Aaaargh - do I need a gearbox rebuild? V50 Monza


I am currently trying to resurrect a 1981 V50 Monza. I bought the bike with no electrics installed but with a little help from various folk in the MG community it is now running.

There is still a lot of work to do but I am trying to adopt a progressive approach rather then trying to do it all at once.

When I bought the “bike” it clearly wasn’t running and the gearchange linkage had been detached. I assembled the linkage because I was tired of tripping over it but with a dead engine I could not select any gears. I thought this may be due to drag in the gearbox or clutch and that with the engine running it would be persuaded into gear. Turning the rear wheel while trying to engage a gear didn’t work.

Along with the bike there was a small pile of spares including a gearbox missing its clutch actuating shaft. Looking at the fasteners on my current gearbox I suspect someone has been in there post factory.

The bike is now running but I can’t select any gears [bike on paddock stand]. I have checked the linkage and its correct and not fouling on any part of the frame or gearbox. The gear pedal moves both up and down. When trying to engage first gear the lever moves a few degrees and then comes up to a hard stop as if movement was stopped by a physical impediment. When lifting the lever up [as if trying to select second gear from neutral] their feels to be more movement as if something is trying to shift in the gearbox but again the gear doesn’t engage. Directly operating the gearbox selector lever produces the same [lack of] results so I don’t believe the linkage to be at fault.

The clutch feels fine - the cable is free and the bar and gearbox actuating levers move freely with what feels like the right level of resistance.

I have just SORNED the bike and noted that it had not been taxed since 1990 so likely not been on the road since then.

Before I go down the road of yanking the gearbox out and taking it to a specialist I have a couple of queries:

I have read that the dry clutch plates can stick together after standing for some time. I don’t believe this is my problem as the clutch feels fine at the lever and when the bike is started the rear wheel rotates slowly due to drag through the gear box on cold oil but when the clutch lever is pulled in it stops rotating. If the clutch was stuck together would I still be able to engage gears on a dead engine?

My other query concerns the detente mechanism on the gear selector - if this was incorrectly adjusted could it internally impede the gear selector?

I suspect that a previous owner with more enthusiasm than skill has been into the gearbox and incorrectly reassembled it but I am keen to eliminate all other alternatives before committing to pulling the bike to bits.

Any help or advice would be much appreciated!



Well, it looks like there’s little you can do now. Clutch seems to work well and I’d focus on the gearbox gear selector mechanism.
First, you got air breather at the top of the gearbox, as you unscrew it, there’s long spring underneath, holding gearchange shaft in correct position. Very little space, but you can undo battery tray and try to see(feel) if your efforts to change gear move the shaft a bit, or not at all. Just remember, spring sits inside long blank tiny tube, which doesn’t need to be removed - it’ll move up and down if shaft rotates.
Here is the pic with two different shafts, yours is the bottom one, and the spring movement during rotation is caused by groves at its right-hand side end:
Now the movement is caused by such a thing, gear selector:
It may happen, that the fat spring cracks, making gear change impossible. BUT, You may try to move a little adjuster from outside, visible here far left:
As you can see, it works as a sort of eccentric bolt, a precise regulator of gear selector action. From outside it is visible just over the adjustable spanner’s end, as a small bolt and big nut:
There is a chance somebody didn’t know how to set it up and made things wrong; the idea is, doing just a narrow angle movement, you change a lot; turning a lot leads you nowhere as you are quickly lost and you can’t remember what was initial setup - so, before you try, put a mark on the bolt. I am not sure, but after removing bolt out it may be impossible to push it in correctly, as its far end won’t jump in between fat spring’s ends.
Give it a try and report if things are any better!

Just a thought - as long as I remember, if nothing works, and you suspect the beefy spring, you may find a big play in gear change shaft on the gearbox, as cracked spring no longer holds all the gear selector stiffly in place, so the movement up and down would be substantial, without any reaction inside. Your description of gear change attempts suggests cracked spring indeed. Have a go on the second gearbox, try to peek into breather, try to select gears, you just need to use first short arm, same as on the pic with eccentric bolt.

Hi - thanks for your post!

I’m into the gearbox now and it is just a pile of parts on the workbench. The issue turned out to be that the selector shaft on which the selector forks are located was seized into the casing and would not rotate . Dismantling was problematic as access to roll pins securing the selector forks was difficult [as the forks could not be repositioned to allow clear access] so one roll pin was mullered on removal.

