5 speed shift drum shimming

[amamet]

can someone tell me if this sounds right, I read the shimming articles over at greg benders site:
I have a 1.0mm shim at the front of the drum and a .6mm shim in the rear for a total of 1.6mm of shims.  with everything out except the drum, i reinstalled the gearbox cover and confirmed there's is lateral play. I left the 1.0mm shim at the front and with a 1.0mm shim added in place of the 6.0mm shim at the rear, all play is gone and rotates freely.  so thats a total of 2.0mm of shims and a .4mm difference from what was in there originally. The shift forks have wear on the back side, mostly at the top (see photo).  if I increase the rear shim to stop pressure on the backside to a .8mm shim then replace the 1.0mm shim with a 1.2mm shim for a total of 2.0mm of shims will that be right? in theory?

[Kiwi_Roy]

Where is Old Jock when you need him?
http://www.thisoldtractor.com/guzzitech.dk/pdf/gearbox-rebuilding-john-noble.pdf
I seem to recall in my box it was not so much the shift drum shimming but the clutch input bearing was free to slide back and forth under the 3 hole retainer plate. I made a spacer to take up this play.
----------------
Off topic a bit you can see where something was thrashing around chewing metal off the box, I assume it was probably due to a plastic bearing cage fixed by the previous owner.

There were far more serious issues in the final drive, bits of broken tooth prevented it from turning.

[LesP]

Here is a picture or six.
#
I will say, don't overthink it, the box by design does not allow that anyway (Remember it is helical so will try and rack the shift drum, the purpose of shimming I guess (dog condition will impact that) but the dogs should be doing the work and they are non performance design.
All you are trying to do is avoid over/under engagement one way or the other from the pairings when shimming the drum but will be a compromise (Perhaps Moto Guzzi left the drum under shimmed for that reason, for leeway)
The main thing to me (and the reason I checked with a bore scope) is you can cycle the fork/dog  carrier in and out of each gear (pair) with no de-tent, in other words you can get the box in a neutral dog condition for each gear and cycle it in and out for no clash (over engagement)
The catch is the two pairs are independent of each other because of the drum tracks... I will say there is reasonable margin at full engagement (carrier dog depth to gear dog deep) because I looked... It would have to be out (drum shimming) quite a bit for a binding shift (imho)

My head hurt  trying to figure that out......... If you have contact at the fork rear that suggests over engagement with shifts with the forks to the rear, to remove some of that the drume needs to go forward so less shim at the front of the drum compensated at the rear shim (add) ???????
If you have 1 mm front and rear, would the 0.8 mm not be at the front and 1.2 mm be at the rear to take load off the rear of the fork when the fork is in the gear engagement to the rear of the box... (0.9 mm and 1.1 mm might be a hedge your bets because that 0.2 mm comes into play when the forks shift in the opposite direction.
AS per this pic below.


(I think some folk would say just leave it at a 1 mm shim each end with  smooth rotation of the shift drum)

[acogoff]

     All I can add is to make sure you are doing this with the gasket in place.

[amamet]

well, according to the article, the gasket should "not" be in place when taking out end float.  he later says the gasket ensures there is a little end float when its all put back together after you've removed all the excess play

[amamet]

heres the article if anyone wants to read it

http://www.guzzipower.com/Files/Gearboxshim.pdf

[Wayne Orwig]

well, according to the article, the gasket should "not" be in place when taking out end float.  he later says the gasket ensures there is a little end float when its all put back together after you've removed all the excess play

So you ARE doing the test with NO gasket, and it is free, but no play, correct? That is how I try to get it.

Wear on the rear of the forks means the drum is pressing rearward on the forks. So the drum needs to go forward. So the front 1.0 shim needs to be reduced, correct?
To me it sounds like you need about a 0.8 front and about 1.2 rear shim.

Maybe.

[amamet]

according to the article I need to come up with .4mm worth of shims to remove end float. since the fork pushed back too far with the .6mm shim, its increased to .8mm.  at that point I add .2mm to the front to remove all play, 1.0mm shim to 1.2mm shim. At least thats how I interpret the article.  I had a total of 1.6mm and I need 2.0mm.  I don't have any shims yet so I haven't gotten any farther than that

[Wayne Orwig]

1) I understand that you say you tried 2.0mm shim total, with NO cover gasket, and it turns free with no play. If so then 2.0mm total should be fine. Correct?

