CT-103 presents notes on the COMPETITION CLUTCH installation and function.

Question:
>"Just a short note in regards to the clutch pressure plate conversion. Have you experienced a very heavy clutch especially with a comp. pressure plate due to the slightly larger diameter of the "A" type master cylinder and possibly the different fulcrum points of the clutch arm."

The MGA and MGB master cylinders are both 7/8" bore size. If you happen to have the misfortune of having an MG Midget 3/4" bore master cylinder in the MGA, that makes for very long pedal travel for both clutch and brakes, to the point where you may have to pump the pedal twice quickly to make it work. In that case you should install the correct 7/8" bore master cylinder to restore proper operation of the clutch.

The fulcrum points of the clutch release arms are not much different when it comes to mechanical advantage and physical travel. The MGB unit has the fixed pivot point a little farther forward, and the release bearing sits farther forward, but the lever arm lengths are about the same, so there will be no difference in either force or pedal travel on that account.

The stock MGB pressure plate has a slightly stronger grip on the disk than the MGA pressure plate, but the spring force is not much higher, so the difference in pedal pressure there is also minimal.

The competition pressure plate is an altogether different story. The spring force and grip on the disk is much higher, at least 30% greater, and the pedal force requirement is similarly higher. This takes just a little getting used to. About a week of driving about in a normal manner, or one or two autocross dates, and it should be old hat. Thereafter it shouldn't bother you unless you get caught in creepy traffic and have to hold the pedal down a lot. Or when you drive someone else's car you may first wonder why their clutch is so soft.

Because of the stronger springs, the competition pressure plate also comes to full grip in a little shorter distance of pedal travel. It can go from full grip to full release while the pedal travels from about 1/4 to about 1/2 stroke, and once you get used to it you may never press the clutch pedal more than about 2/3 stroke. This shorter stroke helps to reduce the maximum force you will encounter, so even though it starts out stiff at the top of stroke it doesn't increase in force all that much as it goes to full release. But then when you go back to drive a standard clutch you have to pay attention to using more pedal travel to get full release.

Being an autocrosser I was quite happy with the competition pressure plate, especially when using the sticky race tires. With the low torque engine and the sticky tires a good launch off the start line involves winding the flywheel up around 5 grand for some good energy storage, and then synchronizing the motion of the loud pedal with the sensible release of the clutch. If you overdo it you can smoke the sticky tires too much and not move off as quickly as you might like. To prevent the engine speed from dropping out of the power band during the first couple seconds of the launch you either slip the clutch or you spin the tires enough to surprise a few people (at least the right rear tire). Laws of economics generally dictate that replacing tires is easier than replacing the clutch, so you get to have some fun with a clear conscience.

With a 6000 rpm power shift into 2nd gear the stock clutch might squeak the sticky tires a bit along with a compromised slippage of the disk. The competition clutch will keep the sticky race tires spinning a bit for half a second or so until the engine speed drops to match ground speed. Power shifting into 3rd gear, the stock clutch would never squeak the tires at all, and the clutch disk would have to dissipate most of the excess energy of the flywheel through slippage and heat until the engine speed would drop, which is really hard on the clutch disk. With the competition clutch a power shift into 3rd gear will turn the sticky tires over much the same as the 2nd gear up-shift with very little slippage of the disk. The result there is a brief kick in the back like a slight nudge from a booster rocket, and a touch of rubber left on the pavement, until the engine and flywheel speed drops, and more of the stored energy is transferred to acceleration rather than generating heat and wear in the clutch disk.

I used to do more than 20 autocross dates per year, usually with some fun run laps tagged on the end for practice, and often with a second driver. With the stock clutch and sticky tires I could end up needing to replace the clutch disk as often as once a year. Using the competition pressure plate one clutch disk lasted three years and had only minimal wear, but it was finally retired when the rivets holding the friction material started to work loose. About that time the steel thrust ring in the center of the pressure plate diaphragm was also getting a bit loose and starting to rattle a little, so the pressure plate was also retired. Thereafter I went back to using the stock MGB pressure plate, but I tend to take it a little easier now shifting into 3rd gear at speed.

During the 2nd and 3rd years of using the competition pressure plate for serious autocross I also had to replace the worn u-joints in the propshaft, which I have otherwise never had to do with the stock clutch. All of that flywheel energy has to go somewhere, and the strong clutch and sticky tires conspire to put a significantly higher load on the driveline. That's a pretty good deal for serious competition, but somewhat overkill for street driving. Unless you plan on regular serious hot shifting and intended tire spinning in 2nd and 3rd gears, I would suggest that the stock MGB clutch is perfectly adequate and a very good combination of parts for daily driving.

$.02,

Barney Gaylord
1958 MGA with an attitude