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MG Midget and Sprite Technical - 1275 clutch slip.

Thought I would put this out as a new thread because it might apply to others as well.

The thread detailing my problems with a type 9 conversion is active if you want to know more about the background of the problem.

I think the problem is the original clutch pressure plate but I'm not sure so want to put it out to a wider audience.

I took the opportunity of buying a new clutch outer from Moss on Friday afternoon so I could compare it with the one already fitted.

On the new one I put a straight edge across the face (see image) and used some drills to measure the distance between the face and the driven plate. I could get a 1/8" (8/64") diameter drill under the straight edge but not a 9/64" diameter drill.

I did the same on the original clutch and found I could easily get a 5/32" (10/64") diameter drill under the straight edge but not an 11/64" diameter drill.

Is this significant? Is it likely such a small difference could cause my clutch slip problem?

Rob


Rob aka MG Moneypit

When you bolt the cover to the flywheel, the pressure plate is pushed back against the springs in the cover, compressing and creating the friction against the driven plate.

The further away the pressure plate is to begin with, for the same spring pressure, you'd expect less friction. So I would assume there must be a greater spring rate in the new cover if the distance is increased.

Is the spring rate in your original cover (which has a greater uncompressed clearance), less than in the new cover?

As it happens, I've been looking at my driven plates on my midget 1275 clutch. There's only 1/8"(8/64ths) in each of the linings to begin with. So maybe a few 64ths does matter, if the springs aren't stronger to compensate for it.
Lawrence Slater

Hi Lawrence, the two pressure plates were bought as standard 1275. The original was bought off eBay (from Sussex MG I think) the new one bought from Moss on Friday. Both are unbranded, but then branding doesn't seem to mean much these days.

I could probably rig up something to measure the spring rate, maybe do that later.

Are higher spring rates easily (and not too expensive) available?

Rob
Rob aka MG Moneypit

You really need to be able to measure the clamping force when compressed.
Dave O'Neill 2

Good point Dave. I think I have a spare flywheel somewhere.

Rob
Rob aka MG Moneypit

when you say 'unbranded' are there no manufacturer's marks or numbers whatsoever?
David Smith

I just measured two 1275 clutch covers.

1). From my RWA Midget. Borg and Beck. Looks pretty new. 3.4mm (8.566929152/64ths")

2). From my Sprite when I swapped to a Type 9 circa 18 years ago. Unbranded, but says Leamington Spa. 4.2mm (10.834645696/64ths").

Neither slipped. I don't think the distance is too relevant. There must be something else going on.

As long as there is no pressure from the release bearing, then as long as the spring rate in the cover is good, it shouldn't slip.

How old/used is the original cover? --- I'll have a read of your T9 thread. ;).
Lawrence Slater

Rob, this is a quote from 2008 by Paul Walbran.

"All a longer pushrod will achieve is to push the piston further up the bore. This won't be a problem unless it goes so far that it bottoms out, which may not happen intially but as the clutch wears and pushes the piston back it may do so. If this happens, the clutch is not fully released resulting in premature release bearing failure and/or clutch slip."

This was a discussion about 1275 g/box and clutch problems. However, if your slave cylinder is also "bottoming out", and the piston can't retract any more, you'll likely get slip too.

Lawrence Slater

The original one has no markings at all. The new Moss supplied has printed on it 76253/10 X09J103. No makers mark.

The original was used with a morris minor type ribcase with a standard 1275 clutch setup. It didn't exhibit any slippage. It did about 750 miles before the engine ate the gearbox. When I put the type 9 in I retained the same clutch except for the driven plate which was a new hybrid from David Manners. I had to remove the centre pad from the diaphragm fingers. I fitted a SAAB 9000 concentric slave because there was no provision on the bell housing I had for a clutch operating lever.

When I tried with the type 9 there was clutch slip from second gear up. I checked for seizure of the clutch pedal, adjusted the pushrod length as short as it would go, then cut 1/4" off the pushrod length in successive attempts to cure the slipping. It did improve though only to the extent that the slipping occurred 3rd gear and up.

It was suggested that the slave might be overthrowing the clutch and I did a few quick calculations which suggested that there was twice as much travel with the concentric setup, so clutch overthrow was very possible. To cure that, I modified the clutch pedal so there was about half as much travel of the master piston compared to the original. The clutch slipping improved after doing this but it was still evident in 4th and 5th. I even shortened the pushrod again and it is now quite slack.

So all the steps taken so far have improved the situation but not eliminated it. While driving, the slightest pressure on the clutch pedal will induce slip. The clutch action is very progressive, fully down and I can select any gear easily. Drive take-up is smooth but the bite point occurs just before you take your foot off the pedal.

So I was left with either the slave is reluctant to back off, or there was something wrong with the clutch diaphragm spring. I took the slave off and was able to back it off manually just using finger and thumb (of both hands) pressure.

So I don't think it is the slave so I'm looking for a problem with the clutch itself.

