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MG Midget and Sprite Technical - Concentric Clutch Release

I know aspects of this have been covered before, and those threads are what have encouraged me to head in this direction.
I want to replace a tired ribcase and standard hydraulic set with a type 9 and concentric. I think I have most of the parts, including a concentric cylinder from ebay, that looks like a Burton one. (See Pic1) Does it look promising?
Should there be a clearance on the release bearing, or is it supposed to have a preload? If so, how much? I know ther are spacers available, although I might have the wherewithal to turn one.
Does the release bearing engage somehow with the diaphragm spring fingers, or just rest against them. (See Pic2)
My plan it to measure the space involved, and mock up the whole drivetrain outside the car with a new master cylinder.

M Crossley

Pic2 MED flywheel and 7.5" clutch

M Crossley

Looks like you have some nice parts there.

Different kits may require a different approach.

On mine the release bearing is in constant light contact with the cover plate fingers, and rotates with the clutch when the engine is running. This position can therefore be used to calculate the combined thickness of spacers and mount that are needed to hold the concentric slave in the correct position within the bellhousing.

Put a straight edge across the bellhousing flange (that bolts to the backplate) You can then do all of the necessary measurements from this and from the backplate whilst the thing is disassembled.

The "nose" that the release bearing slides along needs to extend into the clutch, beyond the spring fingers. Not far enough and the slave will push the release bearing off the front of the nose and all hell will break loose! But not so far that it fouls on the driven plate, which would also be considered a BAD THING!

As I was using the Ford slave I rigged it up with a hydraulic line to the Sprite master cylinder so that I could check how much travel there was when I pressed the pedal. I think the requirement was between 3/8" and 1/2". Less than 3/8" it wouldn't disengage fully. And more than 1/2" it would overthrow and ruin the clutch cover spring fingers. Like all clutch mechanisms, it is the clutch cover that pushes the release bearing and slave piston back, so that as long as the M/C conveniently displaces the right amout of fluid, then the thing will automatically adjust so that the release bearing sits in the right "resting" position.The light forwrds pressure being maintained by a thin coiled wire spring within the slave bellows.

All reasonably straightforward. Needs careful measurement and a bit of clear thinking. It is then VERY exciting when you start it up for the first time, and press the pedal!
Guy W

Looks fine, make sure you have a gap between the roller bearing and the fingers, maybe an 1/8", you need some pedal free play.
Mark Turner

Mark, see what I have just written. On mine there is no gap, no free play. The release bearing is in constant light contact. That is how it is supposed to be.

But different designs may vary in this respect.
Guy W

Looks like were were writing at the same time, however you need to make sure there is a gap with the slave fully retracted, then as the clutch plate wears and the fingers change there natural posn towards the slave you dont end up with a spliping clutch because the release bearing is always pushing on the fingers.

You are right the slave will find its natural position, also depends if there is a light pressure spring inside the slave to maintain pressure of the release on the fingers.
Mark Turner

Mark,
I see what you are getting at. There needs to be adequate travel "in reserve" But this isn't necessarily the same thing as having the release bearing clear of the fingers. That is not how the Ford one (and possibly others) is designed to operate. When assembled from new (no clutch wear) the slave was positioned so that there was plenty of travel in reserve to allow for future wear. But the release bearing resting position is in physical contact with the fingers, held lightly against them by a spring inside the bellows.

This light pressure is maintained throughout the clutch life, but as the driven plate wears the resting position of the release bearing moves back along the guide shaft to compensate. In fact, if there wasn't enough free travel along the guide shaft, the front of it (what I called earlier, the nose) would be too far forward and would be fouling the driven plate.

Mine has done over 60,000 miles like this. It is how it is designed to be.
Guy W

We set these at the recommended 6-8mm gap off the clutch fingers to allow for wear and clearance.
J L HEAP

Different design to mine then. The light spring inside the bellows on mine will push the release bearing all the way to, and beyond, the front of the guide piece. So it is not possible to set it up so that there is a physical gap between the thrust bearing and the clutch fingers. It wouldn't matter how far back you set the slave cylinder, the bearing will be pushed up in contact with the clutch cover. That is how it is supposed to operate. At the foot pedal end of things, there is free movement before the clutch begins to operate, then further travel before it begins to disengage. It's as it should be, and as I said has done well over 60,000 miles like this with no apparent problems.
Guy W

Seen here with a 20mm spacer, and a conversion spacer from short bell housing to the full V6 length.
Guy , is yours the same as this unit or does it have green bellows on it ie a Mondeo unit, as that is a contact all the time type.

J L HEAP

My SAAB type, like Guy's, needs to stay in contact with the fingers.

Most info I found when I was working my version out advised constant contact on the fingers.