Once the gear shafts were out a generous application of heat to the casing where the end journal of the selector shaft locates allowed me to remove it from the casing. There was nothing obvious that caused the shaft to seize in the casing but I have dressed the journal and it all seems to move freely now. Just waiting to source a replacement roll pin [3mm x 16mm] before staring to reassemble…fingers crossed!

Thanks again for taking the time to post.


Good to hear you have found the cause of the problem.


Just hope it all goes back together with no bits left over… :confused:


I am doing same job paralelly, so in case - ask:)

Update. I did the job, what a pleasure, comparing to German school, where every bearing sits hard in its seat and don’t even think of such a job without blow-lamp.
I even made sort of tutorial, making pics step-by-step.
Now, after such refresher, I got idea why your shaft was seized. The end you mentioned is the one which actuates neutral switch. It may happen that somebody used well battered alloy washer, and water found its way in, building up enough rust to block the movement. OR - there was no washer at all, and switch just pushed the shaft blocking its rotation?

Hi - glad you got sorted!

Doesn’t look like water ingress from neutral switch was the problem but a good suggestion nonetheless!

I’m at the “pulling hair out stage” [not that I’ve got much of it left…]. Gearbox loosely reassembled and all looks good - except shift is still problematical. Can get gears into neutral and everything looks “right” but selector pawl seems very hit and miss on engaging with pins on base of selector shaft. Pins rotate freely but on some changes the end of the pawl lodges on the outside of the pin at a point where it seems to jam - its almost as if the “jaws” of the pawl are too short. I’ve got a scrap gearbox so I’ll pull the selector out of that and compare.

Gearbox is still on the bench so unable to test under live conditions with engine running.

OK, tutorial is here, unfortunately in not-that-easy-to-follow language. Take a look

and when you’re in the pics where you can see some red arrows made to indicate the direction of turn, use a battery drill to turn slowly the shafts, socket 22; gears need some spinning before they will finally engage, possibly this is the problem?

OK, I think I know what could it be. Take a look at the same part, pictured from both its ends:

Question is: have you touched it??? If you wonder why am I asking, get back to my first post here in this thread…

Hi, thanks for your post - you have got some great photos there!

Love the wooden wedge to lock the clutch shaft against the input shaft to remove the lock nut - I was lucky in that I had a spare clutch shaft from a scrap gearbox to use but it wasn’t matched to the input shaft. I locked the gears with some copper strip to make sure the teeth weren’t chipped.

I tried using a battery drill to turn the gears but the chuck was just a bit too small to secure properly on the end of the input shaft - I didn’t make the logical leap to replace the lock nut and use a socket in the drill but that’s a great idea and will do that if I have to do the job again…

The eccentric adjuster had been “adjusted” before I split the gearbox - I understand there is a certain amount of mystique and black magic associated with its installation. The adjuster locates between the ends of the selector pawl return spring and rotating the adjuster loads either one “side” of the spring or the other thereby changing the angle at which the pawl presents to the selector drum. I reinstalled it and adjusted so that the selector pawl is in a neutral position relative to the selector drum - both pawl teeth are equidistant from the drum. My thoughts are this will do as a starting position and further adjustment may be necessary. Watching the MG installation video on the Gregory Bender website doesn’t give too much away - you are advised to install the adjuster so that “the eccentric part of the screw is orientated towards the pre-selector nut” [the video shows that the cutaway is arranged parallel to the axis between the selector drum and the preselector shaft]. The eccentric adjuster is then “rotated 90 degrees” and locked. This procedure should mean that the adjuster is arranged so that it bears equally on both “sides” of the return spring.

I fitted the selector pawl from the scrap gearbox - I compared the two and the “scrap” pawl had longer teeth which should give a more positive engagement on the selector drum pins.

I am glad to hear that selection of all gears is difficult on a “static” gearbox. I had tried to get all gears to engage smoothly on the bench using the gear selector but there always seemed to be difficulty. I put this down to:1 - gearbox not spinning as per normal use: 2 - slight misalignment of shafts due to clutch ends of shafts not secured in gearbox cover while testing: 3 - difficulty in using gear selector lever either with grips or socket and extension over end of lever.

Anyway I eventually took the view that everything in the box looked right - I checked the arrangement of parts and gears against the diagrams in the workshop manual and inspected for wear on the teeth, dogs and bearings - all looks OK. I had resolved what appeared to be the immediate fault - the selector shaft seized in the casing so if there is nothing broken and all the individual operations seems to work I’m taking a leap of faith and sticking it back in the bike - will leave off the drive shaft and swing arm until I have tried a bench test with running engine…

Wish me luck and thanks again for your help!