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MG Midget and Sprite Technical - Pops out of third gear

Rebuilt the rib case gearbox in my 1971 midget last winter. I think it actually went pretty well, but I’m an amateur.

Replaced the layshaft, the mainshaft bearings and the needle bearings. Did not disassemble the mainshaft, and the laygear looked okay.

Before the rebuild, the car would pop out of third gear going up a very steep hill, and it would happen two or maybe three times per year. After the rebuild, it wants to pop out of third gear every drive.

I’m in the planning stage to try to address the problem this winter, which just happened to come early here in New England (it was all of 2°F last night). I’d appreciate some advice about what I should be looking at.

I think the only part of the reassembly I might have gotten wrong would be the shims to set the end float. Could that be a factor? I don’t think I could have left out a ball bearing or a spring when reassembling the selector rods, but I’ll double check. If third gear just needs a little more persuasion to stay in place, would a ball bearing a thousandth or two larger be a solution? If there’s a spring or two that help keep the selector rod in place, might new ones make a difference? Am I on the wrong track to think persuading the selector rod to stay in place is the solution?

Reading the archives here, it sounds like the issue might just be overall wear. I realize the benefits of a five speed conversion, but it’s not the path I’m on. There are 114,000 miles on the car, but comments here about the photos I posted showing the condition of the gears were encouraging.

Thank you for any advice.
Mark 1275

I would first check the rubber gaiter at the gearstick is properly mounted and that the carpet isn't putting pressure on the stick / gaiter to push it back into neutral
Dominic Clancy

Mark, this can happen if the 3rd gear selector fork doesn't move the gear hub far enough to fully engage. Check that the pointed end of the selector fork locking screw is correctly locating tightly into the dimple on the shaft.

Also, did you check the inner faces of the selector fork for wear? If there is too much wear on the inner face then again, the fork will not move the hub as far into its correct engaged position as it should and it will jump out of gear when the load is applied to the gear wheel.
GuyW

The other reason could be wear on the thrust washers which locate the gear on the mainshaft. They can wear allowing the gear to move on the shaft therefore not allowing the gear to fully select.

Also check all the detent springs are in place and the correct tension.
Bob Beaumont

Thank you for the replies. This tells me what to look for. I hope once the gearbox is on the bench the problem will be clear. But I'm sure I'll be back here.

If the fork is worn, is there a source? I see Moss offers the 1st/2nd, but not the 3rd/4th fork. Is it a part that often needs replacement?

I hope I don't have to disassemble the main shaft. That looks a bit beyond me.

Much appreciate the ideas.

Mark 1275

Mark, I most likely have a 3/4 fork. I have way more gboxes and parts that I will ever use. I am in the northwest corner of CT. Don't hesitate to tackle the mainshaft. The only difficult part of this g box is getting the springs and balls back together. Let me know if I can help. John
J Bubela

Thank you, John. Good to know you're close by. I just sent an email that I hope gets to you.
Mark 1275

Just an update, but the gearbox is on the bench and the selector rods came out this afternoon. All the plungers, balls and springs were in place. I was wondering if I would find that I'd missed one because the popping out of gear happens so readily, but it's not going to be that easy.

Before removing the rear extension I spent a lot of time observing through the side cover how it shifts, and again didn't find anything obvious. It appears to my untrained eye that the fork moves the gear all the way into third as well as fourth. The fork locking screw was definitely correctly located and tight. There was maybe 0.020" of "wiggle" on the selector fork, which seemed pretty good to me.

It's probably not meaningful, but I did notice two things about the spring and plunger held in by the side panel. There's a detente in the selector rod where the plunger engages when you're in third gear. The detente actually moves a little past the hole, so I'm pretty confident the fork is moving enough to engage third. I have no problem at all with fourth gear, but the detente for fourth also moves past the hole. I'm probably not explaining it well, but it makes me feel there's nothing interfering with the motion of the stick. I'll post a picture of the inside of the selector fork, but this also makes me think it's not worn out.

The second thing I noticed is there's a slight difference in the lengths of the two plungers and springs that are held in by the side cover. I'm sure I didn't keep track of them during my rebuild last winter, and maybe putting the longer of the two into the third/fourth selector rod hole will help a little bit. I'll buy new springs from Moss and see if they're any stiffer or longer.

