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MG MGA - Clutch installation
|After installing a rebuilt engine clutch and transmission I finally got to the point of bleeding the clutch. I have read how troublesome this can be so was prepared for some frustration. After bleeding the clutch many times I still could not get the clutch to release. If I haul back on the clutch arm manually I can get it to release but only after the arm moves back about 1.1/2 inches. I can get the slave cylinder to push it about 3/4 of an inch which seems about as far as it can physically move. By now you all are saying the problem is in the clutch and I agree. Does anyone know what I could have done wrong in the rebuilding of the clutch that would cause this to happen? I suppose I should prepare myself to remove the engine.|
|Very likely the friction disc is in backward. One side of the hub protrudes farther than the other. If you put that side forward it fouls on the flywheel bolts and the clutch will not release. The flatter side of the hub should have stamped on it "This side toward flywheel".|
|I think you are right. You state that the clutch will not release if it is in backwards. Are you referring to using the hydraulics or at all because I can release the clutch manually by hauling back on the arm.|
|This is not a hydraulics problem. The clutch slave cylinder should have about 5/8" travel with full pedal stroke. Actual travel required to release the clutch is normally about half pedal travel (+/- depending on the type of clutch installed). When the disc is backward it fouls on the flywheel bolts, then requiring far more travel of the release arm to let it free. Normal hydraulic action does not have that much travel, so it won't release.|
Seems like your engine will have to come out again. Some of our best learned lessons come from the School Of Hard Knocks.
|Thanks for your replies Barney. I will just have to chalk this up as a life experience.. as you say.|
|So I took the engine out and the friction plate was installed correctly. I cannot see any problems in the clutch installation. Does anyone have any other ideas on what could be causing this problem ?|
|Photos of the parts i question will help.|
Might be a worn arm pivot, short slave cyl pushrod, wrong thickness throwout bearing or basically anything related to the clutch/arm/slave geometries. Compare any new parts with their respective older counterparts and look for differences. Look for wear in any parts that were not replaced.
|All of the clutch parts are new except the pressure plate and the arm. I have checked the arm pivot and there is very little wear. The pushrod is the same length as the old one. I don't have the old throwout bearing but I was wondering if that is the wrong part maybe it should be thicker? Perhaps the pressure plate is adjusted wrong. I put a lever on it to move it manually and it takes ALOT of force make it move. I don't know how much force it should take although it is operating normally when it does move.|
|It does take a lot of force to relase the clutch. I don't know the number, but I had to use a pipe wrench with an extension to manually move it. There really isn't any adjustment on a pressure plate. I know there are some adjustable screws on the release arms. but everything I've read says leave them alone. I have a known good pressure plate on which I could measure the distance from the pressure plate face to the clutch release bearing surface if this would be of interest. |
|G T Foster|
|I am glad to hear that a lot of force is required to move to pressure plate.I am pretty sure that mine is working ok. If you could send me that measurement it would be of great interest. I need to gather all the info I can to get to the source of this problem.|
|Pressure plate springs are about 200 Lb each x 6 springs = 1200 Lb force. Internal arms are about 4:1 lever ratio, so force at the release bearing is about 300 Lb. Release lever is about 3:2 lever ratio, so force at the slave push rod is about 200 Lb. With overall leverage ratio of 6:1, 1/2" movement of the slave push rod will withdraw the pressure plate about .083".|
About 75% of that motion should be enough to affect clutch release, 3/8" at the push rod and 1/16" at the pressure plate. At 1/2" motion of the push rod the clutch must be fully free. If not, then I can only suggest that the friction disc was in backward and fouling on the flywheel to crankshaft bolts. I do not know any other reason why it would require excessive travel to affect release.
At this point I suggest you reassemble the friction disc and pressure plate to the flywheel. Find a way to apply 300 pounds force to the thrust plate, and measure the travel distance of the thrust plate required to affect release of the disc. When the disc is in the right way around, if it require more than 1/4" of motion at the thrust plate, then there must be something dramatically wrong with the pressure plate assembly.
