AMC box jumping out of 1st

I have a wideline Dommie, fitted with a '65 650SS engine, and AMC box. It jumps out of first gear. This has been ongoing for, let's say, some while..... in fact, ever since I have had the bike; long story..... A Noted Expert informed me it must be the 2nd gear on the layshaft popping out of the 1st gear pinion. He sent me a used replacement. However, the 3 dogs on the back of said replacement layshaft 2nd gear, which engage with the 3rd gear, are narrower than on my existing item. I also have a stripped Commando 'box, and the dogs on it's 2nd gear are the same width as my original ones, approx. .85" at their widest point, whereas the dogs on the replacement gear are a mere .65". I fear that if I fit this gear there will be way too much backlash. The internals of the gearbox all appear in good order, with bearings, bushes etc having been replaced, by a process of elimination. The 4 dogs on the replacement gear, which engage with the layshaft 1st gear, all appear to be good, as do the original ones.Can any of you shed any light on this, and why you think it may be jumping out of gear? I am entered for the Prescott Hillclimb, and whilst starting on the flat, a short shift into 2nd works, starting on a hill will not!Thank you in advance. Ian

well there is a possibility that the cam plate and spring and plunger are worn or your gears are not timed in the right so take off outer cover and move the gears by the cam plate via a spanner on the cam plate center bolt at the front of the gear box. and take into top gear and then go down the gear box the top gear your selector arm should be near touching the top of the slot in the inner case if you got down the gears one by one to bottom gear and see if your selector arm near touches the bottom of the inner case slot , then replace outer cover set in second gear your Anna J

As Anna says, the timing of the selector quadrant to the camplate could be wrong in which case the first gear may not be fully engaging and then it jumps out. What happens is, the selector quadrant (the bit with the roller in it) reaches the end of the slot in the inner cover before the gear is fully selected on the camplate. there is a picture in the Haynes Commando manual of the correct position. Re-mesh the gears on the camplate to selector quadrant to get the correct range of movement.

For a good set of pictures on how it should be done, go to oldbritts.com and click on "Commando gearbox re-assembly" it's better than a free workshop manual!

Hi Ian,

Your box jumps out of first because it was never fully engaged in the first place. the camplate has not rotated to the position where the sprung plunger is fully in the detent. Have a look at the oldbritts website. There is an illustrated explanation of the workings of the gearbox. You say you have stripped the gearbox before but have you confirmed the position of the top edge of the roller in the quadrant when the camplate is in the second gear detent?

Has the box been fitted with a roller bearing on the layshaft with excessive end-float or is the shaft loose in the bearing ? I have a feeling that I've heard of this causing slipping out of first. Can you move the kickstart shaft far in and out if you grasp it from the outside ?

The dogs on layshaft 2nd aren't undercut so they will tend to slip under load if they're not positioned correctly.

Are the gears that you have marked or stamped in any way ?

Hi Ian,

Many years ago my Domi 99 did exactly as yours does and the cam-plate plunger spring had broken and the two pieces had started to screw themselves together so that the plunger was no longer in contact with the com-plate, although none of the other gears dis-engaged. Anna mentioned the plunger and spring and I wonder if you've checked as it hasn't been mentioned in further correspondence. If not it's easy enough to check,

Rob.

Ian, I don't know if you have spotted the late Peter Roydhouse's article on gear identification? It's under the singles section so perhaps not ?

It doesn't give a complete answer but does confirm that AMC gears are not marked. This differs from many Commando gears which have an 'AH' marking and best of all, those fitted to upright gearboxes which have a part number and number of teeth.

The layshaft 2nd that you have received with the smaller dogs matches what I have in a dismantled 'Doll's Head' box - these had larger dogs on layshaft 3rd. Peter Roydhouse suggests that these have a different tooth from from AMC so caution is called for.

The layshaft should be a tight fit in the bearing and I don't think that something under .040" is going to cause a jumping out problem even if the shaft is loose. Most Commandos with roller bearings rattle around more than that.

This doesn't help much of course. I'm trying to think why you can't see witness marks showing where the problem occurs but I'm puzzled.

Talking of witness marks, here is a picture of the layshaft 1st gear, with a shiny patch between the 2 right-hand holes, at 3 o'clock. Just the one patch, though, not all the way around. Would anyone have any thoughts on what would cause it, and whether it may have any relevance to the problem?

Ian

Hello Ian whats it like at the other side . and as it been put in the the wrong way round . and is the cam plate worn or coming of its splines as so do come loose on the splines then drop forward only to jam or make it hard to engage gear. and the other thing that springs to mind and offend over looked is the bar that screws into the back of the gearbox it carries the selector dogs it can get out of round or bent or dose not tighten up correctly there something not right with this gear box please post more photos yours anna j

Hi Anna. Thank you for your observations. The 1st gear is fine on the other side, which engages with the kickstart shaft. In fact all the gears look very sound, as are the bearings and bushes. The camplate does not appear to be worn, and is indexing correctly, with the plunger engaging with the corresponding detent in each gear, including 1st. It can't be the wrong way round, as the detents would not engage at all. The selector fork spindle which you mention appears to be straight, with good threads, and of course it engages in the inner cover to keep it in line. Bear in mind that I have been using the bike like this; not much, because of other issues; magneto, split petrol tank etc; but once short-shifted into 2nd, everything is fine. I think if you looked inside the gearbox, the first impression is very good! But, as you say, something is not right. I have now shimmed the layshaft to reduce the end-float to about 10 thou', and am in the process of carefully re-assembling. I am hoping to complete that today, and try it down the road this weekend. I will report back!

Many thanks.

Ian

Hello Ian well lets hope it works after all the hard work you put in to get the gearbox to work right, as For Magnetos well I rebuild my own most mags just need a good clean inside and carbon bushes changing there are 4 carbon brushes to replace two are earth brushes one at the timing cover end the other you cannot see it as its under the the points end somewhere and then you two primary pickup brushes , the next thing to fail is the condenser its in the slip pick-up wheel next too the primary winding in its own little house every thing on the magneto armature shaft can be removed if you have the know how, and some testing Equipment , have a look on you tube there maybe something on there, yours anna j

Update - RESULT!

Having shimmed the layshaft, it did not jump out of 1st once on a test ride today. Before, it would jump out every time once the revs rose. Gear engagement on the pedal feels more positive, too. While I was rebuilding the gearbox, it occurred to me that maybe the selector forks were worn, and got quite a shock when I saw Les Emery's price for new ones. I had a look at RGM's website to see what they charge, only to find out they don't list them. However, I did have a look at their hints and tips page, which mentions that these gearboxes are prone to slipping out of 1st, the culprit being either a worn layshaft 2nd gear, or excessive layshaft end float. So it appears that I was looking in the right area, and got there in the end.

Many thanks to all who contributed suggestions.

Now to tackle the hot-start issue, for which I may start another thread, once I have timed my Matchless G3LC...

Ian

well done Ian now you need to write all this down and sen it too Road Holder and someone else may see the benefit from your hard work and persitance the joy Of any motorcycle club its to try a help others if you can, and all member should remember that its your personnel experiences that your pasting on and not every thing works in the same way as you would do it , and its the same when reading a Manuel not everything works out the same from one bike to the next as there is No two Norton Motorcycle the same there are minor difference's from one bike to other so think on this and use your head, Yours Anna J