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MG MGB Technical - Proper flywheel resurface, Clutch engagement point

Hi all. I’ve got a question about the depth of the step that should be machined when resurfacing a flywheel. I believe the outer edge of the flywheel where the pressure plate bolts up is machined slightly deeper than the friction surface. If that is correct, what should that step measure?

I just got my 68 BGT with a GK engine running again, and the clutch does not engage until the clutch pedal is almost fully released. Also, the pedal pressure seems very light, and I can force it to slip in 4th if I give it a lot to throttle as I release the pedal.

When I had it apart, the clutch components all looked new (the engine was the victim of a bad rebuild that only had 2000 miles) so I did not replace them. We took a guess at the step depth, and I think we did about 0.008 or 0.010 inch, which approximately matched another old flywheel I had.

A diagnosis is a bit more complicated by the fact that I put in an overdrive transmission of unknown history when I put it back together. I need to figure this out pretty soon since I am contemplating taking it on a trip from Kentucky to northern California in a few days.

Thanks, Charley
C R Huff

Charley. The clutch release point may indicate that there is air in the system. If the clutch, when depressing the pedal, releases at the same point as it does when the pedal is being released, there is not a problem. But, it would be unusual for the clutch to release soon after the pedal is depressed. If the clutch does not release, as it is being depressed, as it does when it is being released, it is a sign of air in the system. Very common problem due to the routing of the hard line from the clutch master cylinder to the slave cylinder.

There is a lot of information in the archives about how to get all the air out of the system. I use a special tool that fills the system with fluid through the bleed nipple on the slave cylinder. Others use a pressure bleed system off the master cylinder. Just having an assistant pump the pedal, hold it, then bleeding the slave cylinder--a number of times, will work eventually.

As to the fly wheel, I would have to measure one to give you a determination and that would not be "fact", only what the one I measured was. Who knows how many times one might have been resurfaced over the years? If no one else has this measurement immediately to hand, I will measure the flywheels I can find in my parts locker today and post the average.

Les
Les Bengtson

The hydraulics are supposed to be able to take up any amount of wear in the mechanical linkages between the slave piston and the pressure plate. However when the friction surfaces (2 on the friction plate, one each on the cover plate and the flywheel) wear down too far or the flywheel is machined too much the springs cannot supply enough pressure, which is when the biting point starts moving up the travel, and eventually you get clutch slip in 4th, which seems to be what Charley is complaining of.

If there is air in the system the biting point becomes too *low* due to insufficient travel of the slave push-rod (should be about 1/2"), and in extreme cases the gears will bind going into a forward gear and grind going into reverse. Wear in the mechanical linkages at the *master* will have the same effect.
Paul Hunt 2

Thanks Les and Paul,

I am reasonably sure that my problem is not air in the system because I think that would cause excessive pedal travel before the clutch disengaged. In my case, it disengages after only about a quarter of the pedal travel. I bled it with vacuum applied to the bleed nipple.

I have not yet checked to see if the slave cup is bottoming out, but since I installed the slave after the engine and transmission were in the car, I doubt that is the problem.

Regarding Paul’s comment about excessive flywheel machining not allowing enough spring tension, I am thinking that the tension would be restored if the bolt face for the pressure plate (clutch cover) were machined more than the friction surface. I am thinking there should be a minimum/maximum specification for this “step” but have been unable to find one in the books, the archives, or from parts suppliers. I tried to find out while the flywheel was in the machine shop, but was not successful.

I certainly would not want to pull the engine again just to make another guess, or to find out that it was not the problem.

Charley
C R Huff

Charley. Three flywheels measured and the central section averages .003" higher than the outer section. (I suspect that the only reason it it higher is to allow the center section to be resurfaced rather than the entire surface.)

Paul is correct that the most common failure mode is with the clutch engaging just off the floor. This is commonly caused by worn parts--carbon throw out bearing, the slave cylinder pushrod, and the master cylinder pushrod. The later is available from Moss and most of the clutch master cylinder rebuilds I have done recently have required that the pushrod be replaced as the holes have doubled in size front-to-back.

Failure of the clutch pedal in the high position is, most commonly, an indication of bad seals in the master cylinder. There is sufficient sealing capability to depress the throwout bearing sufficiently that it slightly disengages the pressure plate, but the poor sealing does not allow the pressure plate to be fully depressed and the clutch fully released. Thus, when the pedal is near the top of its travel back to the at rest position, the minor force against the pressure plate is removed, the clutch seems to work properly and the impression is that it is grabbing near the top of the return stroke of the clutch pedal. Air in the lines is the most common cause of this problem, but it can be master cylinder bore/seals also.

You might also want to make sure you have all of the copper sealing washers between the banjo bolt to the clutch hard line and the clutch master cylinder. If not, air can be drawn into the system and fluid expelled out into the shelf below the two master cylinders.

Les
Les Bengtson

I actually said that wear in the linkages at the *master* end causes a low biting/release point. The hydraulic system is designd to take up all reasonable wear in the linkages at the slave end.
Paul Hunt 2

Les,

Thanks for taking the time to measure all your flywheels. I feel a little better knowing that yours are in the same ballpark as mine.

