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MG Midget and Sprite Technical - 948 frogeye clutch

Please, please can anyone help??? I'm having a real problem in getting my frogeye clutch to work properly and it's driving me mad!
Even though I feel I've tried everything I just can't seem to get the clutch to disengage fully. Whenever I drive the car, getting into first gear or reverse when the car is at a standstill is very noisy, even though with first gear I use the trick of pulling back into second to get the gears moving. This mild crunching of gears can't be doing the gearbox any good, and it is also very embarrassing!
The car is as original, being a 948 frogeye with smoothcase gearbox.I replaced the clutch, release bearing and slave cylinder during the rebuild last year. I also fitted front disc brakes, changing the master cylinder to suit ie one with three quarter inch bores.I appreciate the slave and master cylinder are now not matched, but I'm not sure if this mismatch is enough to be cause of the problem.
An easy answer is that there is still air in the system, but I have bled the system over and over using an easibleed, without seeing any improvement.
Thinking about the slave/master mismatch I'm wondering if the effective length of action in the master cylinder needs to be made longer. This is on the assumption that as the the bore is slightly smaller the piston needs to travel further to move the required volume of fluid?
I've tried adjusting the the length of the pushrod which actuates into the master cylinder, but this hasn't made much, if any, difference.
The last thing which might be relevant is that I've noticed that on the back of the clutch pedal I'm using, is a spigot pointing forwards which limits the travel of the pedal when it reaches the end panel of the footwell. Although this might be a bit extreme, should I be grinding off this spigot so that the pedal can travel a little further and hence increase the travel in the master cylinder?
I'm sure there must be an easy and obvious answer to this, but I'm currently going round and round without much success. Any help would therefore be greatly appreciated.
Many thanks
Phil
P J Dent

Hi Phil,
It might be a good idea to change the slave and it's pushrod if the partnumbers are different. However, my money is still on air in the system. I have done just that, going round and round in circles trying to cure it. In the end I got a welding clamp and used it to ensure that the slave pushrod was pushed back as far as possible, then bled it again (for the umpteenth time) and low and behold got two bubbles out, tiny ones each about the size of a grain of rice. After that it worked. I had used an easibleed for all my attempts but it was the welding clamp that sorted it.

Rob

Rob aka MG Moneypit

I had a similar problem, 1098 engine with a frogeye clutch and flywheel + ribcase. In the end I had to lengthen the pushrod by a about an inch or so. This fixed the problem.

Simon
SA Wood

Phil,

The frogeye master cylinder is 7/8" and has an area of 0.60 square inches and the later 3/4" MC has an area of 0.44 square inches so that is a reduction in fluid volume displaced for a given stroke and therefore slave stroke of 27%. So sounds like you either need to increase the MC stroke or fit a smaller slave to suit if that is possible.
David Billington

Fit a slave from a 1098 discbrake car.
Onno K

The bore of the 1098 and 948 slaves are both the same. The later 1275 slave has a bigger bore. A standard slave should be fine. I would first make sure that there is no air in the system. Secondly I agree with David that the stroke of the master cylinder needs increasing. There are two adjustments on the frogeye pedal box. To increase the stroke you also need to back off the pedal stop screws. This makes the clutch pedal move up a little more allowing the pushrod to be lengthened and thereby increasing the stroke. You may have to adjust the brake pedal stop so both pedals are at an equal height
Bob Beaumont

Firstly, thank you all for your comments and I now feel a bit more confident about what to do next!
Thank you Bob for confirming that my slave should be ok. That is a relief.
I had already backed off the master cylinder clutch pedal stop screw, almost as far as it would go, but I will now back it off to it's limit. At the same time I will extend the pushrod as far as the thread will sensibly go, to compensate. Maybe this will give enough extension of piston travel to allow for the 27% reduction in volume of the smaller diameter master cylinder.
I will also look at increasing the pedal travel by reducing the spigot on the back of the clutch pedal, so that the pedal will go right down to the floor (or nearly). I have several old pedal boxes and I have noticed that one of them does not have a spigot on the clutch pedal, so there must be different pedal boxes for different cars? I can't be certain that I am using a frogeye pedal box!
Finally, and probably most importantly, I've just got to get to grips with bleeding the system. It sounds as though making a determined effort to push the slave pushrod back as far as possible when bleeding is the answer!
Anyway thanks again, and I'll let you know if I've succeeded in due course!
Phil
P J Dent