It will be a process, and my observations up to this point are:
-I haven't found anything obvious.
-I might need to dig deeper than I'd like. I was hoping not to have to remove the main and output shafts again.
-But it won't do any good if I don't find something to change. Just reassembling will leave me with the same problem.
-And even though I don't have a specific question, I feel better posting here.

Thanks for your interest.



Mark 1275

The shorter spring also looks to be bent. I shouldn’t think that helps.
Dave O'Neill 2

I agree, Dave! Glad it's a part that's still available from Moss. But so much of the spring protrudes when you take the side cover off, unless for some reason it's hanging up because it's so distorted, I'm thinking it still pushed the plunger into position. But will definitely get two new ones and see how they compare. I wish I knew if the bent spring was in the third/fourth hole, or the first/second, but I don't.

Here's the inside of the selector fork. Don't have anything to compare it to, and it's hard to capture the wear in a photo. If I can get a better photo, I'll post it.



Mark 1275

I do have a new selector fork, but can’t remember if it’s 1-2 or 3-4. If it’s the latter, I will measure it.
Dave O'Neill 2

I am certainly no expert on these gearboxes but wonder if you are looking in the wrong place for the fault. The detente balls and springs should hold the selector rod and therefore the selector fork in the right positions. This makes sure the selector fork takes up it's correct position and then prevents it from moving with the cars vibration.

The fork and selector hub must have some clearance as the hub spins. Surely it is the 3 small balls and springs inside the hub which actually hold it engaged and resist the axial pressure caused by the helical gear thrust. I would check that they are all present and also check the end float on the main and lay shafts.
GuyW

Also check the 'teeth' on the gear and the selector hub. They should not be rounded off or worn excessively. 2nd and 3rd gear get a hard time so wear is quite prevalent on these gears. As a start compare them to the input shaft gear. This tends to wear slightly less. Also the thrust washers (semi circular) can wear allowing the gear to float on the main shaft. I usually replace these anyway. It means stripping the box a bit more!
Bob Beaumont

Guy,

Those balls don't see any thrust from the helical gears, that is dealt with solely by the input shaft and mainshaft bearings and the laygear thrust washers.
David Billington

I have a simialr issue with my 79 MGB in that the it pops out of third into neutral on a deceleration. Iv'e been told it is more than likely a worn out synchro.If I hold the gearshift in third when I decelerate, there is no problem.

The car has 132,000 miles on it, so the worn synchro is a plausible diagnosis.

Curious to know the outcome of your case.

Cheers

Gary
79 MGB
gary hansen

Gary,
an over tight rubber gear lever gaiter can cause or exasperate the problem, so if you've put a new one on recently then try having it loose or take it off to see if it makes any difference.
Nigel Atkins

ETA: (lost from last post)

Regular g/box oil changes can help the g/box generally, especially with engine oil in the g/box, every two years or 24,000 miles by the book, helps too to see the condition of the oil coming out and if there are any bits of metal in it.
Nigel Atkins

Very glad I posted. Much appreciate the responses.

I took another photo of the fork and some measurements, but I left out some important dimensions, so I'll post that later.

The idea that I may be missing balls and springs under the sleeve sounds like my next objective. When I had the gearbox apart last winter, that sleeve/selector probably came right off. If the balls and springs could escape at that point, it might have happened. It's odd it would happen without me noticing something, but it's what I'll look into next.

Just curious, is it possible the lack of those balls and springs would affect only third gear and not fourth?

It worries me that I'm perfectly fine in fourth, and have a real problem in third. I'm curious if this picture showing some of the gear teeth sheds any light on their condition.

I hope it's not a synchro because I think that gets over my head. Do they need to be pressed on and machined for a 1275? Does a synchro problem fit the description of a third gear-only problem?

I might be deluding myself, but the change in the popping out of gear was so dramatic, my hope is that it isn't something that wore very quickly, instead it's something I did wrong in the reassembly.

I'll be back in the garage this evening and hopefully get the mainshaft out.



Mark 1275

Mark, David B corrected me about those 3 balls not taking load from the helical gear mesh and I bow to his superior experience. I would still check the end float on the main and lay shafts though, as I suggested.

The other thing that occurs to me is that when out of the car it may fully engage the third gear. But if there is wear in the gear lever pivot point or if it has somehow been assembled wrongly maybe then in use it isn't moving the required amount forwards to properly engage 3rd gear so that it kicks out again. That could also explain why the fault is only with that gear.
GuyW

Made some progress this afternoon. Not exactly what I was hoping for but I'm starting to understand more about your responses.

