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MG MG Y Type - YB rear axle and carb dash pot

There's nothing like using your Y type every day to show up any faults..currently I have two:

The rear axle on my YB (different to YA and similar to TD)clunks when I press and release the accelerator pedal. Had new UJs so wondering whether this is a sign of axle wear?

Secondly I have to top up the carburettor dashpot quite frequently...not sure where it goes to but the car doesnt like it being empty.

Any ideas?

Hi David

Pull your half shafts out and check the splines. Sounds to me like you have a case of spline wear. If you look in Hints and Tips I think there are some pictures in there of David Hague's YB that had a similar problem and the splines were shot.

Also, with respect to the dash pots - is this a recent phenomena or one that has existed for a while? If the former, is your ignition timing correct? Also, what weight oil are you using in the dash pot? I have heard and use 20/50 is perfectly acceptable and maybe you are just too light?

Paul Barrow

I did use a thin oil in the dashpot but have changed to 20/50 now. It still seems to vanish quickly (over several weeks)which cant be right as its supposed to last ages..must be a leak but cant see it or does it burn off - you know when the dash pot is low on oil as it affects acceleration- slowing response at the gas pedal...timing is fine.

Checked the brake drum and splines on the half shafts on the rear axle - seems fine. How much free play should there be when you rotate the prop shaft by hand with the car stationary? I can rotate it and get the axle to clunk by twisting the prop shaft either way....the car is fine if you take gear changes easy but its not difficult to get the clunk by accelerating and decelerating quickly. Definetly from the axle but not sure if its jusy normal wear - the YB hypoid axle has done over 200,000 miles I would think.

The universal joints tend to 'clunk' when worn although its almost impossible to feel by twisting them. If the diff is not too noisey they then can be overhauled and clearances set up. Take care in removing the diff they are heavy.
B Mellem

fitted new U/Js....who is an expert on these diffs?

At one time new uprated half shafts were for sale...I need to know what the likely fault is though.

With the dash pots - do you see signs of the oil having been ejected out of the "escape" hole on the top of the plunger?
Do you have the proper brass topped plunger or the later plastic type?
Are you seeing signs of oil being burned in the engine?

You have a YB so the shafts are the same I think as TD so I have no idea on acceptable free play. If the splines are showing signs of wear then that is your problem. Alternately it could well be badly fitted UJs.
Paul Barrow

The question of a diff specialists is not at the moment I can answer since my YA spare unit is away with one, but removal of the diff is not a thing I would undertake first. If it is running quietly then chances are that its OK. Undoubtedly there is a certain amount of back lash between the gears but only an expert can say, if however the pinnion is moving check the flange nut, it should be tight, I mean real tight, on the YA there is a split pin/castle nut.

As for the U/Vs I'm not convinced new replacement parts are always up to it, only you can tell from the history of the fault.

By the way I have found that on both YA SU and my Anglia Stromborg dash pot oil tend to end up with only a thimble full but still damp the piston even after a year of neglect. I put the disappearing oil down to the high vacuum in the chamber on closed throttle overrun.

As it happens I am up to my eyes with rear axles at the moment first with the YA and now the Anglia diff being changed to a 3.75, must be the time of year.

B Mellem


I know TD's but not YB's, however if the axles are the same, I would guess that the drums are the same? If so, the splines on the drums wear over time causing a clunk. The axle nut should be tightened to at least 150lb-ft of torque and better yet up to 200 pounds. If they become loose then the noise develops on acceleration and deceleration because of spline wear. You can check this by pulling off a drum and taking a look. With the kind of noise that you are mentioning, it will not be hard to visually see the wear. Usually it is the drums that are the problem since the axle has some hardening to the steel. I doubt that the problem is the diff. It has always been the drums in my experience, but there is always a first.

Mark Strang

i will check the torque on the drums this week and see what happens - many thanks for all the input.


As I suggested before, and Mark has reinforced above, look at the Hints and Tips page on for the hint from David Hague entitled Rear Hub Splines - (peculiar to YBs). There are some pictures there.

Paul Barrow

Hello David. Normally on a YB/TD differential the back lash between the gears is set between .008 to .015 so if the wheels are locked with a parking brake, you should hardley be able to turn the pinion back and forth. Worn U-joints generally allow the drive shaft to go off center and you get a vibration while driving. Most of the clunks that I have seen come as suggested from worn splines at the axle, brake drum. A lot of people will sand the washer to get the castelations to line up with the hole in the axle, wrong. As said above 150+ ft lbs, I tighten the nut as far as I can with a 1/2" breaker bar then put a 4 foot pipe extension on the bar and thghten to the next castelation on the nut.

Butch Taras
Glendale, Ca. USA
R Taras

This thread was discussed between 23/11/2012 and 25/11/2012

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