2) You said the the forks are worn on the BACK side, so the forks (drum) are being PUSHED back. And there is a 1.0mm shim on the front now. To me that means you need LESS front shim to move the drum forward. From 1.0 to maybe 0.8mm, on the front. And about 1.2mm on the rear to get to 2.0mm total.  Correct?

3) If you increase the front shim from 1.0 to 1.2 as you first state, that will press the drum rearward even more causing more shift fork wear, Correct?

[amamet]

That’s what I first though Wayne , but after reading the directions, they mention going up to the next size shim  In the rear of the drum if the wear is in the back side. That’s why I’m thinking .6 to .8  The front increase is just to take out the float. I assume there’s more rearward pressure than fowards. the extra .2 mm at the back stops it from pushing the fork rearward Against the dogs. I emailed Pete roper and asked the same question to see if I’m interpreting his directions correctly. Hey, this is all new to me so I could have it ass backwards  pretty easily :)

[amamet]

To quote:
“ The way I tackle this is:
1. Measure the thickness of the shims you start out with, the one at the front
and the one at the back of the selector drum, that the factory put in. Note
them down.
2. Make sure the large washer under the rear shim is in place.
3. First of all to start adding shim to the selector drum until I've eradicated
all end float but the drum will still spin freely. It doesn't matter where this shim goes for now, what I'm trying to do is find out how much more it needs to take the end float out. Just swap shims until all the end float is gone.
4. Subtract the thickness of the original shims from the ones that have eradicated the end float and you'll know how much extra thickness you're going to need.
Having got that you have to work out at which end or in which proportions on either end you are going to add it. To do this you look at the selector forks and see if there is wear. Usually you'll find that there are wear marks on the rearward facing flank of the 1st/2nd fork and ‘maybe’ on the same face of the 3rd/4th. This isn't guaranteed but it's the ‘commonest’ factory cock-up! In this case you need to move the drum towards the front of the box by adding shim to the back. Try adding the next size up shim to the back of the drum and then adding enough shim to the front of the drum to get the thickness required to eliminate the end float.”

[Old Jock]

I'm pretty certain you're all saying the same thing more or less, the drum needs to move forward

I think what the article is telling you is if you have wear on the rear faces and slack on the drum remove that slack by placing addtional shims (or a thicker one) to the rear of the selector

Its not an exact science from my limited experience, you had 1.0/0.6 before and need 2.0 to remove all float (minus gasket)

You've really 3 choices as I see it

Remove the slack all on the rear, so 1.0/1.0 mm

Go up a size on the rear so 1.2/0.8 mm

or as Wayne suggests alter the position 0.8/1.2

I'd try them all for all the time it takes and probably start with 1.0/1.0, then go to 0.8/1.2 finally 1.2/0.8

Compared to getting the box out in the first place it's really not that big a deal to try them all then shift up and down the box, you should be able to feel when you're moving the right way. You'll see later on in the article Pete tells you to try it out then make further adjustments as required.

As LesP said there is some leeway so it's not absolutely critical, if you did go wrong you might end up with the same wear you had before, and any move forward would help

Be interesting to see if Pete gets back to you and see what he has to say

John

[LesP]

If you have 1 mm front and rear, would the 0.8 mm not be at the front and 1.2 mm be at the rear to take load off the rear of the fork when the fork is in the gear engagement to the rear of the box...

As posted yesterday, you are moving the drum forward which takes the forks with it, reducing rub on the rear of the fork.
That's it ?

All you are doing is removing some end play from the shift drum, there is nothing you can do as far gear engagement because the shift drum tracks dictate the dog engagement. (unless the shimming was drastically altered)
In other words even if you got the top fork to centre on both its gears that does not mean the other fork will do the same being in an independent shift drum track.

You add a pooftenth of shim to the drum so it has minimal end play and make sure it does not induce more rubbing, that's all

You could just leave it at 1 mm / 1 mm and it would be no worse, if it had a 1 mm front shim as dissembled the drum could not go forward even if the rear of it is shimmed additionally so nothing changes (as far as any rubbing)
 
Any rear fork rub would simply stay the same (unless you reduced the front shim to move the drum forward)
Look at the picture of my gearbox ?

You are talking 0.2 mm on a fairly robust gearbox putting it politely, don't get caught up in the hype where this shimming is made into something it is not.
Its not like the shift fork to drum track tolerance is to microns.