An M.G. Locost Build

November 27, 2015

We had a recent scare with the clutch on our Locost. It stopped clutching, or actually de-clutching. It clutched fine. But over the last few months, shifting became almost impossible. When we desperately needed to, we could pump a little life into the clutch by viciously thrashing the pedal, but mostly we just had to match revs. We thought bleeding the clutch might fix it, but it turns out bleeding an M.G. clutch is something only very few people on the Internet claim to have mastered, and we think even those people are exaggerating based on the billions of other M.G. owners who, like us, report being mystified by the process.

  Crusty donor clutch master still in use today
click to enlarge

When we first got our donor MGB back in January of 2012, the clutch master cylinder was dirty, rusty, and dry as a bone. The clutch pedal went to the floor. The car had many other problems, all described in excruciating detail elsewhere in this journal, and the clutch was not considered worthy of immediate attention. So we filled the master cylinder reservoir with fresh Castrol LMA, and worked on other things. Weeks later, when we finally got the engine running, we were as shocked as anyone that the clutch worked perfectly. Somehow, it bled itself.

So of course with the clutch problem resolved we carefully installed everything into the Locost, making sure not to disturb any moving parts, and as expected, it worked perfectly for more than two years, long enough for us to believe it would last forever. But of course it didn't, and this time it wouldn't be as simple as adding more fluid, since the reservoir was already full. So we took a couple of shots at bleeding the clutch, once using the old fashioned two-person method and a few more times with a modern vacuum bleeder. And of course none of this worked.

Cutch slave works like new  
click to enlarge

When your clutch isn't working, either the master or slave cylinder is leaking. If the master cylinder is leaking, fluid slips past the seals instead of going to the slave. If the slave cylinder is leaking, fluid slips past the seals instead of pushing on the clutch lever. In both cases the symptoms are the same, the clutch lever doesn't move. The only difference is, if fluid leaks past the master seals, it ends up back in the reservoir. If fluid leaks past the slave seals, it ends up on the floor. So it's easy to tell which part has failed, by looking at the floor.

Our floor was clean, or you know, not clean but at least not coated with hydraulic fluid, so we knew the master cylinder was at fault. We also knew it could be either replaced or rebuilt. Replacement is difficult and expensive, so that was an easy choice. As a bonus, according to the Internet, the clutch master in an MGB can be rebuilt without removing it from the car, exactly the sort of information we wanted to hear, since during the design phase of our build we somehow missed the step where we made the clutch master easy to remove.

Unfortunately—and this is something you probably already know—the Internet is not always correct. We pulled everything apart, disconnected the clutch pedal and plunger, and removed the circlip that keeps the guts of the cylinder from spilling out onto the ground. Surprisingly—and this is the part where the Internet let us down—the guts of the cylinder did not spill out onto the ground. Not even close. Our 40-year-old cylinder had built up a ridge of solid corrosion just ahead of the piston, which worked as well as or better than the circlip to keep everything bottled up inside.

  Cutch master now a sealed unit, circlip or not
click to enlarge

So without any way to replace anything, we had to leave the old parts in there, put everything back together, and then try to figure out our next option. We thought we might be able to use some kind of tool to remove the ridge, but doing that without damaging the bore and/or cutting ourselves badly seemed unlikely. It might've been easier and/or possible to ream the bore with the master cylinder out of the car, but if we could manage to get the master cylinder out of the car, we'd be better off just replacing it. So that wasn't really an option.

While contemplating other options—which turned out not to actually exist—we left the car sitting for a few days. During that time, the same mysterious process that fixed the clutch on our donor three years ago fixed the clutch on our Locost. All we did was fill the reservoir with fluid and wait a couple of days. We can't fully explain it, or even explain it at all, but it's possible that while fiddling with the piston trying to get it out, we moved and/or disturbed the seals enough to get them working again. Yes, it's a stretch. We don't believe it either.

And yet the clutch works. The release point is close to the floor, but it always was, and with 40-year-old links and clevis pins, I think we can understand why. Ordinarily we wouldn't recommend this procedure as any kind of permanent fix, just rotating the piston slightly with your finger, but it only takes an hour or so, and it's free. A new clutch master is something like eighty bucks, so this would be better, at least from a budgetary perspective. The important thing is, the clutch is fixed, at least for now. How long it'll last is anyone's guess, but if the over/under is anything more than three months, bet the under.


Our Locost
Our Build Plan
Building a Locost
Build Summary
Workshop Manual
Construction Manual
Non-Locost Stuff

Please Note: Our database is currently unavailable so you will not be able to browse through log entries. This happens sometimes, and it usually doesn't last long. We're sorry for the incovenience. Please try again later. Or in a few minutes.