MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Evaluating 3rd Gear

Still trying to address why my '71 midget occasionally pops out of 3rd gear. It only happens going uphill, and only when I'm giving it some gas.

I bought a used 3rd gear on eBay last summer and figured I'd install it no matter what so that I'm changing something to see if it improves. Now I'm having second thoughts.

I don't know much about gearboxes, but I'm assuming the big teeth on the gear are just fine on both the existing and the replacement. So I'm looking at the smaller teeth. I think the teeth should be kind of shaped like home plate in baseball.

The small teeth on the replacement in the first image appear more asymmetrical, and I assume they're worn compared to the small teeth on my existing 3rd gear (second image). Although I was kind of looking forward to removing the third gear, I'm not so sure I'd be improving my situation.

Should I swap them anyway?

I've also just ordered a used, replacement 3rd/4th selector fork. Not sure what I'll get will be an improvement, but at least it should be a change.

Thank you for any thoughts.






Mark 1275

The original almost looks better to me Mark. You might want to change the 3rd/4th synchro sleeve that engages with the "dog teeth" if it looks worn. Also, always change the baulk (synchro) rings anyway. The 3rd gear itself looks OK to me. Probably, the synchro sleeve is throwing back out due to wear in itself and maybe the selector.
Bill Bretherton

Thank you, Bill. I'll look at the synchro sleeve tomorrow evening and see how it looks. I will replace the two synchro rings - they're available from Moss.

Now I've gotten myself into a bit of a jam. Curiosity got the better of me and I removed the locking ring that holds the 3rd gear on the main shaft. Took a little fiddling but it came off.

How the heck do I get the locking ring back on? I've got thin screwdrivers, a very thin ice-pick type tool, and can not find a way to hold the plunger in to get the locking ring back on. Is there a trick to it? I tried for the better part of an hour. Do I do some filing to the locking ring to get a little more clearance?

I should have just left it alone.
Mark 1275

Mark,

If memory serves correctly I believe that the little hole between the teeth in your second picture is for that. You line the hole up with the spring loaded retaining pin, insert an appropriate tool and slide everything back together. It's been 20 years or more since I've had to do it so I may be quite a bit off.
Martin

If the selector fork is worn it may not be pushing the gear far enough into engagement.
b higginson

I'm not particularly experienced with gear boxes* but I see no reason why a worn synchro should cause it to pop out of gear - can someone explain how this happens? Worn selector seems more likely.

*I have bust a couple on my Midget, but I swapped the whole box out and only disassembled the remains out of curiosity - in both cases, I had taken teeth off the actual gears.

AdrianR

Mark

I've just dug out some old bits and have tried to remember how I fitted the lock ring. The picture shows the 3rd gear (without synchro ring and needle rollers) on the mainshaft (3rd motion.) The screwdriver blade is pointing at the plunger hole. I think you put the gear with synchro ring on the shaft then put the plunger/peg in the hole with its spring. Then comes the fiddly bit. You have to hold the peg in whilst you fit the locking ring. The ring turns after it's past the synchro hub teeth on the shaft and I think the peg then locks into a "space" on the ring. I think I used an angled dental pick but it's not easy as the peg is almost hidden. It takes time and patience and a swear box.....
Bill Bretherton

And picture.....


Bill Bretherton

Adrian
Yes, might be the selector not moving far enough but AIUI the synchro hub needs to "lock" onto the "dog teeth" on the gear and if they're both worn it can possibly throw out before it's locked. But I would value another opinion as I've never found definitive information about this and I've done a lot of searching and reading.
Bill Bretherton

Dog teeth. Why? Because dogs canine teeth face inwards, and once they grip, the more you pull, the more they embed.

In a gearbox, the so called dog teeth, are cut with a slight but noticeable reverse taper. The more power through them, the more they 'bite'. But as the taper wears, the less they grip. And if the taper wears so much, as to go parallel or the other way, then more power forces them apart, and disengages the gear.

It's a constant mesh gearbox. The gear sets, on the mainshaft and the layshaft (or countershaft), are always in mesh. But in nuetral the first motion shaft( input shaft) is not coupled to the output shaft(tail shaft).