I think it is likely that the clutch master is suspect because the car sat for three years while the engine was out. I did have to rebuild the clutch slave. I will probably have to address the master at some time, but since it seems that I have more active pedal travel than I need, I think I have another problem as well.

I did talk to someone with a similar problem to mine, and he finally solved it with a competition clutch. He felt that there were some weak pressure plates on the market for a while. Something like that could be my problem because the effort to depress the clutch pedal is surprisingly light.

Thanks again,
Charley
C R Huff

Charley. The fact that someone "solved the problem with a competition clutch" is only useful if a full analysis of the problem was conducted and the original clutch system found to be defective. This would be a test beyond my means--and I have a fairly extensive collection of tooling and friends who are professional mechanics with well equipped shops.

Let us look at some of the factors involved.

First, there are two main brands of clutch available for the MGB, the Q-H and the B&B. The B&B was the original, factory clutch system and is now being produced by Delphi. I have used a number of them over the years with no problems. It is possible you got a bad B&B/Delphi clutch, but the odds are against it. The Q-H clutch assemblies have been noted, in the past, for having problems. Thus, knowing the brand of clutch you installed is a good starting place for our further investigation.

Second, the clutch plate/driven plate is marked, on one side, as the flywheel side. It is possible to assemble the clutch plate backwards which always causes problems. The situation you describe could well be a symptom of the clutch plate being installed backwards, but one has to remove the engine to discover this.

The clutch master cylinder is an obvious suspect in this type of thing. Sitting up for three years is not a good thing for the master cylinder unless it was removed, cleaned throughly, and packaged in such a manner that it could not rust internally. A bad bore in the master cylinder can cause the symptom you describe. So can a bad bore in the slave cylinder. In both cases, I would expect some fluid loss to be taking place, but it may only be a minor amount and not noticed over the short term.

The last possible cause is air in the lines, either through not being bled fully (always a difficult task to get all of the air out) or through some form of ingress of air into the system. Air can get into the system if the bleed nipple does not make a tight seal in the slave cylinder, if the flex line is not tightly sealed to the slave cylinder, if the flex line is not tightly sealed to the hard line, if the hard line has a minor crack in it, if the hard line is not tightly sealed to the banjo assembly, or if the banjo assembly is not tightly sealed to the master cylinder. In all cases, I would expect some loss of fluid to be present and, by looking for the fluid, you might be able to determine where the possible leak is.

I am not sure how many clutches I have installed over the last several years. But, it is at least five. None of which demonstrated any problems with the clutch and pressure plates. All were either B&B or Delphi. Thus, I do not believe that a bad pressure plate is your problem.

Les
Les Bengtson

Les,
I don’t know the brand of the clutch that was installed because it was done during the previous engine rebuild that was done 2000 miles before I bought the car.

However, it is a good bet that it was the cheapest available based on the rest of the job.
The rebuilder cut corners by not timing the custom grind cam and then throwing the old lifters in on top of it, some of which did not fit with push tubes. Apparently they broke a compression ring, so they installed one that was missing about ¾ inch, though to their credit they went to the trouble to grind the sharp edge of the break so that it would not score the fresh cylinder wall. They saved time and trouble by not cleaning about a cup of junk out of the water jacket, so much that the block drain was useless. They put in new valves, but did not bother to do a valve job. Apparently they just hand lapped them into a head with worn out seats of varying depths. Then, for the icing on the cake, they put in the wrong oil pump. Got any idea what oil pump looks just like an MGB pump but is about 5/8 inch shorter? The result is that, with a bit of kick back on shut down, the pump drive will lift out of the pump, and thus deliver zero oil since it is unlikely to fall back into the driven slot.

I am reasonably confident that I installed the driven disc with respect to the flywheel side since I am aware of the need, but it is possible that I was asleep at the wheel. I will certainly rebuild or replace the clutch master before I pull the engine. It probably needs it, like you say, since it sat so long. I’ll be working on the car today, so I will check to see that the slave is not bottoming out. If none of that helps, I suppose I will pull the engine, examine the clutch parts, and try to compare them B&B/Delphi version. At the moment it does not slip unless I force it to by coming off the clutch in 4th with a lot of extra RPMs, so I plan to put some miles on it to check for anything else I need to debug.

Thanks, Charley
C R Huff

Your clutch should not slip in 4th gear. This indicates a worn friction plate or weak pressure plate diaphragm. The flywheel is machined flat meaning that the pressure plate fixing surface is level with the friction surface. If yours is higher this means that pressure is being relieved from the disc and possibly the cause of the slippage. Removing metal from the surface of the flywheel makes no difference to the clutch biting point as the system is self adjusting and the carbon bearing simply follows the pressure plate having been pushed by the slave cylinder spring.
Iain MacKintosh

Charley,
Wow, seems like you got one of the worst DPO's working on your car!