|Any chance the friction disc is stamped "THIS SIDE TOWARD FLYWHEEL" on the wrong side of the assembly? The flatter side of the disc goes toward the flywheel while the more extended side of the disc fits inside of the pressure plate.|
|A friend and I are about to do as you say and reassemble everything and check the motion of the thrust plate. The friction plate is marked correctly. After looking at it I am not sure how it could be installed backwards as it doesn't fit very well.Do you know the thickness of the release bearing? I was wondering if I have the incorrect part and if it was thinner it would allow the arm to sit too far back and the slave would run out of travel before releasing the clutch???|
|I have had trouble bleeding the slave cylinder on A's and B's. I take it off and hold it upright (ie with the bleeder screw uppermost). My "professional" friend avoids this by tramping quickly on the clutch pedal with the bleeder tube attached, saves time (and works!).|
I measured the pressure plate face to throw-out bearing face last night. Mine measured about 2.58 inches.
|G T Foster|
Thanks that will help greatly in my quest to find the problem. You don't happen to have an MGA release bearing that you could measure the thickness of? Mine seems too thin. I am wondering if Moss sent me the wrong one.
Thanks for your help
|The graphite pad is 5/16" thick when new.|
|I measured a new Moss (India) release bearing and a used English made release bearing. They both measured about .800 inches from the face of the carbon surface to the centerline of the pivot point. The thickness of the carbon (graphite) pad on the new Moss unit was about .200 inches, far less than what Barney suggests. It was almost the same thickness as the worn English made item. I did not use the Moss release bearing as I didn't like the look of it. I think the carbon faces should be closer to what Barney suggests. Still, this wouldn't be enough to explain what you are seeing. |
|G T Foster|
|Gerry...thanks for all your info.|
|Many years ago I replaced a clutch (not an MGA and non hydraulic) and then found it impossible to disengage it even with the pedal down to the floorboards. The engine came out three times trying different things until I hit upon the problem. This was the splined bush fixed to the centre of the friction plate which was not pressed in concentrically and there was run out when revolving. It only needs to be off by a few degrees to require an extra half inch movement to completely disengage. Itís easy to check by inserting a close fitting rod into the friction plate and spinning it round. I took it back to the supplier replaced it with a different one and all was well. After this painful experience I always do this quick check on the friction plate before installing.....................Mike|
|I have encountered one difference in my quest so far. The MGA parts manual gives the part # for the clutch arm as 11G3193, my arm has the #11G3194 stamped on it. I have loaded some pics. Can anyone tell if this is the wrong arm?|
Also Mike I did the test and the friction plate seems fine. Thanks
|Here is another view
|That wouldn't by chance be a MGB clutch release arm? The A arm and the B arm look somewhat similar, but there is a difference. I converted to the MGB clutch assembly, if I recall correctly the MGB clutch release arm has somewhat smaller dimensions in the pickle fork area. |
|I don"t know. I have tried to look up part #'s for the B but they seem quite different from the A.|
|It is characteristic of BMC numbering that the blank forging, casting, etc, carries a number one off from the part number. The part number is assigned at drawing stage, including any finish machining and secondary processing. The blank is then produced to suit that part number, but has a subsequent number. In this case, it is the finish machining and fitting of the pivot bush that accounts for the difference. In some cases, there are variants produced from the same blank, so a part numbered 1231, might have a blank numbered 1232, and other variant parts numbered 1233, 1234, etc. Or the forging number may have come first, as AAA123, with variant parts numbered as AAA124, AAA125 etc. Some forgings are marked as 1234R/L, with the parts being machined to hand, and the part numbers being consecutive as 1235, 1236. The general rule is that cast or forged in marks are not the actual finished part number, but are (usually) close. There are cases where an existing forging is machined to a totally different part, so the numbers have no apparent relationship; some 60-70's era cars have parts made from early 50's forging blanks, still with Austin A40 blank numbers. Stamped in numbers are likely but not necessarily actual part numbers, sometimes with no relationship to the cast in numbers. This situation is especially common with special tuning parts, which are often modified forms of standard ones. Stamped numbers take precedence for identification purposes.|
You have part number 11G3193, complete with bush, which was made from blank 11G3194. While it may never have been supplied as a service part, the machined arm without bush may have been 11G3192, or 11G3193A, or similar. Remember that the factory had to track parts in all stages of manufacture.
