Gearbox rebuild problems (2014)

Hello,

i am currently reconditioning the gearbox an a project commando i recently bought and (as i expected) im having difficulties here and there.

The gearbox didn't work basically...no gear change at all...opened it up and found that the cam plate was in the wrong position which explained that! Also the layshaft was bent. Therefore i took the opportunity to replace all bushes and bearings. Now i have the gears in and functioning as they should (nice and smoothly). However...when i replace the inner gearbox cover everything stiffens up and the gears start to make grating noises when i turn the back wheel which is also very stiff to move. This occurs before i even put the nuts on to tighten up the inner case completely!

This morning i pulled the bearings out of the back of the gearbox once more, thinking that one or the other isn't sitting correctly, and reseated them both....they both appear to the naked eye to sit nice and straight (as they did before). Also i ensured that the layshaft isn't too tight in the kickstarted bush and can move freely.

This mornings efforts made absolutely no difference!

Has anybody any suggestions or has anyone encountered similar symptoms when rebuilding the Gearbox?

Seems like it's been the victim of savage mechanic(s), anything is possible, can you post some good images of the parts? Inner cover on, inner cover off, gear set layed out, etc..

It makes me think that the shafts are bent or that the case and cover is not indexing.
It may be that the case and cover are from two different boxes and are not in line.
Check shafts for true first.

My thoughts same as Onder's, but he beat me to the post!

Slick

We either already know what AMC innards look like or can see em in the parts book. Mistakes I've made was flipping one of the cogs the wrong way, 2nd or 3rd and miss indexing the shifter knuckle teeth with cam teeth. Also have swtiched the 2 shifter dogs in wrong. I've run with bent shaft but only thing that did was wear our bushes fast so clutch wobble reappeared but worked fine till putting a belt drive on. A video would be more helpful entertaining.

The layshaft had 1mm bend in it! This i replaced.
The mainshaft appears to be ok...stays in position on rotation.

When i remove all the gears except the last 2 gears on the layshaft and the one on the mainshaft sleeve and fit the inner cover it fits easily and there is no stiffening and no undue pressure on the kickstarter which turns easily. When the gears are in its a struggle to get the cover on at all and everything goes tight...

If the Mainshaft was bent like the layshaft was you wouldn't one experience an especially tight point and an opposite loose point per cycle. This is constantly tight.

timsha said:

The layshaft had 1mm bend in it! This i replaced.
The mainshaft appears to be ok...stays in position on rotation.

When i remove all the gears except the last 2 gears on the layshaft and the one on the mainshaft sleeve and fit the inner cover it fits easily and there is no stiffening and no undue pressure on the kickstarter which turns easily. When the gears are in its a struggle to get the cover on at all and everything goes tight...

If the Mainshaft was bent like the layshaft was you wouldn't one experience an especially tight point and an opposite loose point per cycle. This is constantly tight.

Click to expand...

Give those last gears going in that bind it up, a very good look over. From your initial post, I take it these gears were those you found in the GB, but we do not know if PO replaced them with some aftermarket crapola. Look for any evidence these are not genuine AMC parts.

There still could be an alignment problem with the inner cover. I would guess that as little as 0.002 inch offset in the bearing bores could result in your condition.

I agree that if the mainshaft were bent, there would be a loose, then tight cycle.

There are two different sizes of one of the gearsets and it is possible to put a mismatched pair in the box, I think the difference is between the 1973 and the later 850. The box will go together nicely until you put the inner cover on then it will bind up as the shafts are actually no longer parallel. I found this out the hard way as I was also putting togeher a box from a pile of unknown bits. It was second gear that was the culprit Norton went from an 18 tooth to a 19 tooth on the mainshaft with the layshaft cog remaining at 24 teeth but a different diameter depending on whether it was meant to go with an 18 or 19 tooth mainshaft gear. I will have a look at my spares and see whether I still have the part and measure it.

That makes sense....this bike is a 73 MKV 750 (first registered 74) with 235 prefix, so its definitely from that period.

How many teeth should 2nd gear have? this one has 18.

...the corresponding layshaft cog has 24 teeth.

With the inner cover off, the layshaft sags a little under its own weight, so everything turns smoothly. When put into its true position by the Inner Cover / Kickstarter bush the grinding of gears starts....to me your suggestion that the dimensions of one ore more cogs is not right sounds very likely to be the cause.

