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MG MGB Technical - Sticking gears

Hi, this is my first post on this site since changing my MGA for a 1973 MGB GT.
When driving along, and changing gear, very often the gear seems to stick when changing gear. Do you think this is the clutch , slave cylinder or gearbox?
Nigel Munford

We might assume that your change of cars is quite recent in which case it might be reasonable to think you are not yet used to the clutch - could you be confusing sticky gears with late engagement by the clutch ?
Thinking of the box itself, the linkage from the gear-lever is mostly internal but there is a plastic widgit at the bottom end of the lever and these need grease or replacement once in a blue moon. More likely in this area is the rubber grommet under the gear-lever gaitor - they tend to break up and can impede the change.
Another unknown is the oil in the box - has someone filled it with heavy gear oil. This might make it slow to change gear until the box is warm.
I might start your investigation by watching how much the clutch slave moves when someone depresses the pedal. I would renew the fluid while you are about it,
Roger Walker

Roger makes some good points and I agree with some but at the risk of upsetting him (again) I don't think the plastic bush at the base of the gear lever needs greasing as it's split.

A 73 shouldn't have the rubber gear lever grommet but if it has (under the gaiter cover?) sometimes they can be too stiff and even pull the lever out of gear on the overrun.

The oil in the gearbox is important and it's level, easy to you it up to be sure if it's low.

The weight of the oil is less important than its age and use/abuse. Normally engine oil is used in this gearbox and should be changed every 2 year or 24k-miles whichever is the sooner. Engine oil will break down sooner than gear oil generally.

At the risk of upsetting traditionalist here again, gear oil can be used in the MGB gearbox (even BL said so, see the attached image) but even if they didn't the correct type of gear oil, that doesn't attack the 'yellow metals', could be used anyway as in the same way that engine oils aren't the same now as 50 years ago (even if they have the same name as back then) so gear oils have moved on too.

After the easy stuff like that then as suggested a good look at the clutch hydraulics, starting with the pedal.

Personally I change the oils and fluids ASAP on a car unknown to me and do thorough hot and long drains of the engine, gearbox and rear axle and refill with quality oil, can make an unexpected big difference sometimes (I also clean the cooling/heating system but that's another matter).


Nigel Atkins

As a 73 you should have a dipstick behind the console and you can use that to smell if not feel what is currently in there. Gear and diff oil has a strong sulphurous smell, compare it and the feel with what is on your engine dipstick.

Mine has occasionally needed a firmer push to get it out of 4th for over 30 years. I'd rather have that than a tendency to jump out of a gear, so it's a question of how often yours does it, and how much extra force it needs, to determine whether it is worth investigating.

For a long time people have said gear oil makes the gearbox 'heavy' especially when cold, but they are going by the viscosity numbers which tend to be in the 80/90 range compared to engine oil. They use different ranges and the actual viscosity overlaps. The V8 is specified to use gear oil and that has been no heavier than my roadster with engine oil even in the coldest winters. If very heavy oil had been put in there perhaps to disguise noises then it would be heavy all the time especially when cold.

Clutch issues even if intermittent would be far more noticeable from difficulty selecting gears at a standstill, or a low biting point when pulling away.

paulh4

80 and 90 are ranges not fixed points so some 80s could be 'thicker or thinner than other 80s same for 90s.

The quality, age and fill level of the oil can make a difference. Many "classic" car owners think all oils are the same and just go for the cheapest or some thing they think is from the 60s and 70s. The oil in the gearbox could be decades old (despite the engine oil in the gearbox specified to be changed every two years). Despite what many may thing oil 'wears' and quite quickly from new quality, and engine oil in the gearbox will wear more.

A good quality suitable gear oil in the gearbox shouldn't make the gearbox feel "heavy" and oils can be selected for the frozen north of England or the UK to help more with the cold, not less.

A suitable good quality gearbox oil can help with a worn or perhaps even faulty, gearbox, perhaps only a very little but it can not cure the problems.

I've actually changed the oils in my gearboxes (and rear axles) and used oils other have told me were wrong and I've never had the gearboxes fall apart on me, the rest of the car, yes.

As before first step is to check that the oil level isn't low and if it is top it up and see how that goes. I'd personally do a hot long drain and refill (cleaning the overdrive as per Driver's Handbook) and that way I'd know the oil quality and when it went in - and I could also check what drained out and for bits in it. Even if it made no discernible difference it'd not matter to me as I'd have changed the oil anyway for the reasons stated.


Nigel Atkins

All good, but---First of all to diagnose the issue we need to know exactly what the issue is-----
My vision of a sticky gear might and probably is different to someone else's idea of a sticky gear-
example-
My Grandfather said years ago the gears in his Morris 8 got sticky with hot weather----I drove it a few times and couldn't fault it---Then one day he turned up at home and said he'd fixed it--he'd put a small plastic bag over his gearlever knob---The issue had been the knob got sticky when the sun got on it--
willy
William Revit

Sometimes with my MkII MGA, and I haven't been able to identify a pattern yet, it's difficult to shift out of a gear (usually third but occasionally first or second). I know that it's a completely different transmission, but this is what I'd call "stick[ing] when changing gear." I don't think mine is a clutch issue but I'm certainly not sure. The car runs and drives great and this fault(?) is a minimal irritant so I haven't had much incentive to try to find a cause and cure.

Jud
J K Chapin

Excellent example and story Willy. With most stuff you start with 20 questions and work from the answers given but it's difficult on a forum/message board with just written info.

Perhaps your grandad was just testing to see if you'd make a good medic, whatever you learn from that experience.
Nigel Atkins

Jud description of sticking gears is much better than mine, and described my problem perfectly.
Nigel Munford

All still applies. Driving it more and frequently and regularly on reasonable length journeys (20+ miles one way) will help loosen the car up and run better, until you don't drive it for weeks on end and then you'll be starting over again with somethings.
Nigel Atkins

Jud's description is similar to mine and as I say has been doing it for 30 years. Definitely not clutch in my case, how far in and how long you would have to dig to find and resolve it would be an unknown quantity. Only you can say whether you can live with it or not.
paulh4

NigelM
Just a thought---The MGB clutch slave cylinder down on the bellhousing has a light spring inside it-
The job of this spring is, because there is no actual clutch adjustment, this spring keeps a very slight preload on the clutch lever which in turn keeps all the clutch release mechanism at zero clearance-
When it's all working properly, as soon as you start to press on the pedal everything starts to move----but
If it just happens, and it does, that this spring has rusted away or broken the piston in the cylinder can retract away from the arm which results in less clutch operational movement when the pedal is pushed-
It can be checked by going below and pushing the arm towards the cylinder then releasing it--The piston should return by itself until it takes up the clearance between the piston,pushrod,arm--It takes a few seconds to draw fluid through on it's own--If it doesn't return under it's own steam and trap the pushrod then the spring has probably died causing your intermittent issue---
William Revit

This thread was discussed between 28/08/2022 and 02/09/2022

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