Clutch problems

 In MGA-MGB, Midget, Transmission And Rear Axle

Clutch drag is when the car will not engage gear properly without grinding the gears, especially in reverse.

Generally, this occurs because the pressure plate is not retracting enough to clear the clutch plate.    There can be a number of causes of this, some in the hydraulics others in the clutch itself.  This is not an extensive list, but covers the common causes and some obscure ones.

Check first!

Because of the variety of causes, some of which entail removal of engine, it’s a good idea to sort out whether the problem is in the hydraulics or the bellhousing.   There is a magic number here, being the amount of movement there should be at the slave cylinder.  In an MGB it is 11mm but I can’t remember what a Midget should be and our ones here have all got non-standard clutches so I can’t measure at present.

If your cyl/pushrod movement is less than spec by 1 mm or more then the problem will be hydraulics (this includes the clevis problem).   If it is up to spec, then there’s nothing for it but to remove engine and find the cause.  More than spec in a 1275 Midget and you may find it is the plate hitting the flywheel (see below).

Internal causes of drag (ie Inside the bellhousing)

If the travel at the slave cylinder pushrod is the full amount and the clutch still has a problem, it is internal (ie inside the gearbox bell housing) and the engine needs to be removed to find the cause.

Most common internal cause is a failed release bearing (which is usually accompanied by a swishing noise), a collapsed diaphragm spring (easy to tell from out-of-true release ring), and failed clutch plate (if bits of lining break off and wedge themselves in unhelpful places)

Most common pressure plate assy failure is for the spring to collapse on one side, which results in the release ring being tilted so as noted above it is no longer parallel to the flywheel.  This condition shows up as a shimmy in the pedal when lust lightly resting your foot on it, and causes the clutch arm to be “batted” further back than normal. This uses up a chunk of its travel in just getting back to the release ring leaving insufficient to disngage the clutch.

Occasionally it can be caused by a piece of lining breaking off the clutch pate and wedging itself in the works – usually because of using lots of RPM!  This latter symptom can also be accompanied by clutch slip as the lining is no longer in the place needed to transmit the engine’s torque.

I also had trouble with clutch drag when a roller bearing was fitted.   I couldn’t see why, but returning to carbon fixed the problem.

Finally, if it has been a long time since the clutch was used, moisture in the plate can cause it to rust on to the flywheel.   Solution here is to put in gear and start the engine.   This may be enough to break it free, but make sure you have room for the car to move off in case it doesn’t!     If the clutch is still stuck, try accelerating hard with your foot down on the clutch.  If this doesn’t work, try full throttle on – throttle right off, then on, then off, so a jerking motion is set up.  All the time with your foot on the clutch of course.  And if that doesn’t work you need to remove the engine to attack it (a step which I have not yet had to do for this problem).

1275 Midget clutch – a special case

One of the endearing traits of the 1275 clutch is that it is very sensitive to the amount of pedal travel – too much travel causes drag as well as too little.   There is not much room to spare before the release ring in the centre of the cover hits the driven plate and pushes it forward hard onto the flywheel and creates drag that way.   This condition can be confirmed by seeing if it engages gear more easily as you lift your foot off gradually from the flat-to-the-floor position.  The problem is accentuated by linings which are too thick and can also be cause by lengthened pushrods.

External causes of drag (ie external to the gearbox)

Any less than 10mm in the test described above indicates a problem in the hydraulics/pedal etc. Air in the hydraulics will lead to this symptom and is most commonly caused by failing seals allowing fluid to leak out.

Sometimes it can be just that the release bearing in the clutch is getting worn and the hydraulics have taken up the resulting slack.  This draws more fluid into the system and as the reservoir is quite small it may well use up the spare fluid.  For this reason, if you can’t see any fluid leaks at the pedal or slave cylinder it is worth just topping up the reservoir, give the clutch a vigorous pump and see if it improves.  If it doesn’t come right up, you may need to leave it overnight for the all the air to bleed back up the system, (a useful tip generally when bleeding clutches) or even do a full manual bleed.

