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Side-Change Gearbox Stuck in Top Gear

Crusty

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I was out in my BN7 on Saturday and I had to stop at traffic lights. When I went to shift the gear lever into neutral, it wouldn't move and the gearbox has remained stuck fast in top. There were no nasty noises or any signs of trouble before and up to then the gear box shifted perfectly.
I was able to continue home in top gear without any difficulty.
Does anybody have any idea what is the cause and how I can fix it.
I would be grateful for any advice.

Crusty:sorrow:
 
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Re: side change gear box stuck in top gear

Was the box recently rebuilt? There are three (I think) spring-loaded balls and springs that go into the dog(?) gear to hold gears, if one or more is somehow jammed that would cause the problem. I assume you've tried to 'pop' it out with as much force as you felt comfortable with; too much and you could break a shift fork. If you're not able to pop it out I think you have no choice but to remove the side cover and have a look (I think you can do that in situ, no?).
 
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Crusty

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Re: side change gear box stuck in top gear

Hello Bob,
The gear lever is stuck fast. I have used a fair bit of force but I have been afraid if I use too much I might make the problem worse. It was suggested that I rock the car back and forth and this might free it up , but no success there either.

I am not sure what to do now. I could take the side cover off, and you are right , it can be done in situ. My worry here is that I wouldn't have a clue what I am looking for or even at. If something has fallen off or out, where has it gone to. I think I will do as you suggest and see if I can find someone who knows about these old things to make a home visit.

I am dreading removing the gear box from the car though.

Thank you for your advice You have given me a starting point.
 

Keith_M

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Having just rebuilt my sideshift, I can offer a little help. Bob is right that the problem could be with the detente balls that engage the shafts holding the forks. There are three balls that are spring loaded on the side of shafts, one for each shaft. You will see these three balls and their springs when you remove the side cover. There are also two balls between the shafts, one above the center shaft and one below. Those two balls prevent you from moving one shaft while another shaft is in the "engaged" position. It's possible that one of those balls has gotten jammed somehow so that you can't move the 3rd and 4th gear selector fork shaft. You should be able to access the three spring loaded balls by removing the side cover, but I think you will have to remove the whole tranny to get at the two balls between the shafts because you have to remove the forks and shafts to get to them.

If the problem is not those detente balls, then it could be something more serious with the 3rd and 4th gear shaft or the sliding hub that selects one of those two gears.

Good luck,
Keith
 
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I think the first thing to try would be to drain the oil into a clean pan and see if any metal comes out. You will have to do that to remove the side cover anyway, and if chunks of metal come out you'll know you have to pull the gearbox no matter what.
 
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Crusty

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Thank you both for your comments. I will see how I get on this weekend and report back.
I expect most of the Forum is thinking Thank God it's not me!!!!
 

Jack T

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Thank you both for your comments. I will see how I get on this weekend and report back.
I expect most of the Forum is thinking Thank God it's not me!!!!

So how did you make out? My bet is that something happened at the synchro hub, preventing the engagement sleeve from returning to neutral.
 
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Re: side change gear box stuck in top gear

Was the box recently rebuilt? There are three (I think) spring-loaded balls and springs that go into the dog(?) gear to hold gears, if one or more is somehow jammed that would cause the problem. I assume you've tried to 'pop' it out with as much force as you felt comfortable with; too much and you could break a shift fork. If you're not able to pop it out I think you have no choice but to remove the side cover and have a look (I think you can do that in situ, no?).
DING! DING! DING!

That's where my money says the problem is (from the guy that drove from just south of Atlanta to just north of Tampa, using only 4th & 4th/ovd).

If that is (was) the case, then the xmsn has to come out and be disassembled to rectify.

The cause__again, if that turns out to be the issue__is that too much clearance existed between the 3rd-4th synchro hub and the 3rd (constant mesh) gear. This allowed the inner hub portion to slide too far rearward as the engagement collar traveled too far forward, releasing the spring-loaded balls. It is the spring(s) sticking up that prevented the collar from being returned to its neutral or 3rd-gear position.

IMG_7294.jpg


IMG_7305.jpg


With only three (3) posts, it is unlikely that we'll ever find out :(
 

Jack T

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Re: side change gear box stuck in top gear

DING! DING! DING!

That's where my money says the problem is (from the guy that drove from just south of Atlanta to just north of Tampa, using only 4th & 4th/ovd).

If that is (was) the case, then the xmsn has to come out and be disassembled to rectify.