First, I did measure the bronze selector fork and the dimensions on my note relate to the surfaces that touch the sleeve/selector.

I also removed the mainshaft. I was hoping to discover evidence one or more of the springs or balls in the synchro assembly wasn't there, but now I'm convinced they are. The springs are clearly visible. When I move the outer part (sleeve?), it locks firmly into position about a third of the way off the gear within the sleeve. Same in both directions. It takes so much effort to move the sleeve in either direction, I'm now sure I didn't move it during the disassembly last winter. I'd remember it.

I'll likely post more pictures of what I see with the mainshaft out, but I'm convinced the synchro rings on either side of the 3rd/4th synchro assembly are pretty much idiot-proof, so I couldn't have installed them improperly.

I do change the gearbox oil regularly. I only drive about 1300 miles per year (regrettably) and change it every year or two. I definitely did find lots of metal bits as the the laygear and it's bearings were disintegrating, but don't remember any metal bits this year.

I'll recheck the end float of the laygear now the mainshaft is out, but with it in place I could see very little movement. IIRC, it was maybe 0.003" or 0.004" when I put it back together last winter.

Should I be checking end float on the mainshaft? If so, not sure where to measure. I see there are thrust washers, but also that they're not available from Moss. (Numbers 20 and 30 in the illustration in my catalog.)

Thank you for the ideas. I'm still hoping to find something. By the way, in my situation it never pops out of third when decelerating, only accelerating.



Mark 1275

Mark,

Looking at the picture of the innards of the gearbox, it looks like second and third aren't centered on the lay gear meaning that the main shaft is too far back in the box which means that the second/third slider has room to float thus popping out of third. Did you have the rear housing when you took the picture (which might explain why the main shaft is back that far)? But that's just my theory.
Martin

I see what you mean, Martin!

I'll post two pictures, the first is a crop of the one where you observe the misalignment.

The second is from a year ago.


Mark 1275

And this picture is from disassembly a year ago.

When I toggle back and forth, it looks out of line back then as well.

I didn't have anything apart that would disturb the front to back alignment of the laygear for either photo.

Now you've made me curious. I did have the problem in a very small way going back many years. It would pop out maybe a couple of times per year, and only up two particular very steep New England hills.

The laygear's front thrust washer is the same thickness from Moss, and the rear thrust washer is ordered to the right size. But... one of the main bearings from Moss came with washers to make up for the difference between the original and the replacement.

It's something I'll definitely check when the mainshaft goes back in.



Mark 1275

Not having seen the spacers, I'd say that they would go onto the main shaft before you install the bearing thus moving the main shaft forward a bit. Probably a bit of trial and error to get it right which can be a pain.

Martin

Martin

Did you check the condition of the engagement 'teeth' around the 3rd gear and selector/synchro assembly??
Its unlikely end float on the layshaft will be the cause of jumping out.
The original rear mainshaft bearing is no longer available. the one supplied is narrower hence the spacers.the spacers go in front of the bearing ie between the carrier and the gear.
There is a 6 thou shim between the rear extension and the main housing to take up the thickness of the gasket. the stops the bearing floating back and allowing more end float on the mainshaft This can result in jamming in bottom gear.
Bob Beaumont

Bod is the only person who has mentioned the dog teeth on the speed gears, it is in fact the shape of these teeth, and the shape of the coffin shaped teeth inside the synchro hub outer sliding member, that locks the gear and hub together and stop it jumping out of gear, not the 3 balls and springs in the hub. loads of gearboxes don't use this method of construction.( no spring loaded balls in the hubs)
Andy Tilney

Indeed.

Previous history of crunching when selecting a gear - due to worn synchro rings, normally - will damage the dog teeth, which will eventually lead to jumping out of gear.
Dave O'Neill 2

That's right Dave that's exactly what happens,
Andy Tilney

Hope everyone had a good Christmas. I'm lucky enough to have three young granddaughters, and the youngest still wonders how Santa got into their garage if he doesn't know the code.

Very much appreciate the ongoing guidance. Feels like I'm understanding more and will hopefully get to a solution.

I'll post three pictures. This first one is the teeth inside the synchro hub that engage with the small teeth on third gear.

How do they look?