Phil,
If you remove the rubber where the clutch fork goes trough, You can see how far the fork goes. (somebody else is moving the clutch pedal)
If the fork goes till the end (against the bell housing) its not air , a small bore master cylinder or a short push rod ( tree sizes do exist). Than it it the dimension of the release bearing. (several exist)
I had to add a spring to push the slave back (see photo) It did not disengage enough bij itself.
If you helper is pushing the clutch pedal 2 or 3x and the clutch fork goes further it is air.
About bleeding is enough in the archives. It is a pain in the ass.
Flip


Flip Brühl

Thank you Flip, that is helpful. I will try that, and just keep my fingers crossed that I don't have to take the engine out again to change the release bearing.
I have also not forgotten Simon's comment about lengthening the pushrod. It's good to know that I'm not the first to have this problem!!
One thing I failed to mention at the beginning is that I shortened the master cylinder pushrods when I changed to the three quarter inch bore master cylinder. I read somewhere that this is necessary when converting, but maybe doing this has also contributed to the clutch problem.
Thanks
Phil
P J Dent

Hi,

The standard frogeye clutch pedal does have the spigot on the back. It stops the clutch overthrowing. The other valid point concerns the clutch release bearing. There is a specific one for each engine size. The 1098 and 1275 ones are shallower. The aftermarket bearings do vary. I have two 1275 ones, one is the genuine Borg and Beck and one from a 'supplier' and there is a difference of almost 3/8" between the two. Not what you want to hear i suspect
Bob Beaumont

Bob I think you meant to say the 1275 thrust bearing is deeper, i.e. there's more offset at the mounting lugs. Agree about the rubbish often sold as 1275 when they are nearer the 1098 dimensions.
David Smith

Shortening the pushrods should not be necessary. I have the 3/4 bore master in my Frog as its got discs. I only made a minor adjustment to ensure the correct clearance.

David oops yes I meant deeper.



Bob Beaumont

Thanks again everyone for your comments.
The latest is that I have checked the clutch slave cylinder action as suggested by Flip, and as far as I can see, when the clutch pedal is depressed, whilst the slave cylinder pushrod extends, it does not press the fork as far as the edge of the bellhousing.
However, I also noticed that before the clutch pedal was depressed, there was more than a quarter of an inch of free play in the slave cylinder pushrod. It is loose!
I appreciate that the slave cylinder should not be putting any pressure on the clutch, when the clutch pedal is not depressed, but I can't believe there should be so much slack?
Has anyone knowledge of what play, if any, that should be present?
This must surely mean that when the clutch pedal is depressed and the slave cylinder piston starts moving, it has to take up this play before the fork begins pressing against the release bearing. No wonder the clutch won't fully disengage when the clutch pedal is depressed?
So I am beginning to lean towards doing what Simon said he had to do, ie in my case, to lengthen the slave cylinder pushrod by say a quarter to half an inch. This could be all that is needed to make the clutch work properly?
Phil

P J Dent

Just to finish off this thread, I can report that thanks to the replies to my initial post, I have successfully sorted the clutch problem described earlier.
Since my post yesterday, and as a last ditch attempt, I spent some time bleeding the clutch yet again. I put a large G clamp on the slave to compress it first, and was hoping that this time I would find some air bubbles coming through. It was pretty inconclusive, and afterwards I found that there was absolutely no difference in the actuation of the clutch.
Then as a last resort, as mentioned yesterday, I followed Simon's advice and changed the slave cylinder pushrod for one about half an inch longer. I happened to have a spare mini engine in the garage (as you do!), and blow me, the pushrod on it was just the right length I needed.
I could not believe it. The car is now transformed, and all crunching of first and reverse gears has completely gone. Yippee!!!!!
So thanks again to those who contributed to this thread, this has given me so much more confidence in the car and it's now all go for the summer! Wonderful.
Phil
P J Dent

This thread was discussed between 12/03/2014 and 14/03/2014

MG Midget and Sprite Technical index

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