Mg Midget 1275 gearbox rebuild

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Murrayminor
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Re: Mg Midget 1275 gearbox rebuild

Post by Murrayminor »

Another set of bearings ordered, along with lock washers etc.

The only reason I can see why the synchro balls parted company with the synchro is the First gear selector moved too far, I was of the opinion the 2mm brass bush would stop this from happening.

A strange thing I did notice was the First gear has its part number engraved freehand rather than stamped, I wonder if someone has modified/interfered with the first gear ring.

Strip down number two coming up this evening.
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pgp001
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Re: Mg Midget 1275 gearbox rebuild

Post by pgp001 »

I have my spare gearbox in bits at the moment, I will be very interested in your findings as to why 1st is over selecting. Mine had a couple of flat washers behind the selector, they are approx 2mm total thickness so you should be OK with that.

I think the engraved numbering is normal as well.

Why do you need to replace the main bearings again by the way ?

Phil
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Re: Mg Midget 1275 gearbox rebuild

Post by Murrayminor »

pgp001 wrote: Tue Jul 21, 2020 12:28 pm I have my spare gearbox in bits at the moment, I will be very interested in your findings as to why 1st is over selecting. Mine had a couple of flat washers behind the selector, they are approx 2mm total thickness so you should be OK with that.

I think the engraved numbering is normal as well.

Why do you need to replace the main bearings again by the way ?

Phil
I had a 2mm thick brass spacer turned when I had other stuff in the machine shop so I was hoping that would do the trick I selected first gear and the balls popped out even with the spacer in place.

That's good to know about the engraving, at least that's one less worry.

I could get away with retaining the mainshaft bearing but it would play on my conscience, so to eliminate any problems for the price of a bearing I can have peace of mind.

I was lying awake during the night contemplating what went wrong, I tend to get like that with car related problems, I over think issues.
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pgp001
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Re: Mg Midget 1275 gearbox rebuild

Post by pgp001 »

It was 1st gear that was making a lot of noise in my spare gearbox, the laygear teeth were very chipped and pitted as were the ones on the 1st gear wheel. The layshaft had also picked up and was causing the needle bearings to run roughly.

Luckily I found someone on ebay selling a new old stock laygear for £85, and I have also just picked up an almost complete 3rd motion shaft with all good gears and bushes etc for £20. So I just need a new layshaft and a few oddments and I should have a good spare box.
Hopefully I will never need it, as the one I have just installed in the car last week was a fully reconditioned one.

Phil
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Re: Mg Midget 1275 gearbox rebuild

Post by Murrayminor »

I'm at a loss at to why the 1st gear over selected even with the spacer fitted.
Maybe I did something wrong whilst rebuilding the gear set, I will know later hopefully when I strip it down....
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pgp001
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Re: Mg Midget 1275 gearbox rebuild

Post by pgp001 »

You didn't acidentally instal the 1st gear ring back to front did you, the position of that gear on its hub is very critical.
There is one internal tooth that has a clearance for the interlock detent pin, you have to make sure that is aligned correctly and also that you have the gear on the right way round.

Have you studied the PDF's about gearbox rebuilding ?, much better than your Haynes manual.
http://www.mirabelleclassiccars.co.uk/gearbox.html

Phil
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Re: Mg Midget 1275 gearbox rebuild

Post by Murrayminor »

pgp001 wrote: Tue Jul 21, 2020 4:29 pm You didn't acidentally instal the 1st gear ring back to front did you, the position of that gear on its hub is very critical.
There is one internal tooth that has a clearance for the interlock detent pin, you have to make sure that is aligned correctly and also that you have the gear on the right way round.

Have you studied the PDF's about gearbox rebuilding ?, much better than your Haynes manual.
http://www.mirabelleclassiccars.co.uk/gearbox.html

Phil
I did initially place the 1st gear ring in the wrong position when fitting it to the shaft initially but soon realised my error, I then placed the cut out in the correct position for the detent pin.
What I think is happening to my 1st gear is that it appears to over select even with the brass spacer fitted to the selector rod, there does seem to be a lot of fore/aft movement or "play" on the inner 1st gear ring (where the springs and balls fit into) and this it seems has caused the detent balls to make there bid for freedom.
I'm now thinking the inner gear is fitted the wrong way round thus allowing the detent balls to be visible, the detent holes are offset so that could be the issue, but I cannot check as I don't have another gearbox with me its at my lockup miles away.

*EDIT*

No that's not the issue as the inner ring can only be fitted one way due to the baulk ring cut outs, back to the drawing board.

I will have the mainshaft out again later so will go through the whole thing again ensuring everything is where it should be and take it from there.
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Murrayminor
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Re: Mg Midget 1275 gearbox rebuild

Post by Murrayminor »

I have conducted some research on the matter of the detent balls from first gear making their escape.
Apparently there are a number of different bearings doing the rounds with the circlip groove in different locations, which would obviously allow the gear to move more than normal.

I'm awaiting some bearings from ESM so I will check those alongside the bearings I picked up from Moss Manchester last week and see if there is a difference.

