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MG Midget and Sprite Technical - Tight Gear Shift?

I was out in my 1275 GAN4 midget at the weekend during the very warm weather, car was fine setting off and the gear changing was normal.
As the trip progressed I noticed the gear shift becoming tighter/stiffer when changing up and down the box until it became a struggle to change gears at all.
I then decided to pull up for a while to allow the car to cool down, the engine temperature at this stage was at 170 degF.
I managed to get the car back home by restricting the number of gear changes, taking it easy and left the car overnight to cool down in the garage.
Next morning after cooling down the car had gone back to the gear change shift being as normal.
Other than this problem being perhaps due to the warm weather conditions, could this be a fault starting to develop within the gearbox?
J Bond

Me too, also with 1275 & ribbed gearbox. Oil checked.
Tim Carter

My immediate thought was to check the gearbox oil level (should be up to bottom of filler plug thread).
Bill B

Heat builds up in the tunnel. Engine heat flows into the tunnel making it worse.

I often used to muse on how to introduce an oil cooler for the gearbox. Perhaps even a cooling fan.

Exactly the same used to happen to my rib case. Sign of a worn or wearing gearbox. Helps to change the oil, but not much.

One of the reasons I changed to a T9 box.

anamnesis

Thanks for the advice so far on this matter, could the existing clutch condition also be playing any part in causing this stiff gear change problem when the engine & gearbox reach top end temperatures?
J Bond

Could be if the clut h isn't disengaging properly. Perhaps the friction plate may be sticking on the splines of the gearbox input shaft.


anamnesis

A thought: try slackening the nuts that secure the gear shift extension to the top of the box, just enough to give some free movement. Move the gearstick through it's full range, selecting all the gears a few times. Then retighten those nuts. There is bound to be some fine degree of slack and this may correctly align the selector housing and rail in the extension. It may be slightly misaligned and as the extension won't heat up as much as the rest of the box this could become accentuated and cause the stiffness in the movement
GuyW

Can those nuts be reached in situ Guy?
anamnesis

Yes, at a pinch. A ratchet combination works best.
GuyW

Similar here on hotter days.
Currently using 20/50 engine oil but would a thicker or more modern oil, compensate for 54 years, 105,000 miles worth of wear?
Something along the lines of Dynolite Gear Oil 30 which replaces SAE 20/50.
https://www.dynolite.com/product/dynolite-gear-oil/


Jeremy MkIII

Jeremy--don't forget that the viscosity of engine oil is rated differently to gear oil-----30 engine oil is roughly the same as 80 gear oil

from the net----

The higher numbers of a gear oil (e.g., 75W-140) do not mean that it has higher viscosity than an engine oil. Interestingly a 90-grade gear oil viscosity falls between a 40 or 50 grade engine oil, and 80 grade gear oil is similar viscosity to a 20-grade engine oil.
William Revit

Interestingly Willy, that dynolite looks good as it's a multigrade 20/60


Dynolite Gear Oil

Gear Oil 30 - Multigrade 20w-60. Replaces EP80, SAE30 & 20w-50.

And doesn't contain any EP type additive that would destroy yellow metals such as bronze, copper or brass.

This oil does not contain any friction modifier additives which can cause slipping in overdrive clutches and preselector gearboxes, such as Cotal or Wilson.

So should be good on synchros too.

Nothing to lose by trying -- probably.

anamnesis

I remember years ago, my mate had an E Type jag (still got it) and the gearbox was supposed to have 30 gear oil. Ha had 30 engine oil in it thinking it was the same but it was always hard to get gears cold and the syncro's were always notchy---When it got changed to 30 gear oil it was a completely different beast, always nice gearchanges hot or cold.
William Revit

Dynolite do not really extol the virtues of their gear oil with regard to stiff gear changes when hot. If the viscosity is much the same as good old engine oil, what would be the likely benefits, if any, for using it?
Tim Carter

I remember a similar discussion here some years ago when I started running mine on 30 grade gear oil.

I don’t do a high mileage these days but it seems to run fine - certainly did yesterday when it was quite hot out there.
Saying that I don’t remember a problem 50 years ago when I ran 20/50 either.

Maybe with the new better oils there is an issue ? Can see it could be a problem with O/D clutches etc.

I use Penrite GB30 from Moss. Good old Aussi oil Willie !

