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MG MGB Technical - Miata Clutch slave cyl. for B ?

Saw some info on the MG Exp. site about using a '99 Miata clutch slave cylinder in place of the MG unit. The Mazda cylinder was modified to fit and the poster said it reguired a custom brass adapter for the hose and shim it for the throwout bearing alignment. Is there any other information on this mod? Is it as simple as that to install? The poster stated that this modification eliminated the spongy nature of the stock unit and eliminated maintence issues. I am currently rebuilding the hydraulics on my '77 and would like to convert my car since it sounds fairly simple.
S.L. LaPaugh

I thought the stock system was fine. I have had almost no problems in 40 years. This is the first I have heard of any need for an improvement.
Ian Buckley

Exactly - why?

How can you have a spongy clutch? The pedal is supposed to go to the floor, unlike a brake. And what maintenance 'issues'? Other than topping-up the master as the release bearing wears there *is* no maintenance. The standard slave should last more than the life of the clutch itself.
Paul Hunt

The guy who posted the mod. was into racing. I suppose being accustomed to a racing clutch, a stock one would feel spongy. I figured it was worth looking into. Thanks for the feedback. I suppose I'm frusturated by not having it runing yet. Thanks.
S.L. LaPaugh

Paul,

I can understand the search for improvement...on my rally MGB, I wear out the slave cylinder regulary....
First of all , a comp clutch is heavier than a road clutch, but most important, the times we declutch (and double declutch during shifting down) is uncountable, to keep the engine in the desired powerband.

André
André De Vos

I still don't understand the use of the term 'spongy'. Light or heavy yes.

Someone else has been having problems with the *release bearing* breaking up after about 3000 miles on a competition car, but not the slave cylinder itself. He opted for a hydraulic release bearing, which does replace the slave cylinder as well but purely co-incidentally. He's now got a clutch that is way too heavy and is looking for ways of lightening it.
Paul Hunt

If the clutch hydraulics were spongy, my guess would be that it was due to air in the system ~ indicating that someone had a hard time bleeding the system.

I am not sure what the Miata slave cyl. looks like, but attached is a photo from someone who sounds like he could be the "guy who posted the mod. {and who} was into racing}. There is always another way to skin a cat, and perhaps this sort of conversion makes sense, altho the making of the mounting required looks like some extra monkey motion to me. However, in just now looking at the photo again, I think this must be for a V-8 or 5-speed tranny conversion. If this IS a Miata slave cyl., then one would have to determine if the mounting hole pattern would work.

Probably the easiest method for bleeding the clutch slave cyl. that I am aware of is by using a syringe (large) and pumping the fluid back up into the clutch M/C, rather than the other way round.


Bob Muenchausen

Air in the clutch hydraulics *will* cause the diaphragm springs to move less than they should, which eventually will cause clutch drag and grinding when selecting reverse. If very bad the pedal will feel slightly *lighter* than normal. But the clutch pedal should always go down to the floor, unlike a brake pedal, so you aren't going to feel the effects of air in the lines at the pedal. A brake pedal should be easy to move to start with, then change from being easy to almost solid over a very short additional distance, which will be a long way short of the floor. A servo can result in slightly more of this movement than without. If there is air in the brake lines then the pedal will take longer to get from starting to be harder to solid, i.e. it feels springy, and *that* is spongy.

That slave is not unlike the V8 slave, which is quite different to the 4-cylinder, albeit on the other side.

I've bled clutches in two different ways without any problems, and I hope to be able to report on a third way soon. Because I'd read so much about the difficulties other people had had I decided to use a bottom up process instead of top down. I connected my EeziBleed to the slave nipple on very low air pressure, and filled and bled the system in one go. Another time I connected a pipe between the left front caliper nipple and the clutch slave nipple, opened both, used used the brake pedal *gently* to fill and bleed the clutch system as before, again with no further ado, and it only took seconds, easier and quicker than with the EeziBleed. With this method you do have to be careful not to let the brake master level get too low, but there should be more than enough in the brake system to half-fill the clutch, then top both off as normal.
Paul Hunt

The Miata slave has a much smaller piston diameter than the MGB slave. The result would be a much longer stroke with increased pedal effort. The longer stroke could cause binding and the inceased pedal effort might feel more race-like, but why?

Ironically, if Miata's have an Achilles' heel, it's the slave cylinder. They are notorious for leaking.

--Carl
Carl Heideman

The Miata slave cylinder has a rigid line connected to it that runs to the firewall. There it connects to the flex hose which is next impossible to access. RAY
RAY

The Miata slave cylinder has a rigid line connected to it that runs to the firewall. There it connects to the flex hose which is next impossible to access. RAY
RAY

The MGB clutch hydraulics are a real pain to bleed 100%.

OK I said it. The secret is out.

Most times the pedal is spongy because the tubing used on the system is large and air is easily held in the loop just where itbegins to decend into the master cylinder bracketry (LHD cars). I understand why this route was taken, but it does not discount the fact that the loop is a notorious air trap.

Also, a worn slave cylinder can contribute to a spongy pedal.
I don't know why this is so - but one time I struggled with trying to excorcize a soft clutch pedal (for weeks) - from a rebuilt slave cylinder, to no avail. I tried everything imaginable to chase out that last pesky air bubble. I even welded up a small stand fixture that allowed me to temporarily position the slave cylinder vertically (just off of the car) so that I was assured that any air had no choice but to find it's way to the bleed nipple. The soft pedal persisted.

Only after I had installed a spanking new slave cylinder did the pedal feel perfect to my left foot.

Looking back, I suspect that a bit of air was being inhaled past
the slave piston seal when the pedal was released - through
tiny unseen imperfections in the bore surface.

Installing a new slave cylinder corrected this.

However, if you are still hankering for a slave cylinder with a
longer throw (higher effort) - then perhaps a slave cylinder
from an MG midget will fill your need.

The Midget used 2 versions (both have the same, smaller
bore). One of the versions is a straight bolt-on as it has the correct mounting flanges (ears) for this. You may have to space the cylinder (= shims) away from the bellhousing to properly align it with the throwout arm. I'm not 100% positive
on this, but I believe the hose screws in without need for an adaptor.

Daniel Wong

"Looking back, I suspect that a bit of air was being inhaled past the slave piston seal when the pedal was released - through tiny unseen imperfections in the bore surface."

Exactly so, and pressing the piston back into the bore maually then just letting it go so the internal spring pressure pushes it back out is a major cause of this. When this happens the piston is trying to pull fluid down the pipe from the master reservoir and its tiny bypass hole, and all that happens is it pulls air past the seal because the is no fluid pressure pressing the edges of the seal to the cylinder wall. Removing the cylinder from the bellhousing and doing it is even worse, as the seal is then operating much further forward in the cylinder on a part of the bore that has never been used, and almost certainly has some corrosion and muck unless new.

I'm convinced this is why we had so much trouble bleeding after a clutch change when it hadn't been before - the engine out having allowed the slave piston to move right forwards onto corrosion. Like Daniel we had to resort to a replacement slave.

If you *do* manually push the piston back to push any air trapped in the loop at the top of the metal pipe then you shouldn't just let it go afterwards, unless you only let it come forwards *very* slowly, and ideally have someone operate the clutch pedal which you attempt to hold the piston back so it is fluid pressure that is pushing the piston forwards and keeping the edges of the seal pressed to the cylinder wall.
Paul Hunt

This thread was discussed between 07/01/2009 and 17/01/2009

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