MK3 jumping out of first gear

Likely you need a new layshaft first gear, where the holes for the dogs on the sliding layshaft gear are worn, plus the dogs on that sliding gear will be worn, opening up the box for inspection is needed to reveal the culprit. Could also be something is stopping the quadrant from fully moving into first, so check the quadrant does not touch the window in the inner cover when in first. 

My advice would be to get the clubs gearbox video by Mick Hemmings, it’s a pretty simple box to work on and he explains everything well.

I suppose there are some questions here:

• Has this just started or has it always been like this?
• Has there been any issue with oil the gearbox, e.g. no oil or a lot of water in the oil?
• Is it any better if you hold it in gear with your foot for a second or so?

Seems most likely that the gear is not fully selecting. When perfect you should only need to press lightly on the gear level and the selector ramp will do the rest. Could be a sticky index plunger, sticky cam plate pivot, gearshift train (long and winding on a Mk3) not moving freely - the sort of things that might happen to a 45 year old bike left standing for many years (which it must have been if it's only done 10,000 miles).

Of course, if the box has never been right, been badly rebuilt or run without oil you'll have to pull it apart.

Fiddling with the clutch adjuster might affect a slipping clutch but won't make any difference to whether it stays in gear.

There are no questions and there is no possible - it is not right so it needs stripping and fixing. Do not risk a gearbox lockup, either way it could hurt you or your wallet. 

Interesting you say about not going into gear fully, as i sometimes keep my foot on the gear pedal in first and it does not jump out of gear. Do you think it is not going into gear fully and will correct itself with use? many thanks Graham

 

If it the quadrant hitting the window in the inner cover before the full travel is achieved then it will never resolve itself. However it would be evident all the time and not just started, so my thoughts are back to worn dog holes and worn dogs. While you are in there you best check the inner layshaft bearing and change if its the ball bearing with brass cage to roller bearing or ball with fancy plastic cage. The roller bearing will need the layshaft shimmed for end float to ensure gears are engaged correctly or you will be back to square one. In fact that's another one to consider, a roller bearing was installed and the layshaft not shimmed.

C'mon Ashley, let's not scare the poor boy, jumping out of 1st is not going to lock up the gearbox - unless there are other signs too, such as kickstart lever flailing around / sounds like a bag of nails / oil looks like metallic paint. If any of these are seen, yes, stop riding and strip the box.

First gear is selected when the 4 dogs (pegs) on layshaft 2nd gear engage with the four holes in 1st gear. When fully "in" gear, the dogs pass fully into and slightly thru the holes. It stays in gear because the dogs are slightly undercut (inverted taper) which locks them together under load. With use the dogs and holes wear so the taper goes the other way and the gears are pushed apart under load. The selector mechanism - cam plate & index plunger spring - resists this until the wear is too great and the bike jumps out of gear. The problem also occurs that the 2nd gear dogs are not fully engaged with the 1st gear holes and, sitting right on the edge, easily slip apart. This can happen if the rider is rather half-hearted putting it into gear but a Commando usually completes the shift for you - unless it's worn. Then you have stiffness in the shift mechanism, if the camplate pivot is stiff, if the index plunger doesn't slide freely or has come unscrewed (when you'll have an oil leak)

If the question was worded along the lines of "my rebuilt gearbox jumps out of gear" or "my 100,000 mile bike jumps out of gear" then mis-assembly or wear are the most likely explanations.

But this bike has only done 10,000 miles - barely run in. So, if the reason it was parked for 30 years is not that it wouldn't stay in gear and you're confident that it has not been assembled from a box of bits then I would suggest:

• Select neutral and drop the cam index plunger from the bottom of the box. Clean and check it slides freely and the spring is in good shape. Lubricate and refit. Do not ride the bike with it out and check the box is selecting cleanly after reassembly.
• Remove the neutral switch from the lower front of the box and blank the hole with a bolt (3/8 UNF) until you get things sorted.
• Operate the shift mechnism by hand feeling for resistance. If it's stiff take a look inside the primary chaincase and at the cross-over shaft. Try holding the cross-over shaft while you work the gear lever.
• Fill the box with a 5w30 high detergent diesel engine oil and run on very light load for a few miles, shifting up/down thru all the gears. Empty, repeat, then fill with 75w90 synthetic gear oil.
• Reposition the gear shift lever so that the action is higher (or wear heavier boots :) ).

The Haynes manual for the Commando is very good for simple strip and rebuild so I don't know that Mick Hemmings tutorial will add much.

