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MG MGB Technical - Clutch and Gearbox

Hi guys

I have a bit of a problem and am not sure what is causing it.

I have a 1974 CB MGBGT.

1.Whats strange is that, on the odd occasion, it is jumping out of fourth gear.

2.Also the clutch "bite" point keeps changing.

What i mean by this is that i can be sat in traffic, go to pull away and the bite is at it's usual position. Then when i go to pull away again the bite is much lower down.

If i put the car in neutral, foot off the clutch and try again the bite comes back to its usual place.

Important info: Clutch fluid level is normal, gearbox has been topped up with oil.

I think these are two seperate issues.

Can anyone help??

thanks Adam
Adam Cooper

As to the clutch, there are several places in the linkage that develop wear. Some of the pins which connect these linkage points may have developed wear patterns such that when the pin moves a bit, it shows a different face to the connecting parts and presents a different amount of play. Check for worn parts where the pedal connects to the master cylinder and where the slave cylinder connects to the clutch fork.

As to transmission jumping out of gear - I've never rebuilt a transmission but it sounds like you're about to.

While you have it out, inspect the clutch arm pivot inside the bell housing. That's the third of three common wear points in clutch linkage.

Best Wishes
Matt Kulka

Inside the transmission is a ball and spring that keeps fourth gear (and others) engaged. The spring allows you to engage/disengage without any problems. If the spring gets weak, it can cause the gear to disengage. It is an inexpensive part to replace, it just takes alot of effort to get to it since it will require access to the tranny internals.
Rich Stinchcomb

Adam, As Matt mentioned and I forgot to mention in the General post section, be sure to check the clevis pins at the slave cylinder as well as the one at the pedal assembly. If the pins are worn, the push rod (one at the slave cylinder, one at the master cylinder)hole enlarged, or the hole at the top of the clutch pedal is worn or oblonged this can cause the clutch to do funny things. Have someone push the pedal while you are under the car and watch the movement. The pushrod should move about 3/8" (10mm) for correct clutch operation. If the rod does not move at least this measurement, check for worn clevis pins/pushrods.

Another cause of jumping out of 4th (but not 3rd) is that the input shaft bearing is worn, or either the front cover is loose or incorrectly shimmed (allowing the bearing and shaft to move forward).

Crazy thought... have you checked that your engine and gearbox are bolted together properly? Check the bolts and nuts that hold the gearbox to the backplate, and check that the backplate is secured to the engine.

I've had worn clutch linkages, but the bite point has remained constant (if incorrect) rather than moving around. Mayhaps something loose would cause both these symptoms?

Oliver Stephenson

For the clutch, all of the above re worn bits, plus, a dodgy master cylinder can behave like this. On the gearbox, any bad bearing on the 1st or 3rd motion shafts can do it. One easy check is to be sure the nut at the end of the 3rd mo. shaft is tight. You have to disconnect the driveshaft to check/tighten. I have seen them come loose so that the shaft moves for and aft in the bearings, which can make it jump out of gear - and numerous other evil things that don't show up until too late.
FR Millmore

Sorry to go a totally different way, but undo the rubber boot. If it is 180 degrees out it will pull the shift shaft out of gear. It is easy to check and cheaper.
Frank Baker

This thread was discussed between 15/09/2005 and 16/09/2005

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