Having dealt with a lot of marginal cars over the years that were 'rebuilt' on the cheap - it sounds like you may have a used pressure plate or a rebuilt one.

Back in the days when I had no money and lots of time, I would re-use decent looking clutches on my cars. Invariably this lead to a lot of problems. So many in fact that it became standard practice for me to replace all clutch components, no matter how good they looked. Many of the pressure plates were marked as rebuilt. My opinion of them is they are invariably junk!

When we bought the B that got rebuilt for my Dad's retirement present, the DPO proudly told me that there was a 'new rebuilt clutch' in it. With his buddy's parts discount it had cost him $5.00. Yep, $5 for a complete 3 piece clutch kit! Once the car got home the clutch got replaced with a brand new one and the 'new rebuilt' promptly went in the trash.

Many years ago QH had a bad reputation while B&B always worked. I only use B&B clutches - period.

Tom Sotomayor

Thanks Iain and Tom,

Sorry to be so long getting back to you. I’ve been busy on the road putting miles on the car. I think a weak pressure plate is probably the cause of the problem, but since it only slips if I force it to do so, I decided to go ahead and give it the road test. It passed the first half of the test just fine, making the 2750 miles from Louisville, KY to Eureka, CA without the hint of a problem. Of course I don’t want to brag too much since it still has to pass the second half of the test by getting me back home again. Thanks to all for the help and advice.

Charley
C R Huff

"Many years ago QH had a bad reputation while B&B always worked. I only use B&B clutches - period."

I recently had both a QH and B&B friction plate in hand. I can gaurantee that they had been manufactured in the same place. They even had the same makers stamp on the plates. One had then had the QH part number added, and the other a B&B part number. Probably not historically the case, but I think now both companies use the same ultimate manufacturer and so there will be no quality difference.

Iain C

I D Cameron

If the master cylnder is hanging up inside so that the piston does not return completely to releive the fluid pressure in the line pressure will build from heat and preload the clutch not allowing it to fully engauge.
John H

"It passed the first half of the test just fine, making the 2750 miles ..."

That's the kind of test drive I like :o)

A missing pedal spring can contribute to the factor John mentions, although it would have to be stuck pretty hard for clutch pressure back through the hydraulics not to shift it. However brakes can suffer from this is the brake light switch is screwed too far in, that *definitely* stops the master piston coming back far enough.
Paul Hunt 2

Well Paul, I really had hoped to get the engine and the rest of the car back together a little sooner so that I could do a bit more local testing before the west coast trip, but then I always think I can do things faster than I really can. As it was, I got about 150 miles on it before I headed out, and I decided that that would have to be good enough. It did the trip in 3 days and 2 hrs, which is not really too bad for starting in the early evening instead of fresh in the morning.

Now that you mention it, the clutch pedal spring is missing. I don’t think that is the problem, but I will try to catch up with one out here before I head back around the 20th. This is a bit of a Johnny Cash car (I don’t know if that translates on your side of the pond, but it means it was put together out of bits from many cars) and digging through the boxes I only found one pedal return spring, so I gave it to the brake pedal.

I’m glad you brought that up about the brake light switch, since every now and then the brakes do seem to drag. I have the 1/8 inch free play in the pedal, but maybe that is introduced by push rod wear rather than free play at the switch. I will check it, though I suspect the problem is sludge on the caliper pistons from sitting for three years.

Oh well, if I waited to get it perfect before I drove it, I’d never go anywhere.

If you are not familiar with it, try to down load the Johnny Cash song about building his Cadillac. I’m sure you would enjoy the story.

Thanks, Charley
C R Huff

Wasn't familiar with 'Johhny Cash' (applied to a car), but found this http://www.cowboylyrics.com/lyrics/cash-johnny/one-piece-at-a-time-2214.html - very good. In the UK 'Bitsa' is more common - 'Bitsa (i.e. 'Bits of') this and Bitsa that'. Funnily enough the local paper last week had a story on a chap who nicked 100 grands worth of cylinder heads from Land Rover (just down the road) a few at a time.

Building site labourer goes home every night with his coat draped over his wheelbarrow like it's covering something up. Every night the site foreman whips the coat off to find ... nothing underneath. At the end of the job the foreman says to the labourer "OK, I know you were knicking something, what was it?" The labourer says "Wheelbarrows".
Paul Hunt 2

Hey Paul,

Glad you found the Johnny Cash version of a Bitsa. It is a good old song. It's getting to the point these days in the States that when you say a "Johnny Cash car" people don't know what you mean any more.

It finally stopped raining here in Eureka, California today so I got some work done on the GT. One more dry day and I should have it ready for the retrun trip to Kentucky.

I didn't check on the brake light switch yet, but it seems that if it were too tight, it would take up the slack in a worn push rod as well as the normal slack.

I had a pleasant surprise when I went to adjust the rear brakes. The adjusters weren't rusted up solid. About time I got a break.

Charley
C R Huff

This thread was discussed between 28/12/2007 and 11/01/2008

MG MGB Technical index

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