I thought they would look a lot different from the teeth that engage with fourth gear, but to my untrained eye I didn't see much. Maybe I will when I compare enlargements of the two pictures.

I understand this is a part I can't buy from Moss, so I'm hoping these fall in the category of good enough. If better pictures are needed I can try.



Mark 1275

This picture shows the small teeth on third gear. These are the dog teeth?

Maybe I need to provide pictures that show more of the teeth around the circumference.

This is a part I can still buy from Moss. Not cheap, but it sure would be great if it's a part I'm capable of replacing, and it's the source of the problem.


Mark 1275

And this last picture is one of the synchro rings.

These are both easy and affordable to replace. It would make my day to learn these are the source of the problem.

I've owned the car since 2001 with 88k miles on it. I'd say until this past summer I never really ground the gears going into third, but this year it happened a few times. And when it pops out of third gear it doesn't sound good.

I'd sure appreciate input on the condition of these parts.

Thank you.


Mark 1275

Mark
I'm wondering if the needle roller bearing between input shaft (first motion) and mainshaft (third motion) is worn. This bearing has a hard life and readily wears, especially if oil gets low. Also, the journal it runs on (on end of mainshaft) wears. This combined wear would mostly affect third gear due to its location (fourth is not affected as the mainshaft is locked to the input shaft in fourth).

I'm guessing, but this wear might cause the mainshaft to move around axially whilst in third gear especially and, combined with worn dog teeth and synchro hub teeth as Bob mentions, might cause your jumping out of gear whilst accelerating problem.

You really need to remove the mainshaft, third gear and synchro hub to inspect these parts.
Bill Bretherton

From the photos the teeth look about right. There is a good bevel on them and they don't look worn. The image of the synchro ring is a bit dark to make out the details but I assume the teeth on it are also in good shape.

As Bill says the needle roller bearing in the end of the 3rd Motion Shaft gets a hard life so its always worth replacing in any rebuild although I have not found it the cause of jumping out. The synchro ring only affects selection. When placed on the gear cone, there should be at least 2/3mm between the bottom of the ring and the gear. The repro rings are generally of inferior quality so its best to keep the originals if possible.

It suggests to me that the problem lies elsewhere. I mentioned the gear should not have any end float on the main shaft. there was also some discussion about the location of the main shaft in the casing. Are all the detent balls and plungers present and correct?
Bob Beaumont

Just a wild card - when I had a similar problem with my 68 Mk111, I took it to a specialist; a guy who does nothing but classic car gear boxes and majors on all "A-series"
While he was re-building the box, he gave me a new spigot bush to fit. There was no trouble getting the old one out as there wasn't one there !
M J Chapman

I doubt a missing spigot bush would cause the problem although it would cause more axial play between input and mainshaft near to where third gear is. I already referred to excess play here caused by a worn needle roller bearing but, as Bob suggests, that probably wouldn't cause the problem. The dog teeth and synchro hub look ok so it's hard to suggest the cause. I doubt the ballbearings alone would cause it. My understanding concurs with what Andy says i.e. it is the synchro hub and the dog teeth "locking" together that keep the gear engaged (assuming they are aligned correctly). Any wear of these might lead to premature disengagement. In the absence of significant wear maybe there is misalignment.
Bill Bretherton

Much appreciate all the ideas. I hope to get out to the garage this weekend.

I'm disappointed replacing the synchros won't make it all right. I'll post another picture or two just in case it's possible even a lesser quality new pair would be an improvement.

I should mention the needle bearing between the input shaft and mainshaft was replaced just this past winter. The spigot bush was replaced in 2002, but has only about 24k miles.

It's encouraging that the gear teeth look okay. I realize I need to understand better how things mesh in third gear, and why the heck the sleeve/synchro doesn't stay where it should.

Dave, you weren't able to find a 3rd/4th selector fork to measure, did you?

Thanks for sticking with me. I hope to find the smoking gun yet.
Mark 1275

Mark, I don't have an answer to this but the misalignment of the layshaft and mainshaft gears was mentioned earlier, but no explanation given. Looking at your two photos, in both, the layshaft gears are well forward compared to the gears it is meshing with. And it appears that this has not always been so as one can see the light catching on the tell-tale marks on the mainshaft teeth where the mesh was more central on the teeth. Whatever the reason, if the whole of the mainshaft is too far back in the casing, then when third gear is selected the syncro hub won't be pulled fully into position to fully select the gear and it is likely to kick out under load.