The difference between MGA & MGB forks is the relative pivot locations to suit the travel of the different clutches; the MGB diaphragm clutch takes less release travel. The pivot point on the front gearbox cover is also moved; if you have the wrong one for the gearbox front cover, the release bearing would be off center on the 1st motion shaft.
|Excellent information. Makes for very interesting reading.After spending all afternoon working on this problem we have come to the belief that I may have the incorrect slave cylinder. Dopes anyone know what the bore on the slave is? Mine is 1 1/4 in, which we feel is too large.|
|1 1/4" is correct, same for MGB too.|
You could have a 3/4" Spridget master cylinder fitted. However, I am inclined to agree with Barney - you have plenty of arm travel, it is a problem with the disc or pressure plate.
Put the disc on the input shaft and rotate it to see if it is bent.
Measure the thickness of the disc at numerous places - all should be the same. Somebody ought to have the number for a new one.
Assemble the complete clutch to the flywheel and see if the release table is parallel to the flywheel. Usually if there is a PP problem, the table will not be parallel, but this is not definitive. Might post a picture of this - some of us might be able to tell something from what it looks like.
If you make a temporary (LONG!) steel line from master to slave, you can check operation without putting engine in and out. I've done this with short steel stubs and nylon tubing.
|Have you changed the gearbox front cover - the one inside the bell housing. If you have got a B one instead of an A one the clutch will not disengage. The B pivot lugs are longer to accomodate the thinner pressure plate of the B. Late A and early B both have a lip oil seal and are identical except for the pivot length.Casting number on the A one is 22H27|
|A J Dee|
|I took the m/c off and measured the bores and they are the correct 7/8. The disc is flat and we can get the clutch to release with a lever at around the 1/4 in mark as Barney said it should. All is good til we get to the arm movement which is not sufficient. We have the required travel of the slave arm so that's ok.|
I am now exploring Andy's idea about the cover plate. The stamping on mine is H3137 A or cA..not the number you gave me Andy. When I overhauled the transmission I needed to replace the rear seal. I ordered ever type of MGA rear seal from Moss they had and none fit. In the end a friend machined one for me. Now with this info on the cover plate, could this transmission be from a B. Since it was out of the car and sitting in the front seat when I bought the car it is impossible to know where it came from.How do I confirm if this is an A or B transmission?
|It cannot be an MGB transmission complete, because it will not mount in the car, unless the frame is modified. It could well be a mixed breed.|
Let's back up:
What year is the car, what engine, what clutch, how many splines on the input shaft, is the propshaft a sliding spline or a bolted flange drive?
Barney has a lot of clutch and gearbox ID and swap info, study on!
|If you are not sure what the gearbox is, send a picture. But that it not likely your problem here.|
At this point you have an apparent conflict of information. You have now determined that the clutch will release with 1/4" motion at the thrust plate. You also have ample motion of the slave cylinder and push rod. The only part left in between is the release lever which has a leverage ratio about 3:2. Normally the release lever only needs to move about 3/8" at the push rod end to release the clutch, and that should be the case here as well.
First the least likely scenario. If the input end of the release lever moves more than 1/2" and the output end moves less than 1/4", then the pivot point would have to be moving. The only way that can happen is if the gearbox front cover is broken or the bolts are very loose so the plate can move, which I seriously doubt is the case.
Second, more likely and needs to be checked: If you installed a new spigot bushing in the crankshaft. it may be undersize after installation (slightly crushed). This can make the bushing bind on the gearbox input shaft. Result is, when the clutch does release the shaft is stuck in the bushing so the gearbox input continues to spin with he crankshaft, and you can't shift gears, same as if the clutch did not release.
For MGA and early MGB 3-main bearing engines, the spigot bearing is 5/8" nominal diameter. Shaft is .002" undersize at 0.623", while bushing should be nominal bore size at 0.625" ID (when new). A proper clutch alignment tool should fit into the bushing, and you should be able to pull it out with your pinky finger. But the plastic alignment tool can also be undersize. By coincidence, the valve rocker shaft is also 0.623" diameter. If you have an old rocker shaft lying around you can try that in the bushing as a size gauge. The should rotate freely in the bushing with two fingers
If the bushing is undersize you can run a 0.625" reamer into it to clean it out. Or you can burnish it by driving in a 0.625" OD rod or tube (drill rod for instance), and pull it out with ViceGrip.
As follow up to prior comments, the MGB clutch uses a larger diameter release bearing and must have the matching MGB release arm. The gearbox front cover must also be from early MGB to position the pivot point in correct location. Details here: http://mgaguru.com/mgtech/clutch/ct_102.htm
This thread was discussed between 06/08/2011 and 22/08/2011
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