I'm aware the mainshaft bearing now comes with a washer to make up for the inner bearing not protruding as per the original bearing, but I'm more concerned about the circlip groove and its proximity to the casing.

This may all prove fruitless but I have to know and can then rule out the bearings being at fault.
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Murrayminor
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Re: Mg Midget 1275 gearbox rebuild

Post by Murrayminor »

Well how much of an idiot do I fell just about now??

I assembled the mainshaft forgetting to install the 1st gear locating pin.

Without this tiny pin it allows the 1st gear ring to slide fractionally past the end of the inner ring thus allowing the balls to escape.

Now assembled correctly the 1st gear stops when the pin reaches the raised tooth on the outer ring, this coupled with the 2mm spacer will not allow the selector rod to force the outer ring over the inner ring.

Now just a case of reassembly, I have left 2nd and 3rd gears on the shaft and working back to the bearing and casing so it "should" all go back together correctly.

I will need a couple of hours quiet time in the garage to concentrate and not forget any minor parts.

Fingers crossed.
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pgp001
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Re: Mg Midget 1275 gearbox rebuild

Post by pgp001 »

Well you live and learn. Glad you found the problem "Before" installing the box in the car !!

I have now got all my bits laid out on the bench ready for re-assembly this weekend. Biggest job will be machining the new tail shaft bush.

Phil
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Re: Mg Midget 1275 gearbox rebuild

Post by Murrayminor »

pgp001 wrote: Fri Jul 24, 2020 12:04 pm Well you live and learn. Glad you found the problem "Before" installing the box in the car !!

I have now got all my bits laid out on the bench ready for re-assembly this weekend. Biggest job will be machining the new tail shaft bush.

Phil
I dropped my extension piece off at a local machine shop they reamed the tail bush to the mainshaft, reamed the reverse idler gear bush to the shaft and made the brass spacer for the selector rod all for the princely sum of £20.

It would be nicer to have the ability to use a lathe but that's a step too far for me.
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philthehill
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Re: Mg Midget 1275 gearbox rebuild

Post by philthehill »

Your ability is only limited by your perceived lack of skill. You can do anything if you put your mind to it.

I am not trained on lathe work but have picked it up as I have gone along.

There are some good cheap lathes out there at the moment. They do not have to be complicated or new. The two lathes I have are each over 100 years old and have plenty of life left in them yet. In fact one has been hardly used and I know the full history of both.

Phil

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Re: Mg Midget 1275 gearbox rebuild

Post by oliver90owner »

I am not trained on lathe work but have picked it up as I have gone along

Me neither. Doesn’t stop me using my lathe and two milling machines. Your local model engineering society might be a good place to find help for machining, with the ‘cost’ being free - or a bottle of wine, for example. Cost, space and time are other possible constraints to taking up the hobby, mind.

Phil uses old lathes; my machines are a little younger, at around 50-70 years old.🙂. I only started kitting out about 25 years ago... to make/repair/restore bits for old machinery, where spares were no longer available.

Your twenty quid expenditure would not cover the cost of buying new reamers and the brass for the bush, so you certainly got it done for a good price, from that point of view.🙂
Murrayminor
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Re: Mg Midget 1275 gearbox rebuild

Post by Murrayminor »

philthehill wrote: Fri Jul 24, 2020 1:33 pm Your ability is only limited by your perceived lack of skill. You can do anything if you put your mind to it.

I am not trained on lathe work but have picked it up as I have gone along.

There are some good cheap lathes out there at the moment. They do not have to be complicated or new. The two lathes I have are each over 100 years old and have plenty of life left in them yet. In fact one has been hardly used and I know the full history of both.

Phil
You may be right there Phil, I may actually take some time to try my hand at some point.
I worked in an engineering works for avery short period of time many years ago and was in awe of the lathe operator who turned some lovely pieces out on a huge lathe, I seem to recall there were three very big lathes in the engine room as it was called.
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Murrayminor
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Re: Mg Midget 1275 gearbox rebuild

Post by Murrayminor »

Another couple of hours misery with this gearbox.
It was all ready, the main shaft was going in but the reverse gear was fouling it's progress, I lifted out the main shaft again and snap went the gear, it didn't go all the way so the balls popped out but far enough for one to pop out when I examined the shaft.

I'm at a loss, my first thought was to buy a ready reconditioned midget box and have done with them altogether but then I wondered if the pin in the 1st gear set was worn from abuse and that is allowing the 1st gear to over select.
Upon looking at the gearset I can see the three balls but I can also see the lock pin which "should" have held the gear in place.
Attachments
Mainshaft
Mainshaft
20200728_180707.jpg (2.01 MiB) Viewed 1395 times
1st gear
1st gear
20200728_180508.jpg (1.91 MiB) Viewed 1395 times
Gearbox internal
Gearbox internal
20200728_180437.jpg (2.73 MiB) Viewed 1395 times
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Murrayminor
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Re: Mg Midget 1275 gearbox rebuild

Post by Murrayminor »

The first picture shows the mainshaft.