The next most common cause of insufficient travel after failing hydraulics is excessive play in the clevis pin at the top of the pedal:

Clutch Drag/not disengaging – a 10 min miracle cure! (sometimes…)

If your clutch is not fully disengaging, before you rush in & spend lots of money & time renewing the clutch and/or hydraulics check the slack in the clevis pin which joins the pedal to the master cyl pushrod. If there is movement there, (sometimes there is lots!) it will reduce the amount of movement generated at the clutch fork in the bell-housing. When this condition is present the first 2 or 3 inches of pedal travel feel extremely light – even lighter than air in the hydraulics.

To check if this is your problem, measure the movement at the slave cylinder pushrod as the pedal is depressed (see above). If less than 10mm, and you have free play at the clevis at the top of the pedal, this will highly likely be the cause of the drag.

A new pin & pushrod should fix it, and very cheaply and quickly at that – you don’t even need to remove (let alone strip) the master cylinder if you have the new pushrod at the ready and keep the piston from popping out while you do the switch. (An assistant can be useful to help hold the piston with a long skinny screwdriver) In severe cases, the hole in the top of the pedal will be significantly worn too, in which case it will be necessary to remove the pedal, weld up the hole and re-drill it.

A less common cause of insufficient travel is the wrong size cylinders.  MGA master cylinder should be 7/8″, MGB 3/4″, Midget/Sprite are 7/8″ (948cc engines) 3/4″ (1098cc engines) and 0.7″ (1275cc engines).  Slave cylinders are MGA and MGB 1.25″, 1275 Midget 1″, 1098 Midget I think is 7/8″ but need to check as the grey matter is getting challenged!

Bleeding clutches – they drive you mad!

Because there is no non-return valve in the clutch master cylinder, it can be very difficult to draw new fluid into the master cylinder when the clutch has been apart.  The lack of a return valve means the air in the pipe and slave cylinder is not isolated from the master cylinder so the suction generated by the master cylinder piston stroke  is dissipated right through the system rather than confined to the master cylinder.  The amount of suction remaining is not enough to draw new fluid in.

This is almost invariably the case when bleeding a clutch, so it is not a sign of a faulty cylinder!

Solutions are:

  • Fill the slave and master cylinders with fluid prior to fitting.  This can be a messy solution, particularly for the master cylinder if any fluid dribbles out while installing as it ends up in the engine bay attacking the paint or down in the footwell.
  • Push the slave cylinder piston as far into the cylinder as you can, and wire it in to hold it there during the initial part of the bleeding process.  This reduces the amount of air in the system.
  • Pressure bleed.  Either with a made-for-purpose pressure breather or by getting an assistant to stroke the pedal up and down while you blow for all you are worth into the reservoir (via a suitable hose which can be sealed over the reservoir opening).
  • Vacuum bleed – another made-for-purpose device, attach it to the bleed screw and let it do the job.

Some times the clutch gets to be reasonably working and just needs to stand (eg overnight) to allow the last of the air to bleed back up and out into the reservoir.

Pushrod length

A common mistake is to assume that insufficient travel is down to the slave cylinder pushrod being too short, and to lengthen the pushrod in response.  In the MGA, MGB and Midget, this is a complete waste of time and may cause other problems.  These clutches are self adjusting – ie rather than retract the slave cylinder piston right back into the cylinder after use (as does the Mini for example) the piston is left to lie where it came to rest once the clutch is disengaged.  It is thus in the right position for the next use of the clutch (assuming there are no internal issues which might bat the release arm – and therefore the piston – back further than normal).

Lengthening the pushrod on any of these clutches simply just pushes the rest position of the piston further back into the slave cylinder and does nothing for the clutch arm travel.  It may help if the piston is about to pop out of the slave cylinder, but there is always another cause for this – usually a worn out release bearing, but I have also seen the odd bent clutch arm.  If you lengthen the pushrod under these circumstances, you might find that when the offending component is rectified (for example, a new release bearing fitted) then the piston will bottom out in the cylinder before the clutch is fully disengaged.  This will result in accelerated wear of the release bearing, and may also be enough to cause clutch slip.

So the moral to the story is: Keep the slave cylinder pushrod at its original length!

The master cylinder pushrod is another matter, if it is too short it will restrict travel.  However, this only happens if someone has blindly fiddled with it before!

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