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Author Topic: 74 mgb tranny  (Read 6211 times)

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74 mgb tranny
« on: March 18, 2008, 05:03:28 AM »
The 74 mg I bought my wife has me a little perplexed. I can shift it into gear without depressing the clutch, then when I do depress the clutch the gear engages????? How is this possible? Am I basically screwed with a rebuild or is this a case of an "easy" fix? Is it a matter of fluid seeping behind the slave? Also the clutch is extremely hard to depress and it does not return the the right position right away, it takes time for the pedal to return to the up position. Help please. ???


  • chfwrench
  • AMGBA Club Tech Staff
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  • Membership Number (if known): 91-10014
  • MG information: '73 red B roadster
Re: 74 mgb tranny
« Reply #1 on: March 19, 2008, 11:11:48 PM »
From the sound of your symptoms, you should first check the clutch slave and master cylinders before diving into a trans rebuild.  It's not wasted time or money as, with what you are seeing, I would want to replace or rebuild the hydraulics when doing the transmission anyway.

The gearbox is a fairly robust item and takes a lot of abuse before failing.  That said, it is probably original and almost 35 years old, so some expectation of work at some point should be made.

First, some questions:

- Once in gear and engaged, does the car drive easily; no noises from the gearbox, it remains in gear under power.  If this is the case, there's a good chance the gearbox is reasonably sound.  Worn or damaged gears and synchros usually manifest as difficulty getting into gear or popping out under load.

- Do you have any difficulty shifting while driving?  A bad clutch or hydraulics may make it hard to make shifts, acting like crash-boxing in all gears when the clutch fails entirely.  Worn internals start to show the lower gears first as they are the most often used.
- Does the clutch slip?  Do you find under acceleration that the engine spins faster than you are getting power to the wheels?  This could be a worn clutch disc, contamination of the clutch material or a failure of the hydraulics not allowing the fluid to return to tghe reservoir and hanging-up the slave holding the clutch depressed (slow return to normal, as you described).  There's also another slight possibility that dirt and grease have accumulated in the boot on the bell housing restricting the motion of arm from the slave cylinder to the clutch.  i've seen it, but not that often to the extent that it effects the clutch operation that greatly.  It usually makes the pedal a bit stiff.

There are kits to rebuild both the slave and master clutch cylinders.  The kits are very inexpensive, but do not address the wear, corrosion or damage that can occur.  You also must replace the flexible line that runs to the slave cylinder.  This is very important as the rubber coating deteriorates and clogs the metal lines and cylinders, the debris often acting as a check valve hindering operation.  The kits and lines will run about $50 plus shipping.  For about $160, you can replace the system outright and get all new components plus the job done faster.  Both Moss and Victoria British offer complete kits that include the flexible line to the slave cylinder (Victoria British P/N CRK110, $159 plus shipping for your car).   I recommend this route.  Again, the money is not wasted as, even if you have to ultimately go for a clutch or trans rebuild, you would want to do this as well.  However, from the sound of your symptoms, the reason could very well be in the hydraulics.

The bleeding of the system can be time consuming and somewhat frustrating.  You will need help to pump the pedal, but, because of the many twist in the line that can catch air, this way often does not work and has to be repeated several times.  The best method, which can be done alone, is to use some sort of pump (hand-held Mighty Vac or the like) and push the fluid up from the bleeder on the slave cylinder.  This pushes the air more naturally up to the master cylinder resevoir rather than trying to force it down to the slave bleeder. 

I hope that helps you.  Good luck and keep us posted on what you find.

Art Isaacs
AMGBA Tech Staff


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