Rob

Rob aka MG Moneypit

Great idea of using drill bits as measuring bits...im a fan

As a pure amature, I just cant imagine the differance in space would cause the slipping...it just seems that would fall in the margin of error

Id think machine shops remove that much off of flywheels just to resurface them

Good luck

Jeff
Prop and the Blackhole Midget

Lawrence

Leamington Spa is/was the home of Automotive Products.
Dave O'Neill 2

"While driving, the slightest pressure on the clutch pedal will induce slip. "

" Drive take-up is smooth but the bite point occurs just before you take your foot off the pedal."

All of which suggests that the release bearing is somehow still pressing in the cover. Try flexing the cover in with your hand, and you know how much mechanical advantage the hydraulics are giving you. So if the slightest foot pressure results in slip, there must be somehow some significant pressure on the diaphragm all the time.
Lawrence Slater

I think I'm back to the question, "Is it the slave resisting or is it the diaphragm spring not strong enough?".

I'll have to find a way to measure both diaphragm springs.

In the meantime. I've seen uprated clutches for 1275 Midgets. Does this mean the diaphragm spring is stronger, the driven plate uses harder materials or both?

Is there a better clutch that will fit in place of the standard one? That will fit without having to modify the flywheel?

Rob

Rob aka MG Moneypit

I've only in over 35 years, used the standard clutch. I've only ever had slipping when down to the rivets on the lining, or when I've reused the cover too many times when just replacing the friction plate.

Mind you my engine 1275 is essentially standard.

How much more power are you pushing out? Is that your problem? But the bite point seems to suggest the relaese is too close to/pressing on the cover constantly.
Lawrence Slater

I think the engine is about 85BHP, I'll have to find the printout I got from PB's rolling road but I seem to remember that figure.

The release bearing on a concentric setup is supposed to run in contact with the clutch but as you say is it pressing too much on the diaphragm fingers? I'm struggling to find a reason why. If I undo the bleed nipple I don't get any release of fluid so that would indicate it is not a hydraulic problem so it must be mechanical.

Maybe it was a bad choice using a SAAB 9000 concentric with a standard 1275 clutch and maybe it might be solved using a clutch with a stronger diaphragm spring?

Rob

Rob aka MG Moneypit

The standard 1275 diaphragm pressure plate seems good for more than 100bhp.

The 1380 in our Minor is coupled to a Toyota 5-speed box, so the centre plate is a hybrid - 1275 outer and Toyota spline, but the pressure plate is standard 1275, as there is no upgraded version available from AP.

The engine output is 112 bhp on a rolling road, and we have never had any clutch slip at all - mind you, it still has the original mechanical linkage, none of this fancy modern hydraulics technology!

Rob,

If the new clutch cover has the clutch face closer to the flywheel than the old one, with the centre plate fitted it will have to move further against the diaphragm spring as it is fitted. The result will be a greater clamping force on the centre plate than the old one - assuming that the spring rates are the same.

I think I would look at the release mechanism for probable cause?
Richard Wale

Maybe this is better posted here?

http://www.apracing.com/Info.aspx?InfoID=127&ProductID=3081

Installation & Technical Information

The information contained in this section covers the relevant technical and installation details for the range of cover assemblies. This information includes:

- Mounting Holes:
- Dowel Holes:
- Mounting Hole / Dowel Hole Position:
-Set-Up Height:
- Diaphragm Spring:
- Release Plate:
- Clamp Load:
- Driven Plate Thickness:
- Torque Capacity:
- Maximum Rotational Speed:
- Maximum Release Travel:
- Release Bearing Type:

It is important that the correct type of release bearing is used for each cover assembly configuration. If a release plate is fitted a carbon thrust bearing should be used. If a release plate is not fitted and the diaphragm spring has straight fingers then a round nose ball type bearing should be used. If a release plate is not fitted and the diaphragm spring has curved fingers then a flat faced ball type bearing should be used.
The maximum recommended travel for the release bearing to prevent the diaphragm spring being over stroked. The maximum recommended rotational speed for each cover assembly. Given in rpm. The torque capacity for the clutch will vary depending upon which type of driven plate is to be used. The table gives the figure for all the various types of plate that can be run with the particular cover assembly. Given in Lbs / Ft and NM. Two thicknesses are given, the ‘new clamped’ thickness and the ‘minimum worn’ thickness. ‘New clamped’ is the thickness of the driven plate when first installed but with the plate in the clamped position. The ‘minimum worn’ figure is derived from the clamp load characteristics of each individual cover assembly, and can be used as a guide to the life of the driven plate. Whilst the driven plate thickness is between these two figures the clamp load stated will be within specification. When the thickness of the driven plate drops below the minimum worn figure the clamp load will be reduced which may result in clutch ‘slip’. The amount of clamping force exerted by the diaphragm spring (identified by colour on spring fingers). Given in Lbs and Nm Informs you if a release plate is fitted to the diaphragm spring fingers.The colour identifies the spring strength whilst the ‘design’ details the finger form, straight or curved (curly).The dimension from the flywheel face to the diaphragm spring fingers or to the top face of a release plate if fitted.
The angular dimension between any given mounting hole and a dowel hole, provided that they are both equi-spaced on their relevant P.C.D.Number of, diameter, pitch circle diameter and spacing.
Number of, diameter, pitch circle diameter and spacing.
Lawrence Slater

This thread was discussed between 08/03/2015 and 15/03/2015

MG Midget and Sprite Technical index

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