The cause__again, if that turns out to be the issue__is that too much clearance existed between the 3rd-4th synchro hub and the 3rd (constant mesh) gear. This allowed the inner hub portion to slide too far rearward as the engagement collar traveled too far forward, releasing the spring-loaded balls. It is the spring(s) sticking up that prevented the collar from being returned to its neutral or 3rd-gear position.

IMG_7294.jpg


IMG_7305.jpg


With only three (3) posts, it is unlikely that we'll ever find out :(


Yeah, was rebuilding my box and that happened to me, fortunately while it was still on the bench. Without the bell housing attached the input shaft pushed out far enough that the balls came out too far. Bad enough I had to take it all apart again, but glad it wasn't in the car.
 
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Crusty

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Re: side change gear box stuck in top gear

I haven't been able to anything about yet. I was away for a few days in mid March in Melbourne for the ( cancelled) F1 meeting. I had to go into self isolation because of the coronavirus for two weeks when I came back. Now we are in lockdown for at least four weeks which is more self isolation. I dont think I can do this on my own and while the lockdown continues, access to any assistance from friends is not possible.

Crusty
 
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Crusty

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Re: side change gear box stuck in top gear

The gearbox behaved faultlessly with no indication of any trouble or nasty grinding sounds coming from the gearbox. Just a stubborn refusal to shift out of 4Th gear.
I am dreading to think where the ball (or balls) have gone to and if they have damaged any of the gears.

crusty
 

pdplot

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Re: side change gear box stuck in top gear

FWIW, in my 60 years of experience fooling with cars, 9 out of 10 times it's something simple and stupid. That 10th time will cost ya.
 

Lawk

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Re: side change gear box stuck in top gear

The gearbox behaved faultlessly with no indication of any trouble or nasty grinding sounds coming from the gearbox. Just a stubborn refusal to shift out of 4Th gear.
I am dreading to think where the ball (or balls) have gone to and if they have damaged any of the gears.

crusty
Hi Crusty,

Did you determine the problem. I just returned from my first drive of the season with the gearbox of my BN7 stuck in 4th gear. Is at an extension of the Covid 19 virus or just that the cars are getting old.

Hopefully it is just an issue with the selecter rods and the interlock there otherwise its pull out the gearbox on a freshly painted car.

Ken
 

Lawk

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Re: side change gear box stuck in top gear

An update on my previous post to this thread.

After driving our BN7 for 30 years I decided it was time for a repaint this winter. Saturday we took our freshly repainted BN7 out for it's inagural drive and half way home the gearbox got stuck in fourth gear.


A quick google search pointed to the balls and springs on the 3rd/4th gear synchro hub escaping as the synchro hub travelled too far forward.

Yes, time to remove the interior, the newly painted doors and the gearbox. Fifty years ago I could do this in a couple of hours on my 100/4, now it takes about 5 hours.

Disassembling the gearbox confirmed the suspected issue, which was very inexpensive to fix, but how do I ensure that it never happens again particularly since the gearbox and I are getting older, the gearbox with looser joints and me with stiffer joints.

The solution was very easy. Put a positive stop on the 3/4th gear selector rod to limit its forward postion when shifting into 4th and not just rely on the spring and ball detent. .

I accomplished the by simply putting a spacer tube over the selector rod to create a positive stop should the I shift to aggressively for the spring and ball detent. For my gearbox this tube was 1 7/16 long leaving about 0.020" clearance from the spring detent position.Visible in the attached photo

If rebuilding your gearbox you may want to add such a positive stop.

Ken
 

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There is a small, oval cavity on the rear side of the bell housing that the selector rods extend into. There should be in a pad in the cavity to prevent too much forward travel of all three selector rods; this pad is N/A at the usual suspects and may be missing in some cars causing these 'locked-in' conditions. It's easy enough to fashion the pad out of leather or rubber.

Item#2 in this diagram: https://mossmotors.com/external-gearbox-center-shift
 

Jack T

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That's the same principle as on the side shift. Before I installed the bell housing I measured how far the shift rod should extend when in 4th and the depth of the recess to make sure the rod could not travel too far. I used heavy gasket material as a buffer so there is no metal to metal contact.
 

Jack T

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While we're on the subject of shift rods, can someone explain why the reverse rod gets deformed on a side shift? Three boxes I took apart all had similar damage to the top of the rod where it would be inside the case when reverse is engaged. I had to touch up a couple of them with a Dremel to get them out.
 
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