The second image shows how far the first gear has moved, leading me to suspect the locking pin.

The steel plate in the last picture is a mount I made up so I can hold the gearbox casing in the vice while working on it.
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Murrayminor
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Re: Mg Midget 1275 gearbox rebuild

Post by Murrayminor »

Having thought about it overnight I think the pin in the gear set would be hardened therefore unlikely to be worn, but I wonder if the gear is actually worn beyond tolerances.
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Murrayminor
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Re: Mg Midget 1275 gearbox rebuild

Post by Murrayminor »

I have discovered wear in both the pin and the gear.
Maybe that was the issue all along.
Enquiries have been made to source a new gear and pin.
I'm concerned about the quality of new gears,but it cannot be any worse than a worn old gear.
Attachments
Gear wear
Gear wear
20200729_145902.jpg (2.21 MiB) Viewed 1356 times
Pin wear
Pin wear
20200729_145600.jpg (1.85 MiB) Viewed 1356 times
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pgp001
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Re: Mg Midget 1275 gearbox rebuild

Post by pgp001 »

Hi Dermot

I have just re-assembled my 3rd motion shaft this evening, before I did I spent a bit of time working out just what the purpose of the little rounded end pin is.
I concluded that it is there to prevent the inner hub moving back towards 2nd gear whilst 1st gear is engaged , the only reason the inner hub moves along the third motion shaft is to engage the baulk ring with 2nd gear just before the 1st gear ring engages with it and locks it to the shaft.
When you select 1st gear from neutral, the spring loaded balls hold the inner hub and shift it backwards towards the bearing, and the little plunger rises into the 1st gear ring and prevents the inner hub moving forwards towards 2nd again, you would not want 2nd gear synchro trying to operate when in 1st gear.

There is nothing to physically prevent the 1st gear ring travelling too far backwards towards the bearing and then all the springs and balls pop out, that is until you have the 1st/2nd gear selector in place. The usual thing is to put a washer on the selector shaft so it fills the gap between selector and case to prevent over travel.
This assumes all the bearings and shims etc are in the correct positions in the case.

Just seen and subscribed to your Youtube channel by the way.

Phil
Murrayminor
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Re: Mg Midget 1275 gearbox rebuild

Post by Murrayminor »

pgp001 wrote: Wed Jul 29, 2020 10:33 pm Hi Dermot

I have just re-assembled my 3rd motion shaft this evening, before I did I spent a bit of time working out just what the purpose of the little rounded end pin is.
I concluded that it is there to prevent the inner hub moving back towards 2nd gear whilst 1st gear is engaged , the only reason the inner hub moves along the third motion shaft is to engage the baulk ring with 2nd gear just before the 1st gear ring engages with it and locks it to the shaft.
When you select 1st gear from neutral, the spring loaded balls hold the inner hub and shift it backwards towards the bearing, and the little plunger rises into the 1st gear ring and prevents the inner hub moving forwards towards 2nd again, you would not want 2nd gear synchro trying to operate when in 1st gear.

There is nothing to physically prevent the 1st gear ring travelling too far backwards towards the bearing and then all the springs and balls pop out, that is until you have the 1st/2nd gear selector in place. The usual thing is to put a washer on the selector shaft so it fills the gap between selector and case to prevent over travel.
This assumes all the bearings and shims etc are in the correct positions in the case.

Just seen and subscribed to your Youtube channel by the way.

Phil
Phil
From what I understand, and I'm probably wrong so bear with me, the external detent balls and springs are there to prevent the selection of two gears at the same time.
The inner gear on 1st gear is always in motion with 2nd gear being held there by the baulk ring and only meshes with the laygear once 2nd gear is selected.

On examining my 1st gear outer ring there is clear wear where the pin runs along the raised tooth and as the pin is solid (i.e. no spring underneath) it "should" lock the gear to prevent over selection, which is to stop the gear making contact with the bearing.

The pin and indeed the 1st gear on my gearbox seem worn due to the fact the gear has either been forced or simply worn over time allowing the gear to slip over the pin thus causing the detent balls to escape.

I have a 2mm brass washer to fit to the 1st selector rod which "should" prevent 1st gear from over selecting, but when I assembled my mainshaft and fitted it when checking the movement of the gears my 1st gear overselected and the detent balls almost escaped.

I think they stayed in as I believe the pin had the unworn end in the gear, but there was still enough meat left on it to prevent a full escape of the balls and springs.

So I have decided to obtain either a NOS or at second choice a remanufactured 1st gear and new pin, along with the 2mm washer will hopefully solve that problem.

I have checked and checked so I'm confident everything is fitted correctly the only issue I can detect is the wear on the pin and 1st gear, if you still have your shaft on the bench it would be a good time to check yours or at least compare them to mine.

Thanks for subscribing to the you tube channel, It came about as I was searching how the sill panels went together so when I couldn't find anything I decided to make a short explanation video, I then just carried on rambling.
There are now over 60 people interested or have subscribed which is great and I hope people find them useful as a point of reference.

Regards

Dermot.
Proud owner of my first Morris Minor
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