solve the problem and maybe Im missing something simple. Mechanical expertise is in short supply here in Bangkok and Im a long way from being competent, in fact the only way Ive managed to get this far by following the advice and tips from a lot of excellent sources available on the web. However, this one has me beat so any advice would be very much appreciated.



A Valentine

I don't think you have a hydraulic system problem. Stong springs in the pressure plate keep the clutch disc clamped between the pressure plate and flywheel without any assistance from the hydraulic system. its job is to leverage the pressure plate away from the disc if the hydraulics are not in working order your unable to fully disengage the clutch. Its either one of two things. The pressure plate is weak or the disc is slipping either from contamination or wear. Chuck
Charles O'Brien

I agree with Charles - if the clutch is new and assembled/installed properly it should not slip unless contaminated. There is a slim possibility of residual pressure in the hydraulic system preventing the slave cylinder from fully releasing. This could be checked by ensuring there is the correct amount of freeplay between the master cylinder piston and the rod it contacts once the pedal is in the released position. I believe I once heard that it is possible to install the valve from the brake side of the M/C incorrectly in the M/C clutch bore and its function is to maintain some residual pressure in the line. I don't know if that would be enough to cause the clutch to slip, though but perhaps worth checking before pulling the engine.
Tom Heath

I agree with Tom and Chuck.

Regarding contamination, there may an oil leak from the gearbox front cover/input shaft. The oil can run along the shaft and onto the clutch. Evidence of that would be gearbox oil leaking from the bellhousing with a few drops or more dripping from the split pin hole onto the floor under the car. I believe that this can be caused by wear to the frnt cover in turn caused by not centralising the front cover before tightening the fasteners.

It it less likely to be caused by an engine oil leak (from the crankshaft "seal") as that would tend to be thrown out by the flywheel and not easily reach the clutch.

In any case, if it's not residual pressure in the clutch hydraulics then it sounds like the engine has to come out.

By the way, your paintwork looks fantastic in the picture!
N McGurk

A small amount of oil leaking from either engine rear or gearbox front will not affect the clutch. Al large leak will screw it up, but that much oil would make a big puddle on the floor. Been there.

First suspect is the master cylinder push rod being adjusted a tad too long with no free play. When the master piston does not return all the way to rest position it can inhibit free return of fluid to the reservoir. The clutch may work normally when cold, but as things warn up thermal expansion of the fluid can prevent the clutch from fully engaging. Solution there is to shorten the master push rod until you have a tiny bit of free play in the rod and at the pedal.

Next suspect is a deteriorated hose at the slave cylinder. Common mode of failure is internal swelling that will constrict to inhibit fluid flow. This comes on very gradually, sort of sneaks up on you in a way you may bot notice until it's more serious. The first sign is rather heavy pedal pressure (stiffer than normal) when you press the clutch pedal quickly, and much lower return force when you let the pedal up quickly. As the problem progresses the difference between application force and return force increases.

If it gets bad enough to make application force double the normal amount, then the fluid will not return at all. But before it gets that bad the clutch will be slipping momentarily with release of pedal, and it gets progressively worse. If ignored long enough the friction disc will wear out, in which case it will slip all the time, and the only fix is to R&R the engine to replace the friction disc. If you catch it early all it needs is a new hose.

Another possibility is if the friction disc is worn out the clutch will slip. This also comes on gradually but in much shorter time span. When it slips a little if you continue to drive it will be slipping a lot in short order.

Another more rare problem is a faulty slave cylinder where the piston might be sticking in the bore on the return stroke. I have a pictures of a slave cylinder that had an aluminum) piston (bad design that became badly worn and cocked in the cylinder bore. http://mgaguru.com/mgtech/faulty/ft032.htm
Barney Gaylord

I agree with Barney - I had to fiddle with the length of my clutch push rod (the one up at the M/C that connects to the pedal) four or five times until I got it right.

Loosen the lock screw, and adjust it until it has about 1/8" free play, then tighten.

If you check yours and it has no free play now, that might be your problem!

JIM in NH
AJ Mail

It is fairly common for people to monkey with the slave cylinder pushrod when clutches or hydraulics start to go goofy. Typically, the pushrod is made longer. Then they fix the real problem, and the clutch may work when it is new. As the new clutch beds in, the driven plate gets thinner for the first few hundred miles, which raises the release table relative to the flywheel, which bottoms the slave cylinder piston, which makes the clutch slip.
Since you say everything is new, check the MC pushrod clearance as mentioned. Then, check that you can push the release arm so that it pushes the slave piston deeper in the cylinder; there should be about 1" travel with all new parts. Note that any movement here means the clutch will not slip from this cause; it is NO movement that will make it slip. If that's not it, good chance the pressure plate is DOA.

FRM
FR Millmore

DOA??????
F. Camilleri

One thing that might have been overlooked is the brake fluid.

Because most of the rubber seals in British cars are made from natural rubber or a composite with a high natural rubber content you have to use a brake fluid that will not cause the rubber to swell. A lot of North American brake fluids will do this.

If the rubber seal / cups have swollen, particularly in the slave cylinder the clutch may not be fully releasing and staying partly disengaged due to the slave cylinder sticking because of a swollen seal.

Check to see if this might be the problem in this case since the car has had time for the seals to react to the brake fluid if not compatible.

If this is the case then you will have to replace all of the contaminated seals and fill and bleed the system with the correct type of brake fluid.

Steve
S L Bryant

Frank-
DOA = Dead On Arrival

FRM
FR Millmore

Before fiddling with the hydraulics I would work the clutch pedal a few times and go underneath the car. Feel the clevis pin at the end of the slave cylinder pushrod. If it is bound and tight, then it is the hydraulics or the length of the pushrod holding it back. If it spins and slips back and forth freely (You would also be able to remove the pin and replace it again) then unfortunately the probem has to lie in the clutch itself. While the pin was out you could perform the test with the slave cylinder as FR has mentioned above. I believe you would have to loosen the bleed screw to push the piston back.

Ralph
L Poupard

All,

Thanks for all the advice . . .

Overall, y'alls comments have convinced me to wait for the new hose, slave cylinder, etc and then fiddle with the car for a few weeks and only pull the motor if nothing else works. Even if I cant get it working properly I will certainly learn a few things so it won't be wasted effort.

I have been a regular visitor to Barney Gaylords website ever since I got the MGA back in 2002 and its now my primary reference point for all things MGA so it was great to get a reply to my question from Barney.

Tony Valentine (in Bangkok where MGAs are few and far between)

A Valentine

If it were my car I would feel the clevis pin. It will take the guesswork out of it. That being said, if the slave and hose are old, it would be a benefit to change them anyway. Even if they are not at fault it would help add some reliability. I always try to devise a test before just changing things. It avoids spending a lot of money and doing a lot of work for nothing. Too much guesswork can turn this hobby into a nightmare.

Ralph
L Poupard

This thread was discussed between 25/09/2011 and 28/09/2011

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