I'm not scaring anyone - it's not right, it needs fixing, simple as that. 

If you pull out and the bike jumps out of gear just hope the car behind does not hit you or worse, shunt you into traffic

In my business you would be surprised how many will ignore a known gearbox fault. Do not chance it, I hear of the aftermath often. 

??

Graham

I had a similar problem with my 750.  I started by dismantling the clutch. I found that the grooves in the clutch centre were badly worn and that two of the metal clutch plates were bent - checked on a piece of plate glass. I replaced these and cleaned the fibre plates in petrol.

Re-assembled the gearbox was transformed.  No jumping out of gear and easy to select neutral.

Based on my experience I suggest it might be worth starting with overhauling the clutch before dismantling the gearbox.

Andy

"it stays in gear because the dogs are slightly undercut (inverted taper) which locks them together under load. "

This is not the case for first gear. 2nd 3rd 4th yes this clear distinction about first gear  is always made during my tech sessions for the last 30 years.

"not the case for first gear..."

Well David, knowing the effort you put into these things, I hesitate to disagree with you but I have just measured the dogs on a brand new 2nd gear (06-4640) and, at the base (next to the gear) I get 10.52mm and at the tip 10.68mm. Which in my book is a taper, albeit not as marked as the one on the 3 dogs between 2nd and 3rd.

[edit]
06-4640 : I think my gear must be in the wrong bag 'cos that p/n shows as mainshaft 2nd on Andover's site, Mine is definitely a Mk3 layshaft 23T 2nd from Andover (06-4639) circa 2000.

Stan

Having 13 gearboxes with all the bins of parts, from N to NA to all vintages of commando including MKIII. Then several  more in 12 commandos I own. The factory prints will tell all if the "current" ANIL will show or declare  the print detail. I have examined various other factory prints and the taper detail "should be" quite clearly spelled out. The peg on 2nd gear and the holes on 1st would both be needed to be tapered to be effective.

I defer to the results of factory drawing details....

Your 2000 gear = pre Siefert   AN -BSA Regal ?

Drive in first gear is obtained through the Layshaft first engaging the neighbouring second gear. This turns the layshaft and then the fourth gear pairing to rotate the output shaft.

Perhaps an issue with the camplate is allowing this second gear to drift out of engagement?

Detent / Indent Plunger problem?

Having looked at the dynamics of thrust transfer between two ajoined gears, I have surmised the thrust transfer is at an angle between the driven layshaft and drive mainshaft gear sets. With a worn 1st gear bush there becomes a "crawling effect" while revolving that produces a bit of dog disconnecting thrust. I always refresh the 1st gear bush to reduce the crawling effect by better supporting 1st gear.

Worn dogs against worn holes in 1st layshaft aggrivate this worn bush situation and the result is popping out of gear in 1st gear high thrust situations.

added: Perhaps an issue with the camplate is allowing this second gear to drift out of engagement?

I assert it can be driven out of engagement by the small crawiling thrust rather than a haphazard failure of the shift fork to retain it's position. This would show more of a lack of self engaging dogs on 1st gear.

An easy check to see if it is clutch related or gearbox related is if you go into first gear while moving  and when accelerating is does not jump out of gear then the problem is in the clutch side of things and you need to start there.

If it still jumps out of gear it is not the clutch but is in the gearbox.

Does not tell you what the problem is but it has reduced the possibilities considerably.

Tony

My 750 does this as well if I give it some welly when pulling away quickly. If I put it into 2nd gear and then into first the selector connects properly and it does not slip out of first gear. I know its fannying about but at least you can use the bike before stripping the gearbox for a rebuild. Like others have said the Mick Hemmings gearbox rebuild video is brilliant. NOC shop.

My 1978 MkIII started jumping out of first gear.  It was also a challenge to get it into first gear.  Advice from Jim Redman at Hinckley Motorcycles was to put it into first as I coasted to a halt at traffic lights etc.  I tried this a few times but it still jumped out.  So I started just using second, third and fourth gears.  It's a little tricky starting on a hill but it can be done. At some point I will get around to taking the gearbox out and sorting the problem. But since Jim gave me the advice, in 1982, I have only used the top three gears. I don't think I could adjust to having a normal first gear now :-)

Was the fault ever found, riding in groups or in traffic must be dodgy with a bike jumping out of first gear. No one else around you not a problem. 

Anyone on here who has this problem may find they could end up joining another member who knows what it is like to be rear ended.

Considering this fault can be investigated in less than an hour it makes sense to do it. Potentially a few hours to remedy and have a safe bike to ride, is time well invested.