Carl Heideman
Carl Heideman
10/12/18 8:40 a.m.


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Story and Photos by Carl Heideman

Sloppy driveline swaps have a few telltale signs, but there’s one you may not even recognize as a problem. Have a stiff clutch with a very short friction point? Feels kind of racey, right? Well, it shouldn’t–not unless it’s an actual race clutch.

For daily driving, a soft, smooth pedal with a decent friction point works best. In fact, setting up the clutch actuation is one of the most important details of a driveline transplant. Doing it properly will make your driving experience a lot more comfortable. Trust us, it’s worth the planning.

If you’re just joining us on this project, we’ve been swapping a 1.8-liter Miata engine and gearbox into an MGB. Now it’s time to make sure the clutch feels just like a fresh-from-the-factory Miata clutch.

Both cars feature clutches that operate hydraulically, so there was the temptation to just plumb the MGB clutch master cylinder to the Miata slave cylinder. A few measurements and some quick arithmetic made us realize we’d be better off just doing it right.

The MGB’s master cylinder, while only 0.125 inch larger in diameter than the Miata’s, was also coupled to a pedal that gave a 43 percent longer stroke. On top of that, the MGB’s pedal ratio was smaller than the Miata’s, which would have resulted in a stiffer pedal.

The MGB’s slave cylinder explained it all: With a bore of 1.25 inches compared to the Miata’s 0.75 inch, it needed roughly twice the volume of fluid to move its pushrod the same distance. We’ll detail our calculations later, but here’s the bottom line: If we’d pushed this “easy” button, we would have had an awful clutch–one that could easily break from over-travel.

When we began this project, we set a rule for ourselves: Stick to parts from only the MGB and the Miata. When we’re making a repair five years from now, we don’t want to struggle to remember where a part came from. To keep that goal–and, honestly, to keep things simple–we decided to use the Miata clutch master cylinder and change the MGB’s pedal ratio to match the Miata’s.

After about an hour of math and another four hours of light fabrication and welding, we had all the bits we needed to make our clutch move like a hot knife through butter. Follow along to see how we did it.

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