Hello all’
I need a little advice or wisdom concerning a recent problem with my M20 gearbox’ What seems to be happening is when i’m changing up into 3rd the gear seems to engage then disengage snatch lurch forward then re engage again ??? But it doesn’t do it all the time ? Although when it plays up the pressure on the gear lever doesn’t feel right’
Could it be the selector ratchet ( I replaced the 2 springs last year and have done a few thousand miles with no problems)
Or could it be the selectors or dogs 😬 I’m hoping it’s something simple as i don’t want to strip the gearbox so close to the start of the riding season,
Any thoughts or advice would be greatly appreciated’
Best regards, Tom Green.
email (option): rustytomm20@hotmail.com
Do you mean you have a false neutral between 3th and 4th gear?
Hi Roy’
I don’t think so, it’s like it drops out of 3rd an back in again’ when in 3rd it changes up into 4th beautifully also changes back into 3rd from top with no problems, as i mentioned it doesn’t happen all the time ?
I don’t think it’s a false neutral because the revs don’t rise and the box is still driving, (Although juddering a little )
May have to have the outer cover off and take the rear inspection cover off to have a look to see if anything is amiss, but would be great to know what to look out for,
Regards, Tom.
email (option): rustytomm20@hotmail.com
You won't see much through the inspection cover with the gearbox in place (if you can get it off)....It sounds like you are not quite engaging the gear with the selector dog...You might get it with a SLIGHT adjustment of the gear change linkage rod but make sure you are happy with what you are trying to do before you start...You could make it worse...Has the problem suddenly started or has it always been like that?...
The problem won't be the selector springs you fitted...Ian
email (option): ian@wright52.plus.com
Hi Ian’
Yes the problem has just started, But it doesn’t do it all the time sometimes it engages perfectly? So was thinking along the lines of the selector ratchet or the dogs ? It seems to be worse when the box is cold (if that makes any sense) and i’m loathed to make any adjustments as you say could make it worse’ May have to bite the bullet and give it a rebuild next winter,
Many thanks, Tom.
email (option): rustytomm20@hotmail.com
That may be the solution ultimately...As it is, the amount of wear in the box is an unknown quantity so there would always be an element of 'shooting in the dark' when making adjustments...As the problem isn't too bad currently I'd learn to live with it and keep an eye out for any further deterioration until you get to looking at it....
It's a job that every M20 owner can benefit from as an exercise in mind expansion:laughing:...Ian
email (option): ian@wright52.plus.com
Be sure that the both forks from the adjuster and the pins are not have too much backlash..my setup had to much backlash and i drilled the forks up to 6.5 mm and made new pins for it.
The design from the gearbox does not allow backlash on it at all.
Yes i think a rebuild would be the best thing for it’ as stampings on the box suggests a REME rebuild in 1953 and probably not been opened since? As you say Ian i will live with it for a while longer just have to be careful with it’
I have had the mind expanding experience of these boxes when i rebuilt one for my 720 😬🤯😀
So i kind of know what to expect,
So i will be on the lookout for some NOS parts i can lay my hands on some dogs and as Roy suggests a new selector setup may be beneficial’
I have about 6 months to prepare myself 🤪 😀
Best regards, Tom.
email (option): rustytomm20@hotmail.com
I would take it out of the bike and rebuild it.
You would be surprised how well this model gearbox can still work even with the parts inside being completely worn out. The last one I did had a few problems with shifting but nothing serious to make you think there were problems inside. It's main problem was oil leaks. Once apart the forks were blue from overheating and the dog clutches and a few gears were all in horrible condition. It looked like it had been run with no oil for some time even though when I got it it was full of clean oil. I probably had to replace half the parts in it. Luckily everything I needed was still available new. It now shifts perfectly.
It sounds to me like a selector dog & fork problem
Their is so much slop in the system that the 3rd gear dog is sort of engaging with the end of the opposing dog rather than slotting completely home
They will drive ( sort of ) end to end and when locked in that position it is very hard to get the selector to move the dog is either direction.
It happens to me now & then & I find a touch on the rear brake with the other foot holding the pedal down does the trick.
It was not the best deign on the plaet in the first place & 70 years of use won't make it any better
I have had 2 boxes repaired by regrinding the dovetails in the side of the dogs & reprofiling the top so the ends of the dogs slide over each other when making changes
Regrinding a deeper dovetail means that the dovetail holds the gears in place, not the selector fork
Reforming the slopes on the end of the dogs helps then to align as they come into contact
I have played with way too many different boxes to be 100% sure but I think what worked best was a curve on the sliding dog and the flat slopes on the gear.
Over time the ends wear flat so the do not align properly as they start to touch .
Great advice’ Thanks guys 👍
Regards, Tom.
email (option): rustytomm20@hotmail.com
Hi Tom
My regular ‘44 ride is doing this more often these times but until I rebuild the box on another bike on the bench & get that sorted I won’t go at it
In saying that - I’m learning heaps from building mine - so will be confident enough to tackle it in due course -
Jo’b
email (option): jonnyob1@googlemail.com
Personally, I wouldn't be inclined to follow Trevors course of remedial work...If the box is rebuilt, in good condition and is correctly assembled and set up then good gear selection and general operation is entirely achievable without modification....
If no parts were available to rectify the problems of a badly worn box then that strategy may well be the right one but that isn't the case...Most parts are still available either NOS or good used if needed and there is plenty of help available here when it comes to assembly advice...
Later pattern gears and selector dogs with the improved dog tooth form and the selector forks with the wider track and larger heads on the pins are worth consideration to improve operation when it does comes to a rebuild...
Although 'vintage' in design and slow in operation the gearbox type is both serviceable and durable and in various BSA models and its two different versions must have competently covered millions and millions of miles...:gear: Ian
email (option): ian@wright52.plus.com
Thanks again guys’
I have got a bucket full of M20 gear sets (about 3 i think) Both early & late so i’ll Have a look through them when I get time to see if any parts are serviceable (Probably not they’ll be in a bucket for a reason 😄) but i think NOS parts are the way to go’ plus a sealed bearing on the main shaft’ I changed the oil last weekend almost managed to get a pint in it’ Used it to go to work on this week drained oil again this morning and have lost half a pint so don’t think that helps!!!
Regards, Tom.
email (option): rustytomm20@hotmail.com