Are you sure that the bearing carrier at the back of the mainshaft is fully seated in the casing?
GuyW

Mark

I’ve had a rummage and all I can find is a 1st/2nd fork. Just on the off chance that the inner dimensions are the same, I measured it at .455” at the two ends, which are the narrowest points.
Dave O'Neill 2

Mark, Looking at your pictures of the gearbox innards again, and reading Guy's last post together with Martin's earlier post, it does indeed appear that the mainshaft is too far back. The laygear can't move very far forwards and backwards (assuming the thrust washers are installed) so the misalignment can only be due to the mainshaft being back a little, maybe due to the bearing housing not being fully seated.

I attach a picture of my gearbox innards whilst rebuilding three or four years ago. Although the forks are installed you can see third gear correctly aligning with its laygear mating gear. The gaps either side of the third/ fourth synchro sleeve don't look too much different to yours but different camera angles could account for that. Although, if the mainshaft was too far back it would mean the first/ second sleeve (with selector fork installed) was closer to second gear and might affect second gear selection. Your third gear dog teeth and synchro sleeve teeth look good to me (better than mine)so one wouldn't expect those to cause dis-engagement. I'm no expert and haven't yet installed my gearbox but I hope this helps a bit more.



Bill Bretherton

I think the bearing housing is held in place by the rear extension, so if it wasn’t seated properly, the rear extension wouldn’t mate with the back of the gearbox casing.

As Martin said, it could be to do with the bearing spacers.
Dave O'Neill 2

Mark, compare Bill's photo of the 3/4 selector hub with yours (displayed here)
The red line is the hub selector collar, the blue is the space between this and the 4th gear dog teeth on the input shaft - this is much wider than on Bill's gearbox and really should be minimal, as on the other side of the hub. This is more evidence that the mainshaft is too far back.
Although your photo shows the synchro hub flush up to the gear 3 dog teeth, this isn't the position it would take up when located by the selector fork.

Did you mention that the needle roller bearing on the front of the mainshaft had broken up? maybe there is a rogue needle trapped inside the end of the input shaft, preventing the mainshaft from fitting properly?



GuyW

There should be 2 spacers in front of the rear main bearing. ie between the lst gear and the bearing.
Bob Beaumont

I didn't have much time in the garage today, but hopefully will tomorrow.

The pictures are a great help and I much appreciate them. I'm going to do a few more checks based on advice here, and then reassemble to see if things line up as they should.

I'm confident the rear main bearing housing was dead flush with the gearbox casing. But I also remember the shims for the two main bearings was a part of the process where I could have gotten it wrong. I don't think I had to buy any additional shims from Moss, and I thought it was odd the depth I measured at that step in the process was covered by the shims I had on hand. I don't remember more details, but maybe it will come to me.

If when reassembled the alignment isn't dead on like Bill's picture, I'll know I have something off.

I did have the chance to measure the end float of the laygear. I can definitely get a 0.005" feeler in between the rear thrust washer and the laygear, and definitely can't fit a 0.006" feeler. So I might be off a thousandth from the spec of 0.003" - 0.005", but my washer is 0.121"-0.122", and the thinnest Moss offers, 0.123" to 0.124" isn't presently available. That may well have been the case when I was reassembling, so I used my old rear washer.

I read and re-read the Haynes instructions about disassembling the mainshaft. I think I could probably do it, but so few parts are available for purchase I think I'm going to trust that what I've got is good enough, and maybe the alignment will point me toward the problem.

The needle roller bearings I found broken up last winter were in the laygear. The one between the input and main shafts was intact, but I did replace it, and I verified there's nothing in where the mainshaft fits in.

I think I saw something, maybe here or maybe in Haynes, about there being 0.001" - 0.002" float between the main and input shafts. I don't remember measuring that during reassembly. I hope when I put it together tomorrow I'll be able to figure out how to check that. Is there a method I should know?

