Hi All
Just got some pay in so am looking at sorting the gearbox problems on the M20. The problem I have is that it only has drive in 1st and 2nd gear. It engages 3rd but has no drive and is difficult to get back into 2nd again. I think I was told on here in the past that it was the selector forks, does this still sound the case? Just would be nice to find out it wasn't something simple before I take the gearbox apart again.

Thanks!

email (option): martynhillyard@msn.com

It sounds like it isn't indexed properly, there is a marking on the selector mechanism that has to line up with the little cam plate on the inside of the inner cover, it sounds like it may be out, this will cause the symptoms that you describe
Dave

email (option): davmax@ntlworld.com

Check that the pawl is not worn too while the cover is off.

Hi All just took the cover off. Found that i have 4th gear just not third.

I took a few photos, the 1st is in 2nd gear, 2nd is in 3rd gear and 3rd is in 4th gear. Will do some reading up but wondered if this meant it was another problem.

email (option): martynhillyard@msn.com

Removed the gearbox from the frame to get a closer inspection. Reading the gearbox pdf on the m20 site the pawls are usualy the false neutral problem. Had a look but it seems to be more that the third gear teeth or the "mainshaft dog clutch" teeth are worn down and are not engaging properly. They rub over each other when the sprocket is turned acting as a neutral.

Is it looking like these will need replacing or am I looking in the wrong direction (hopefully i am as it is looking expensive!). Could it be the pawls still or the misalignment of the pawls now have damaged the teeth as a result.Or even the wearing of the selectors giving too much movement causing them to skip over each other?

Hope you can help

email (option): martynhillyard@msn.com

Hi Martyn...These boxes can be awkward and there are a number of different things that affect their ability to select gears correctly...

One of the first problems is that they are designed to function when they are rotating, with the inertia of the gears themselves helping everything to engage. Thus they are difficult to test correctly on the bench.

However, there are some things you can check...First remove the kickstart ratchet assembly from the mainshaft....

Then check the 'end float' on the mainshaft, layshaft and the selector fork assembly...These should all have approx. .005" end float...If they have substantially more than that the box will have to be stripped and the shafts correctly shimmed...This also gives the opportunity to inspect the condition of the selector fork, selector dogs and the cam pins and track on the selector fork shaft....

These also need to be in reasonable condition...I have a lot of serviceable used parts for these gearboxes so may be able to help with reasonably priced replacement bits....

Once you have established that all these things are as they should be you can move on to setting the box up, ratchet assembly wear etc. etc...

However, the things I have outlined are the first things you should check...

Where abouts are you based?....Ian

email (option): ian@wright52.plus.com

Thanks Ian

Will check this out tonight.

Am based near Colchester

Martyn

email (option): martynhillyard@msn.com

Hi Martyn...Too far away for me to help you directly I think...I'm in South Devon...

Also check for movement in the sleeve gear bushes (where the mainshaft comes out of the gearbox)...If you have more than minimal up and down play in these bushes (Test by moving the mainshaft up and down) they will need replacing as well...This also entails a complete gearbox stripdown...

Stripping the box will also give you the opportunity to fit a sealed bearing at the sprocket end of the box...This is essential if you want to control the oil loss which is heavy from a standard military box...

You should carefully consider whether you want to do the 'full' job...The process will be a bit difficult first time out but once your done you won't have to worry about gearbox repairs, or how it works, in the future...Ian

email (option): ian@wright52.plus.com

Thanks again Ian

Your last comment reminded me that a previous owner had rebuilt the gearbox and put M21 bits in for improved seals etc. Although it seems he did a very good job with it then it has been messed around with since leading up to the situation I am in now.

Thanks

email (option): martynhillyard@msn.com

Hi Martyn...M20 and M21 boxes are identical for a given year so there are no 'M21' bits that can be fitted as an upgrade...

I have seen WD boxes fitted with the later main gearbox case from a postwar box. This has a lipped oil seal at the sprocket end (there isn't one in the WD box)...The problem then though is that you lose the filler spout from the WD box so there's nowhere to put the oil in..(later boxes are filled through the outer cover)...

The other alternative is the sealed bearing..perhaps that is what was fitted?...though not an original or specifically an M21 part...Ian

email (option): ian@wright52.plus.com

Just had a closer inspection with a fresh mind. Main shaft is fine. The problem seems to be that there is too much movement between the selector fork and the mainshaft dog clutch. There is quite a bit of movement there, when in 3rd possition the movement to the far right is engaged but the far left is skimming the top.

So it would seem like one of those two is worn, if not both as the mainshaft dog clutch does seem quite worn, as do the 3rd gear teeth but it could be the selector fork wear that caused this.

Im guessing it is a rebuild job as there is no adjusting that i think can be done and probably cannot find out which was causing the movement till it is apart.

email (option): martynhillyard@msn.com

I notice in your photos there is some discolouration of the dog clutch caused by overheating...This indicates the box has been incorrectly assembled/set up in the past.(or is now).
Pressure contact between the selector fork and the groove in the dog lead to friction (heat) and wear of both parts over time..

Also incorrect setting and/or shimming of the shafts can also exert pressure on the pins and cam track of the selector fork assembly..this will ultimately also lead to excessive free play...usually at the point where the cam track changes direction.

'Lost movement' through excessive wear of these components is one of the things that can detrimentally affect the operation of these boxes and hence gear selection...Ian

email (option): ian@wright52.plus.com

The main shaft & lay shaft forks are identical so have a good look at them both and particularly the relative thicknesses.
Liquise the sliding dogs are also the same so should have the same width groove and shoulder .
That is the easy way to work out what has happened.
From there you can work backwards to why.
I sent my gears out to have the dogs dovetailed so that once they contact , they lock together, One of the best things I ever did. It makes the gear changes a bit heavier but a lot cleaner and to a large extent compensated for my errors in assembly.
The initial failure was due to me pounding on the mainshaft to get the clutch hub off.
This shifted the mainshaft to the left bu cracking the retainer in the inner cover ( which I never noticed ) so no changes where the dogs moved to the right went fully home thus the fork and dog wore making selecting 4th very hard and eventually third as well although the latter coung be easily selected bu not getting into 4th then going back to third.

email (option): wariron@tpg,com,au

I have finally got around to taking the box apart.

The probablem appears to be a chip in the high point of one of the selector froks at the 3rd gear position. This means the gears are not being pushed fully into possition therefore not properly engaging. This has led to excessive wear on the gear teeth.

So the parts I am now in need of are as follows:

24-4393 Mainshaft Dog Clutch
66-3212 Mainshaft 3rd Gear
66-3248 Selector Fork

I will also put something up in the wanted section

Thanks!

email (option): martynhillyard@msn.com