To engage a gear, the synchroniser ring slows the mainshaft by pressing against the cone on the particular gear. As the collar slides over the gear, the dog teeth line up, and once fully engaged, they 'bite'.

I've got some pictures somewhere. I'll have a look.
anamnesis

Interesting, I had never thought about that. I can see that wear on the pointy end of the teeth and the inside of the synchro might make it reluctant to engage cleanly but I'd assumed once in it was in, but not so and in which case yes, I can see that a synchro worn like that might cause it to pop out, possibly if you hit a bump while accelerating and get some momentary overrun due to wheel slip.

And thus we deduce that Mark also needs to inspect the inside of the synchro ring.
AdrianR

Adrian, the synchro *ring* matches shaft speed to gear speed - if that is worn the gears crunch. The synchro *sleeve* or collar is what is moved by the selector and is used to lock the shaft to the gear. So 3rd/4th on a ribcase has a synchro sleeve that locks either 3rd or 4th speed to the mainshaft (4th speed actually being on the input shaft). 1st/2nd are similar except 1st is straight cut and does not synchronise. 1st tends to wear and become noisy (also wears its matching gear on the laygear).
Bill Bretherton

It's coming back to me slowly how this works, there might have been some ball detents in there as well perhaps? I wonder if I still have my collection of broken bits as a learning aid but meantime I'd probably better shut up and leave it to those who actually know to advise.
AdrianR

The dog teeth are more accurately called clutching teeth.

Dog clutches are used inside manual automotive transmissions to lock different gears to the rotating input and output shafts. A synchromesh arrangement ensures smooth engagement by matching the shaft speeds before the dog clutch is allowed to engage.

The pictures are taken from a really good video that explains how it works. The pictures appear at 4 minutes 10 sec, if you want to skip ahead. But the whole video is really worth watching.

You'll notice that instead of springs and plungers, these hubs have sprung sliders that do the same job. So this is a REALLY good tutorial for type 9 gearboxes too, they have sliders.

Take a look at the video.

https://www.youtube.com/watch?v=e-Lk3FXHubw








anamnesis

anem,

Very interesting video. I never really knew the details of how the synchros worked. Great posting.
Martin

Great video thanks.

Nothing like my Type 9 as there wasn't oil leaking from everywhere.
Nigel Atkins

Anam, thanks for the great video which confirms my understanding although I hadn't fully appreciated the dog teeth reverse taper. All clear now!
Bill Bretherton

Yep it is good. I found that and some others, when I wanted to rebuild both my capri 3 rail 4 speed, and the type 9 in my Sprite, to put a longer 1st gear in it. Until then I'd never played with gearboxes, and only had a crude understanding myself.
anamnesis

Very helpful video!

I took these pictures before watching the video, and I'm not sure if they capture the wear inside the ring. I'll look again next time I'm out in the garage, but I hope these will help to see if I have a problem here.

I'll have to try that locking ring again tomorrow as well.





Mark 1275

Anam,

Great video, thanks for posting it. I have never done a gearbox but I have a resonable idea of how they work. I learnt a lot more in 4 minutes or so. Very clear with a good description of what is going on.
Ray Rowsell

I have some other extremely good explanatory/instructional vid's. I downloaded them from u tube as mp4. Not sure if I have the links to the originals. But I can email the mp4's to whoever wants them.

Or, does anyone know if an mp4 can be uploaded here?
anamnesis

Mark
Do you have an old school gearbox shop near you? Someone experienced could advise you what to put back in and what to change by inspecting your parts. There's bound to be dog teeth wear on used gears and sleeves, it's just a question of what is "acceptable". Or take a chance accepting you might have to pull it again.