I'll check for the two spacers before I reassemble. I expect I might need to put things together and take them apart a couple more times. The manual says to tap the inside of the mainshaft bearing housing at opposing points to get it started during removal. I haven't been successful with that, so following advice I heard here last winter I heat the heck out of the casing with a heat gun, hold the main shaft and hit the casing with a rubber mallet.
Mark 1275

Please correct me if I'm wrong but I think it's possible to locate the bearing incorrectly on the mainshaft i.e. there is no stop. So it would be quite possible for the mainshaft to be too far back even with the bearing housing correctly located in the casing. This must surely be what happened.
Bill Bretherton

If you have the gearstick of the smoothcase gearbox, you will have a similar problem. It is just a little shorter at the end, which causes the gears not to engage properly.
J.W. Vlaanderen

I've just realised, on engaging brain, that an absence of spacers would cause the situation I described above.
Bill Bretherton

Yes thats the way I remove the rear bearing carrier.
The end float on the laygear is ok if slightly over.
Th rear bearing carrier fits flush to the casing otherwise the rear extension will not fit.
Bob Beaumont

When I asked if the bearing carrier was fully seated in the casing, l meant for the purposes of the photo. Obviously it would be fully seated once the rear extension was bolted up but as the gear selector rods weren't fitted l wondered if the rear extension wasn't either, in which case the displaced main shaft could have been a red herring!
GuyW

I need to take some time to understand what it means, but tonight I found three shims on the back of the mainshaft bearing (between the bearing and the spacer tube) totaling 0.060". And one shim between the front of the bearing and the first gear.

I think there was one 0.042" shim (or washer) both front and back, and two thin shims in the back.

Wish I'd taken a photo of the instructions that came with the Moss bearing for the mainshaft.

I need to do some reassembly and see how things line up. Hope to get to it before Tuesday evening.

Thank you for the continuing input.
Mark 1275

Mark

Moss Motors website wasn’t much help, but Moss Europe have the fitting instructions online.

https://www.moss-europe.co.uk/media/pdf/22A465_instructions.pdf
Dave O'Neill 2

The Moss instructions are a little odd in that any shims on the side facing the spacer tube have no real effect on the position of the mainshaft in the box as the bearing is located by the circlip.
The difference to be made up by the different thickness of the original and new bearing is approx 0.95 thou. That suggests 0.47 thou either side of the new bearing. When assembling mine I fitted a 0.08 and a 0.40 shim to the first gear side and didn't bother with the circlip side.
Bob Beaumont

The spacer behind the bearing will have an effect on the position of the speedometer drive gear.
Dave O'Neill 2

Unfortunately, Dave, the Moss supplied instructions don't really make sense! I got the same kit for my gearbox rebuild. Their diagram shows 0.055" of shims on the rear face of the bearing. Why? Adding shims there only serves to add to the length of the spacer tube and mean that the nut tightens that bit sooner on its thread.

The instructions state "spacers are needed to position the bearing in the gearbox" This isn't really true as the bearing position is determined by the outer circlip position clamped between the carrier and rear extension. The shims are needed to hold the main shaft in the correct position relative to the bearing. Adding shims behind the bearing makes no difference to this at all.

The critical thing is that shims are added in front of the bearing to make up for the shortfall in the dimension between the forward face of the circlip and the forward face of he inner race of the bearing.
GuyW

Guy

According to the Moss instructions, the new bearing inner race is .095” narrower than the old one. Assuming that you only need .040” in front of the bearing to line up the gears, then the .055” shim behind the bearing is just positioning the spacer tube, speedo drive gear and nut where they should be.

It may well be that moving the speedo drive forward by .055” could cause a problem, or the nut may be running out of thread on the mainshaft before being fully tight.

Of course, it could well be that omitting the .055” shim would not cause any issues at all, but why risk it.
Dave O'Neill 2

Dave, I think that is what I was saying. The critical thing is that the mainshaft is shimmed far enough forwards relative to the bearing, which has a fixed position in the casing. The thinner bearing therefore needs shimmng on the forward facing side. On the rear face, the additional shims are less critical. They will determin the speedo drive gear position though as its a worm type gear then meshing with the speedometer cable gear is pretty well automatic, within reason.

When I did mine I did in fact put the surplus shims on the rear, but as much as anything as storage as I had no other use for them!
GuyW

Once again, very glad I posted. Thank you, Dave, that's just what I needed! Even though it didn't make much sense to me, it looks like I followed the Moss instructions.

Here's the interesting part: I had a problem with the speedometer in the spring. I did end up cutting out about 3/8" from the middle of the cable sheath, and it solved the problem. I thought it was just worn parts, but now I suspect the shims had something to do with it.

My plan is to reassemble the gearbox as it was, and see how things line up, particularly between third gear and the laygear.