And no I didn’t put the pictures the wrong way up !






richard b

Probably not much Tim-
The only real issue is most engine oils have truckloads of friction additives in them nowadays that 'might' impact on the performance of the syncro rings.
A gearbox oil would/should have less friction additives allowing the syncro rings to grip as they are designed to do
That in itself is one of the differences between gearbox oils and axle oils that can be heavily doctored

Transaxle (combined gearbox/final drive) oils have targeted additives to allow the syncro rings to work while still having protection for the geartrain

fair bit to it once you dig into the specs of the oils

Penrite is a fairly highly respected oil supplier here Richard--I'd guess they're just blenders that use base oils from somewhere but they do have some good products.

Very politically correct of you to be so kind to have your bottles sideways so the labels can be read without me having to turn my laptop upside down --lol
William Revit

Thanks Willy, it's a multigrade as Anam pointed out so may be worth a punt at some stage.
When cold the gearchange is peachy, and with the cooler weather approaching, the hot weather issue may not be such a problem.
Jeremy MkIII

Well after reading the tech stuff it appears the Penrite GB30 is also a multigrade - learn something every day.
The clue is only calling it GB30 not SAE 30.
Strange though as oils that are multigrades I.e 20/50W - the W indicates multigrades.
As I noted it works ok for me - well famous last words !
richard b

I had my first Midget back in the early '80s. I used to commute 30 miles, mainly on the M6, and the gearchange would tighten up once I'd been on the motorway and got some heat into it.
Dave O'Neill 2

just read that myself Richard-

https://penriteoil.com.au/assets/pis_pdfs/GEAR%20BOX%20OIL%2030.pdf

With multigrades, using 20w60 as an example it means the oil is the same viscosity as a cold 20weight oil when it's cold but the same viscosity as a hot 60 weight oil when it's hot (operating temp)

basically, the bigger the differance between the 2 numbers the less the oil thins out when it's up to temp.
William Revit

Sorry Willie the link doesn’t work for me - get an error 404 ?
richard b

Richard, it goes to 404 initially but then lets you select options on Penrite's website.
Bill B

An image of the pdf


Tim Carter

I know but don’t know what Willie was linking to ?
It’s rather a large site !
richard b

So it looks like these oils offer a wider temperature performance, therefore may well be worth a try.
Tim Carter

Thanks Tim - similar to what’s on the can.
richard b

Richard,

Copy and paste the link and it works fine, the embedded spaces cause an incomplete link.
David Billington

After my restoration I used that dynolite gear oil.
When the car got warmed up was difficult to change gears mainly 1 or 2.

Someone recommended using this stuff for auto transmissions and it has been perfect ever since.


James Paul

Looks similar to this ?

https://www.opieoils.co.uk/p-965-fuchs-titan-atf-4000-multifunctional-high-performance-automatic-transmission-fluid.aspx

Now then, ATF in a 1275 gearbox ?


Tim Carter

Are you saying your gearbox is Fuch'd?
David Billington

They don't lock up when hot when new or rebuilt and using standard 20:50.

Conclusion? Gbox f*cked or on it's way to being.

anamnesis

Agreed, they don't go tight when rebuilt, well mine doesn't and it has 20/50 in it. I'd worry about ATF, you'd think its viscosity was on the low side for a gearbox although the synchro rings would work well presumably. But you need sufficient viscosity to maintain a protective film between the gearwheel teeth I would presume. 30 weight engine oil was specified originally, before multigrade had evolved.
Bill B

What about an additive?

The image is for STP, but there is also Liquid Moly...


Tim Carter

Bill-modern atf's like Mercon v are around the same viscosity as 25 engine oil.
A large number of vehicles use atf in manual gearboxes.
An example is the T5 box used in all sorts of cars, we were told use atf only in these boxes because of the fibre lined syncro rings but then when ford put them in the XR6 and v8 Falcons they were filled with 80/90 gear oil--same syncro rings--------?
With the ford courier utes if there was a stiff gearchange issue the trick was to drain200/300ml of the 80/90 gearoil out and then top up with atf--made a different box out of them.
willy
William Revit

Willy
Isn't 25 weight a bit low though for a ribcase especially when warm? I'd assumed 30 is as low as it should be? Although 20/50 might not get much above 25 or so at gearbox temp if I've understood how multigrade works?
Bill B

Bill--For a ribcase box I'd probably agree , my comment was more to say that atf is spec'd in some manual gearboxes as the required lubricant and should be fine -more in answer to your comment of needing viscosity to lube the gear teeth -atf will lube the gears quite satisfactorily.
Sorry if I've confused things-
But a cup full or so in a gearbox as an additive makes a big difference to gear changes.
One down side of the Ford boxes that spec atf is that when they're really,really hot, sometimes you can get a gear rattle/chatter at idle in neutral which is a bit--annoying

willy
William Revit

This thread was discussed between 19/08/2025 and 22/09/2025

MG Midget and Sprite Technical index

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