Whatever else, do check the layshaft and its needle rollers - they have a hard life. Maybe change the shaft anyway but you'd need a decent one. Also check the end float on the lay gear and change the appropriate thrust washer, if play is excessive. Also check the input and output shaft bearings and change the output shaft (tail shaft, 3rd motion shaft) oil seal (easy).
Bill Bretherton

Mark
The first 2 pics you put up-
First pic , your new 2nd hand gear --looks good, no wear on the gear teeth and the synchro slider looks to have been driving properly going by the wear marks on the synchro teeth on that gear---The gear turns clockwise as sitting in your pic so the drive from this gear will be on the sides of the teeth that are looking up at you on the left side of the pic, and drive the slider which in turn drives the output shaft---all good so far
Then--
The second pic. of your original gear
Rubbish tin job
If you look at the main teeth, up the top there it's gone through the hard facing, it's stuffed, also, if you look at the synchro drive teeth ,on the left side looking at you again, you can see by the wear mark that it's been driving out towards the pointy end of the tooth whereas it should be contacting the full length of the teeth, this is why it's jumping out of gear under load-
There's a few things that can cause this--

Incorrectly machined---highly unlikely
Badly worn bearing inside 3rd gear letting it run out of line under load-Common on MGB as they had a bronze bush but i think your box would have rollers so probably not that in this case, but worth checking to make sure the gear runs nicely on the shaft with no rocking, could have a worn shaft or worn in the gear itself--just have a good check that your new gear fits nice and can't rock out of line
The most likely cause of the problem is that hacked up slider in your later pics-it's really been to war, you need a better one of those, it'll never be any good with that one--
willy
William Revit

I use a hose clamp with a hole and a plastic bag. It took much time to find the balls in the Garage...

Flip





Flip Brühl

Willy
I was hoping someone knowledgeable would chime in! But, looking at Mark's new 3rd gear, why are some dog teeth pointed whereas some are angled one way only? Mine was like that so is it normal? (My rebuilt box works fine).
Bill Bretherton

Willy

Your right, the rollers bearings mean the gears do not run out of line unlike the 'B' boxes. What causes the dogs to wear can be worn synchro rings not allowing the gears to engage sweetly and the teeth grinding. Poor maintenance such as low oil levels/ dirty oil doesn't help.

Agree the fork looks worn too. May be worth checking the detent balls and springs are all ok as broken or missing ones don't help.

Its always worth renewing all the bearings anyway on a box rebuild, especially the needle rollers in the layshaft and the needle roller on the which goes on the end of the third motion or output shaft. Also check the end float on the layshaft is within tolerance of .001-.003 thou.

Bob Beaumont

I'd be interested to know what makes the A series gearbox jump out of gear when you lift off. I had a new BL silver seal gearbox and it started doing it in 1st at about 1.5 years, then 2nd and then 3rd at about 2.5 years, it made driving in stop start traffic a pain as the slightest lift off and it would pop out of gear so you had to hold it in gear. Under drive no problems. I replaced it with a Ford box at 3 years.
David Billington

Much appreciate the comments. Heading out but wanted to respond briefly.

Bill, I think that's good advice for me to find an old school transmission shop nearby. The one near me closed long ago, but there's got to be one in one of the larger cities nearby. I did replace the layshaft and all the bearings in early 2018. I hope the shop can help me solve the mystery of replacing that locking ring.

Willy, very glad to see your post. So it sounds like I should swap over to the new 2nd hand 3rd gear. I haven't replaced the needle bearings inside the 3rd gear, but I should just to be sure there's no wobble. I'm not sure where I'll find another slider, but it sounds like it's a priority and I need to start the search. I'm guessing clearer pictures won't make the slider look any better.

For what it's worth, I do change the oil in the gearbox more than needed. I average only about 1200 miles driven per year - hope to improve on that when I retire in 5 months - and change the oil most years.

Thank you for the help.
Mark 1275

David

One cause of popping out of gear is movement of the main shaft and input shaft. The main bearings are shimmed to restrict end float of the shafts. If they are incorrectly sized the shaft moves and it can pop out of gear. I had this problem a few years ago and replacing the shims cured it! I also found my gold seal box had been incorrectly assembled with a couple of detent springs missing!
Bob Beaumont

And I had a problem with a new silver seal direct from bmc, where it went into 1st and couldn't be taken out. A shim was incorrectly sized, resulting in over engagement, which allowed the detents to almost pop our behind the sleeve. Resolved in a german garage after locking up in a petrol station on the autobahn. It happened again on Crete. The german mechanic thought it might, and gave me some spare shims. I spent over half a day into the evening, on a greek building site under a gantry made from scaffolding, taking out the engine and box, stripping the box as per said mechanics instructions, adding another shim, and outting it back together. It was fine then for the trip all over crete, back up through greece, yugo' austria, germany, holland, and home. Bmc swapped it for another. Circa 30k miles later, it was once again howling like a banshi in 1st. I've never regretted putting in a T9.