Just need those dimensions now to see if you're right!

texasSlick said:

Give those last gears going in that bind it up, a very good look over. From your initial post, I take it these gears were those you found in the GB, but we do not know if PO replaced them with some aftermarket crapola. Look for any evidence these are not genuine AMC parts.

Click to expand...

What should i look to see wether the gears are genuine AMC parts or not?

Most of my gears are stamped "AH" except the following:

2nd Gear only has a stamp "RS85" on it and is 56mm in diameter.
The innermost layshaft Gear (also 56 mm) is stamped with a "6" or a "9". The large Layshaft gear (the one with 24 teeth and the groove for the gear selector fork) measures 70,5 mm (whatever that is in inches) and has no stamp on it.

I had a similar problem when I did mine. I rebuilt mine on the bike with the primary side still intact. The PO had modified the gears ( perhaps to fix this same problem). The problem was on the primary side. The clutch retaining "c" clip was missing. When the gear box was correcty assembled and cover was being tightened it pulled everything to the right. I replaced the clip assembled the primary and the gearbox went together fine.

thanks Deets55, my primary side is off and the circlip is not yet on...i will give it a try to see if that makes a difference in my case. However those shafts are so stiff with the inner cover on that i can't imagine the shaft would slip anywhere.

Sounds like you have a different issue. On mine w/o the clip the clutch basket was was too far to the right. When tightening up on the gearbox the basket was up against the bearing pushing it into the gearbox and binding the gears up.

The ealier gear set has an 18 tooth mainshaft gear and a 24 tooth layshaft gear. The later gear set has a 19 tooth mainshaft and also has a 24 tooth layshaft gear. The 24 tooth layshaft gears are not the same diameter even though they look the same and have the same number of teeth. I will measure the diameter of a couple of 24 teeth gears that I have and let you know what size they are. Unfortunately we are on opposite sides of the earth so you may not get the info at a sensible time if you are in a hige hurry.

Why not put the box together without the second gear pinions in place and see if it goes together nicely and changes through the remaining gears, this might at least let you know that you are on the right track.

I think Dave M is putting you on the right track.

One other thing to check out...look over the dowels or indexing pins to see if anything looks amiss, such as a bent dowel. If Dave's gear measurements do not locate the cause, you might try removing the dowels, then putting the inner cover in place, letting it "float". If that removes the binding, then figure out what is amiss with the dowels....do not bolt up the inner cover without the dowels thinking the problem is solved!

I hope it proves to be gear mismatch as Dave suggests....that would be easiest to rectify.

Slick

I have just measured two sets of used gears that I have in my workshop. The measurements are as follows:

Mainshaft 1st. (14 teeth). 1.696 inches dia at highest point on teeth
Layshaft 1st. (28 teeth kickstart gear) 2.995 inches
Mainshaft 2nd (18 teeth) 1.975 inches
Layshaft 2nd (24 teeth) 1.595 inches

I also found a separate mainshaft 2nd Gear (18 teeth) which measured 2.048 inches. I believe this was the one that gave me the trouble, as you can see it is only slightly larger in diameter, but sufficient to deflect the shafts at the point where they enter the inner casing.

I hope this helps if only by way of eliminating one or two possibilities. Good luck solving this problem.

Hi Dave,

thanks for for help.

I'm getting some strange measurements here....sorry don't have a digital measure and mine is not in inches so these measurements are not 100% but should be accurate to within 0.3mm. Your values are in italics.

Mainshaft 1st (14 teeth).....45 mm (1.772 Inches) 1.696 inches
Layshaft 1st (28 teeth) ....80.05 mm (3.152 Inches) 2.995 inches
Mainshaft 2nd (18 teeth) ....63 mm (2.480 Inches) 1.975 inches
Layshaft 2nd (24 teeth)....70.8 mm (2.787 Inches) 1.595 inches
Mainshaft 3rd Gear (21 teeth)....63.4mm (2.496 Inches)
Layshaft 3rd Gear (20 teeth)...62.9mm (2.476 Inches)
4th Gear Main Countershaft (23 Teeth)...69.2mm (2.724 Inches)
4th Gear Layshaft (18 Teeth)...57 mm (2.244 Inches)

What really stands out is the difference between our Lay / Mainshaft 2nd Gears!! Are you absolutely sure of those dimensions...youre layshaft 2nd is absolutely tiny!!!