I'm surprised and appreciate that such detailed advice is available here.
Mark 1275

As I said in my post the critical shims are on the gear side of the bearing. I didn't bother with the spacer side. I did 2000 plus miles to Switzerland and back in the summer with that box with no problems at all. The bearings were supplied by Peter May as they were better quality than the Moss ones. Only two shims are supplied in any event
Bob Beaumont

I was able to get things back together this afternoon, and I have a new theory about the alignment of the third gear with the laygear.

I've been taking the pictures with an iPad, and because the camera lens is so far off center compared to an iPhone, I think I was just taking the pictures at quite an angle.

This afternoon I lined up the camera better, and it looks like things align pretty well.


Mark 1275

Better, but there is still rather more of a gap to the right of the 3/4 the gear selector hub than to the left of it.
GuyW

This may be an optical illusion but the input shaft alignment with the laygear looks like it may be a bit forward. That could account for the gap that Guy is referring to.
Martin

Is the front cover fitted?
Dave O'Neill 2

I'm following this thread with interest as my experience with the spridget box wasn't good when I got my frogeye on the road. I bought a BL silver seal box newly rebuilt, no miles on it, when I got it on the road in 1985 and the 1st couldn't be fitted as the mainshaft wasn't central in the extension housing, the replacement 2nd failed after a few months and was replaced under warrantee, inspection showed likely poor heat treatment of the failed part. The 3rd gbox I was never happy with but it worked but over the course of about 2 years it started to jump out of 1st, 2nd , and 3rd at the slightest lift off the throttle which meant having to hold the gearlever in place in traffic. I replaced it with a Ford 4 speed box which was totally reliable and had 1st synchro. I haven't been able to come up with a good reason for the poor operation and jumping out of gear but wonder if the gearlever might have been the wrong one leading to poor engagement and premature wear on the dog teeth resulting it being thrown out of gear. No way to tell as the gearbox is long gone now. I did have it jam in 1st once but got it out with considerable force while driving it, what causes jamming in gear? I know it has been mentioned but can't recall the reason . I know from experience that the A series box in good condition is sweet and a delight to use I just haven't run across many. I gave a friend one from an Austen A40 farina I scrapped for her Sprite and it was brilliant. Ford gboxes were common and always seemed good. A guy I used to know, now long gone, said about Ford and BMC, Ford good gearboxes poor engines, BMC good engines poor gearboxes.
David Billington

I once had my standard gearbox lock in first. Can't remember if it was a smooth case or ribbed since this was many years ago. I couldn't get back into neutral so I drove it home that way, pulled the engine/gearbox and tore the gearbox apart. For some reason, first gear had slid back so far that the detent balls had almost come out of their little holes. I got them back in but I don't remember if I did anything else to help eliminate the problem.
Martin

Front cover is removed.
Mark 1275

If the cover is removed the shaft may have slid forward a bit. Push it into the box and see if it moves any. If it does then the gap that Guy mentioned should decrease.

When you reassembled the main shaft, did you change anything like the order of the spacers?
Martin

I had mine jam in first, just driving back from scrutineering at Le Mans. I tried pulling the stick back while driving slowly around the paddock, but it wouldn’t come out.

Fortunately, it is possible to remove the side cover with the ‘box in the car and get it out of gear.

Once out on track, I found that it had started crunching into second gear. When stuck in first, the first gear outer travels too far backwards and the inner hub too far forwards which does allow the three balls to protrude, but it also jams on the interlock plunger inside the 1st/2nd hub, which pushes the hub and synchro ring against the second speed gear, causing wear.

After the race, the newly built engine had to come out, as the new K*nt cam was rubbish, so I pulled the gearbox as well, in order to sort the jamming problem and replace second gear synchro ring. Despite this, it continued to crunch into second, so the gear itself had to be replaced eventually.
Dave O'Neill 2

When I put the main shaft back together, I used only the one 0.040" shim between the bearing and the gear, so no change.

Hope to get back out to the garage a bit tomorrow.
Mark 1275

The 0.040 shim may not be enough. Remember the difference to make up is 0.047. May be better to use the 0.040 and the 0.010
Bob Beaumont

Well, I much appreciate all the responses so far. I'm going to be off line for about two weeks. When I'm back up, I'll focus on the alignments and hopefully find the cause of the problem.

Thank you, and Happy New Year.
Mark 1275

This thread was discussed between 23/11/2018 and 01/01/2019

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