As Bob says the reverse of that could happen, that prevents the gear fully engaging, and when the grip from the dogs is less, when power is lifted, out walks the gear.
anamnesis

It turns out the transmission shop near me just looked closed. I stopped this afternoon and I learned a lot in a short time from a very helpful guy. He confirmed my 2nd hand 3rd gear is better than the one I've got.

So I did the swap this evening. I'm an expert with the locking ring now, thanks to a small piece of very thin spring steel from an old storm window. A little filing so it fits in between the gear teeth, and holding it in a small pair of vise grips allowed me to get the locking ring started. Withdraw the thin, stiff steel piece and I can just tap the ring into place.

I've ordered the new synchro rings from Moss. I don't have access to a replacement sleeve, so I'm going to try sticking with the one I've got.

I'm not sure I ever felt I had the end float of the shafts confirmed, so I may need more help when I reach that point. I remember trying to follow the Haynes instructions, but I just wasn't certain I had it right. I did get the float on the layshaft within spec.







Mark 1275

Mark
Well done. As Willy confirmed (and I didn't...) the replacement gear is better. Maybe the "new" dog teeth will be good enough to grip the sleeve.
Bill Bretherton

Bill--sorry mate, I'm a bit slow getting back to you.
your question about the synchro drive teeth on the gear--
"why are some dog teeth pointed whereas some are angled one way only? "
I'll try and explain--It's a gearchange baulking fix.---There must have been an issue with the gearlever feeling like it was locking during the 2-3 gearchange
Both 2nd and 3rd gears are constant mesh gears and being so are driven permanently by the input shaft and cluster--3rd gear (by ratio to the cluster)always spins faster than 2nd gear-----so
we rev. out in 2nd gear and go for 3rd
The syncro assembly/slider/sleeve moves towards 3rd and catches the synchro ring teeth, pushes the synchro ring towards the taper/cone on the gear and slows the gear(and cluster and input shaft) down to get equal speed between it and the 3-4 synchro hub which is locked to the output shaft (tailshaft speed)-When they get to equal speed the slider can move over the drive teeth on 3rd gear and the gearchange is complete
However, as the sleeve attempts to move over the gear it won't be perfectly aligned and could still be slipping a bit in the cone---If it is still creeping a bit, then the tapers on the ends of the teeth of the gear and sleeve allow the sleeve to get into the gaps in the gear and slide into place. This is where grating noises come from when the syncros are worn and can't slow the gear enough. The problem is that with a faster than normal gearchange, if the syncro ring is working perfectly, and holds the gear at equal speed to the hub while changing gears and has done this and the teeth are out of line the gearchange can balk/lock because all the teeth on the slider are trying to push into the misaligned gear teeth which the syncro is holding out of alignment-
With all the teeth machined equally the slider comes up against a wall and balks/locks but by having several of the teeth machined deeper back on the ends it means the slider is initially only trying to push against a few teeth instead of all of them and is able to force it's way in and align the teeth to make the gearchange complete--
Racing Ford toploader boxes had a real baulking issue -but in their case the teeth on the syncro ring itself would jam against the slider and the fix for them was to either cut every second tooth off the rings or sharpen the angle up on the end of the teeth so the slider could push over them

enough from me, hope that answers your question
willy
William Revit

Best explanation of how a gearchange happens that I've ever read ! Thanks Willy and keep it coming, learning material like that is worth its weight in gold.
David Smith

Willy
Thanks, that certainly answers my question. Superb explanation!
Bill Bretherton

This thread was discussed between 30/01/2022 and 11/02/2022

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.