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MG MGB Technical - Quick gearbox question

During our unusually hot weather in the UK, I have found that when the car is hot after a run, 1st and reverse gear is sometimes a pain to engage. Is there anything I can do to help this or it something to live with.
Wasn't sure if the 20/50 oil in the gearbox was too thin when the weather is very warm.

most find, a good quality, 20w-50 engine oil fine in the gearbox provided it's not left in too long or the level allowed to drop too much. It's not too thin if the oil isn't too old or too worn. Surprisingly even owners that change the engine oil every year neglect changing the engine oil in the gearbox or even checking the level.

A thorough change of oil might help if it's not been changed in the last two years (or 24k-miles whichever is soonest).

First clean around fill/level and drain plugs, spray with PlusGas (not Wd40) and check they move - slightly tighten first to break muck/crud/rust seal then slightly loosen to check they both move.

If good, get the gearbox oil as warm as possible and then drain out for as long as possible to remove as much muck/crud/bits and existing oil.

Check what's drained out for quality of previous oil and any bits in it.

Refill with good quality oil and remember to continue the service in two years time (or 24k-miles if sooner!).

If you have overdrive there a bit more work you should and could do.

If you've been regular with checking gearbox oil level and with the changes then consider when you last changed clutch fluid or perhaps issues with your clutch.

Good luck.
Nigel Atkins

B*gger, forgot the graph to show 20w-50 range against gear oil range -

Nigel Atkins

Thanks Nigel.
I had forgot that 20/50 viscocity is not dissimilar to gear oil.
I do regularly change the gearbox/overdrive oil and clean the filter - usually every 2 years.
I fitted a new clutch slave cylinder about 6 years ago but must admit I have not changed the fluid since. Perhaps I ought to run some new fluid through that first and see if that improves access to first and reverse!

A pain to engage in what way? Does trying to engage reverse grind? Or simply baulk?

If it grinds then you have clutch drag, which will also cause 1st gear to baulk.

If it's baulking without grinding then in 50 years I have never had a gearbox that is completely free from the tendency to do that occasionally, although it is true to say that BL gearboxes (by far the majority of cars I have had) seem to suffer more than others.

Baulking in first usually just requires the gearlever to be nudged down to second then it will go straight into 1st. In reverse sometimes nudging to 1st is enough, sometimes it needs nudging to 1st as well as 2nd. Very occasionally it still baulks so it has to be a case of neutral clutch up and down, then it goes in. This is the roadster, I can't recall it happening on the V8, even though they are basically the same gearbox, so it's probably yet another example (if any more were needed) that even though basically the same our cars do differ, especially after 40+ years.

Whilst it is a bit of a pain especially having to go through the 1-2-neutral process, I just live with it. Not worth the effort of removal and dismantling, in the hope of finding something not quite right especially when the gearbox is then cold and the problem occurs when hot!

No grinding - it just baulks.
Probably best to leave alone then!

You could check for things like carpet/mat getting in the way of the clutch pedal, a squirt of oil on the pedal pivot perhaps, wear in pedal bush.

I find it best to let the revs totally drop when engaging particularly reverse, in any car. If I'm on choke or fast idle then even more attention needs to be paid and have the car completely stationary before engaging reverse.

I got into the habit of engaging first whilst still rolling back from reverse and then my wife picked up that bad habit from me.

Well done on regular oil changes and servicing of the gearbox and o/d. The thoroughness of the change is also important as is frequency. Gear oil is different to engine oil but as you put the viscosity ranges are similar.

Many classic owners don't see the benefit of better quality engine (or gear oils) but an idea I have seen is to have different colour engine oil in the gearbox than the engine to be able to tell which is leaking (the most?). :)

Nigel Atkins

If the pedal isn't going down far enough - for any reason including carpet bunched up under the pedal) - it will grind when selecting reverse.

The MGB engine is so 'dirty' that there should be any problem telling gearbox from engine except immediately after an oil change. Having said that it also depends on where it has run before it drips off, as to how much dirt it has picked up. Any from the first-motion shaft seal will have run through powdered graphite from conventional release bearings, and could be blacker than that from the engine rear seal, both drip out of the hole at the bottom of the bell-housing.

Just reread Adrian's last post after reading Paul's and realise I totally misread Adrian's post, sorry about that - I can't find the door in to SpecSavers.

As for different oils I think the idea may have related to the less driven Bs.

I can't remember on Bs but on my Ford Type 9 gearbox I have enough leaks to be able to easily see gearbox oil somewhere.

On my car the convergence of collection and drop point for engine oil and coolant meets gearbox oil providing a rainbow of potential anxiety.

I hope I've managed to stop the coolant leak, engine oil leaks I'll need to check again, gearbox leaks I can only hope to reduce a bit unless I win the Lottery and have a better altered gearbox installed.
Nigel Atkins

This thread was discussed between 15/07/2018 and 16/07/2018

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