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MG MGB Technical - clutch slave
|Well, here's a curveball...perhaps someone can help....As part of my relentless updating of my '74B, I replaced the ancient hose to the slave and rebuilt the slave...All was working well before...but since I was there with the old hose....I fugured I'd update the slave....And I noticed when I removed the push pin from the cylinder piston...a nut had been welded on to the end that seats in the piston....extending the length of the pin by about 1/2 of an inch...hummm?....And when I put in a new, original length pin, and bled it all out...I dont get enuf lash now to fully disengage the clutch...|
|Something wrong there. The hydraulic system of the MGB is designed to take up any and all wear in the slave cylinder piston, push-rod and all mechanical linkages from there to the clutch friction plate. In other words once you have eliminated all the air and got all the travel of the slave push-rod you are going to get, the piston will only come back as far as the diaphragm springs in the clutch cover plate will push it. If the push-rod were too short, then either the piston will sit further out in the slave cylinder than it normally would but the clutch would still work normally, or it would sit so far out that pushing the pedal would push the piston out of the end of the cylinder and you would lose fluid. Conversely if the push-rod is lengthened all that happens is that the piston sits further into the cylinder than normally, and as long as it doesn't reach the end the clutch will still work normally. If the piston *does* reach the end of the cylinder then the clutch won't be fully engaged and you are likely to get slip. This means that the length of the slave push-rod is relatively immaterial, as long as when the clutch is fully engaged i.e. pedal fully released it is not so long as to push the slave piston all the way to the end of the cylinder when released, and not so short that when the pedal is fully operated it pushes the piston out of the end of the cylinder. To move the driven plate from fully enaged to fully disengaged the ratio of the bores of the master and slave need to be matched. If the slave is too large or the master too small you won't get enough movement of the driven plate to fully release the clutch, or the biting point will be very low (which can also happen when the *pedal* linkages are worn), but the pedal pressure required will be lighter than normal. If the slave is too small or the master too large it will fully disengage, but the biting point will be high (like a worn friction plate), and the pedal pressure required will be higher than normal.|
On it's own, and with the correct diameter master and slave, insufficient slave piston travel usually means there is still some air in the system, and they are a pig to bleed. You should be getting around 1/2" to 5/8" of slave piston travel with no air in the system. I've found either using a gunson's EeziBleed on very low pressure but conencted to the *slave* bleed nipple instead of the master cap, or interconnecting the right-hand caliper with the clutch slave, opening both bleed nipples and using the brake pedal to bleed the clutch, is by far the easiest way. This is reverse bleeding and pushes air *up* the clutch pipe and not downwards. You need to make some space in the clutch master first to prevent overflow, and watch the brake master level.
|Thx...Mr. Hunt....I certainly appreciate your expert commentary on this site...I'm sensing something odd at play here...since the old set-up worked so well and now the proper push pin does not move quite far enough...I've reverse bled the system repeatedly....and get about 1/4" movement...I'm going to look in the pedal box....and try the old longer pin...I must say, not being an expert on hydralics, but surviving several courses in college physics, I agree with your theoretical discussion vis a vis a self adjusting system...and it should make no difference a'la pin length, unless, do you think, I have a wear problem in the pedal box?...|
|Could well be, but to lose 1/4" at the slave is a huge amount, and doesn't explain the business with the slave pushrod. Certainly you need to look under the pedal box and see just what is happening at the *master* push-rod when the pedal is operated and released.|
|Pete, Anonther unknown is the condition of the slave's piston. Is the cone where, the pivot pin rides, excessevly worn down or ovaled? This might explain the lost distanct between the slave cylinder and the throwout bearing yoke and would not be readily noticeable. RAY |
|Was the slave rod extension instituted to cover the effect of a badly worn clevis pin at the clutch pedal/pushrod joint? Or perhaps a very badly worn clutch pedal, mounting bolt pivot point? Or a combination of the two?|
A thorough look around beneath the pedal box cover could provide a solution.
|Neither of those things should have any effect, as the hydraulic system takes account of any and all wear from the slave piston right through to the cover plate as previously mentioned, by positioning the piston in the bore accordingly. It's only when you get so much wear or a breakage that the piston can come out of the bore altogether, or some other problem causes the piston to be pushed back as far as it will go i.e. a too-long push-rod meaning the clutch isn't fully engaged, that you should get problems.|
|Except Paul, if the clutch pedal is skewing on its mounting, it will not be able to apply proper extension to the slave rod. Likewise, to the extent that ovaled clevis pin holes prevent proper extension, the same result.|
I'm not sure I understand how your hydraulic system compensates in those cases.
|Fellows, thx for all the fine comments. This site is splendid...There was MUCH wear in the pedal box and I refurbished with new pins, and re-did the holes in the pedal levers...and clevises...a lot of wear...On the trans side, all looked fine...And, after a long session of bleeding (I owe my good lady wife a dinner out)...I finally got the air out of the system....and suddenly...it started working/articulating better than ever!....this was tough one...but all is well...|
I love these cars, I just did a total restore on an old chevy muscle car, but these mg's are more fun....
|Bleeding the MGB clutch hydraulics is bleeding hard if you only use the pedal. The pipe from the cyl goes up hill for a bit and the fluid just goes past the air bubble. The best way is to pressure bleed it with about 20psi. Or do as I have done on most clutch jobs. Pump the pedal furiously about 30 - 40 times. This aerates the fluid in the system & then you can bleed the system. Leave the fluid to stand & it can be used again. I have had cars towed in because they couldn't get it work properly and just pumped the pedal to get it working This procedure can be done a few times & you won't have to bleed it at all. PS: midget 1275 is even worse to bleed.|
|Roger - *anything* that stops the master piston giving its full stroke will cause insufficient movement of the slave push-rod and hence will cause a low biting point if not dragging. This can be worn pedal bushings or anything else, but the usual cause is the clevis pin and its bushes as they are smaller diameter.|
The hydraulic system is self-adjusting/compensating as follows: The slave piston is only pushed back into its cylinder by the clutch cover-plate springs - nothing else. In fact there is a spring inside the slave which is continually trying to push the piston out, and this keeps all the mechanical components in contact with each other i.e. it takes out any mechanical wear. The clutch slave piston only pushes fluid back up the pipe and into the master cylinder while it is being pushed by the cover plate. If the master piston is still being pulled back by the pedal, then it will simply pull more fluid into the master cylinder ready to be delivered next time, and so push the slave piston and hence the friction part of the cover plate a distance dependant on the master cylinder likages, and not the slave cylinder linkages.
Bleeding with a constant pressure bleeder from the top down is bad enough. I've bled from the bottom up on two occasions and found it very easy, once using a Gunsons EeziBleder conencted to the slave nipple, and once using a pipe to interconnect the left caliper and clutch bleed nipples and using the brake pedal to bleed the clutch. But changing a clutch last week neither of those worked, I even tried reverse bleding with the slave hanging on its flex hose i.e. both ports definitely at the top. I also tried pushing the piston all the way into the bore, but when I released it the internal spring pushed it out again as it should but way too fast because it was sucking in air past the seals. Then it started dripping so a new slave was called for. Even with the new one I couldn't get it reverse bled until I had removed it and bled it hanging on the flex.
This thread was discussed between 06/06/2008 and 17/06/2008
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