I do wish they left a design alone after finding it worked…
Bill1

Yes, JL mine is a Ford Mondeo type. I had said right at the start that mine was different design.
Guy W

Jonathan, Guy,

So there are two different techniques? I assumed that the pictures I'd seen of the bellows equipped cylinders were just better protected.
So with the JLH/Burton type am I correct in thinking that I should measure/calculate the clearance between the pressure plate spring fingers and the contact face of the release bearing when the piston is fully retracted? And that should end up being between six and eight millimeters once I've calculated the correct spacer thickness?
I might be wrong, but the release bearing only appears to have 9.5mm or so of travel between fully retracted to the point where it starts to run off its locating tube, the one that surrounds the gearbox input shaft. I suppose it doesn't need to remain fully on the tube? Perhaps the kit I have is ersatz, and that was why it was on ebay.
How much movement of the release bearing is needed to dissengage the drive through the clutch?

Sorry to twitter on. Perhaps I have totally misunderstood the concept.

Mark
M Crossley

9.5mm should be abought right. Between 3/8" and 1/2" l think l said. I was describing my Ford type (green bellows), but travel to disengage should be the same. Take advice on clearance from those who have used the exact same version as yours. I was wrongly assuming that all concentric were of the constant contact design.
In theory I would expect a light constant contact one would be less prone to wear the clutch fingers.
Guy W

As far as I know, my Burton type concentric, has the bearing in light contact with the fingers. And it looks like it when looking though the inspection hole in the bell housing. I remember Tony Bolton (moor lane garage kit) telling me that he spent a lot of time getting the spacer thickness just right. Too thin and not enough depression of the clutch.

I'm pulling the engine for a rebuild sometime this year, so I'll be able to confirm it then.

And thinking about it, as Guy said, there's no spring return on the bearing apart from the clutch. So the bearing is likely to be in light contact.

Evey now and then a thread such as this comes up with very good information. Definitely one for my private collection of archives. I save it as an .mht file in internet explorer.
Lawrence Slater

PS. Obviously though as said previously, there must be space behind the bearing to accomodate wear in the clutch driven plate, and pressure plate to a lesser degree.
Lawrence Slater

Mark,

The system as shown in my photo requires the piston to be fully retracted and the bearing face to be set at 6-8mm from the clutch splines by use of shims, to allow for wear.( I believe they start at 20mm but thinner 2-4mm shims are available) In practice once the system is filled with fluid the line pressure will place the release bearing face ,very close to or slightly skimming the clutch fingers.
We never used the straight fingers though as they tend to wear the bearing face, we used the fingers with a curl at the end, which presents a larger area of contact.
A common problem with well used Caterhams.

Photo shows the curly fingers on an AP racing Clutch on a Zetec

J L HEAP

"requires the piston to be fully retracted and the bearing face to be set at 6-8mm from the clutch splines by use of shims"

I think we may be saying the same thing, but with a slightly different application.

Using the Ford "green bellows" unit, it is mounted on spacers so that the release bearing will slide back and forth along the bearing guide. When fitted, the light spring pushes the bearing forwards in light contact with the clutch fingers. So there is no clearance. However, if you were to slide it back rearwards along its guide, against the light spring pressure, you would indeed get something like 6-8mm clearance from the clutch fingers. This is the amount needed to allow for clutch driven plate wear.

On mine, the bearing rotates continually, in contact with the clutch fingers. As there is little or no relative rotational difference between these contacting surfaces, there is likely to be very little wear. This is unlike a non-contact bearing which spins up to maybe 6,000rpm and then slows down again repeatedly, at every gear change. And this is exactly when a lot of pressure is being applied as well! This represents a lot of friction, and therefore wear between release bearing and clutch fingers. On the light contact version like mine, this wear is virtually non existent, and the properly designed bearing, being lightly loaded for much of the time, should easily last many tens of thousands of miles.
Guy W

Sorry to labour a point, but sometimes it takes a bit of thumping before I get things straight in my head. Have I got this right?

On initial assembly, with the slave fully retracted, there should be 6-8mm available for the release bearing to slide along the bearing guide tube. This allows for friction plate wear.
Once fluid is introduced and the clutch is operated and then released, the slack will be taken up.
If its a Ford/SAAB type, the light spring will keep the thrust bearing in contact with the cover spring fingers.
If its the Burton/Titan type then, with no pressure on the pedal, the piston will slightly withdraw (like a brake caliper, due to elasticity in the seal)and the race may or may not brush against the fingers. Hence Guys comments about accelerating the bearing, and Jonathans about the advantage of curved finger ends?

So the two things I need to take into account are:-
1) Check that there is 6-8mm of initial rattle room for the bearing.
2) Check that there is clearance between the bearing guide tube and the driven plate.

Anything thats not penetrated, or I've forgotten?

Mark
M Crossley

This thread was discussed between 30/05/2013 and 04/06/2013

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.