75 Broken Teeth on Fourth Gear?

[JeremiahL]

Hello all,
As you can tell I'm new here, I signed up a couple weeks ago and have been soaking up as much information as I can.

I recently picked up a 75 XS650 in pieces that the PO had been rebuilding. He said the bike had no compression when he bought it and he found the rings were stuck in the piston grooves. He lapped the valves, and I think got the jugs and head back on but said he bent a ring in doing so and scored the bore pretty decently. So I'm guessing he got it together and found out he still didn't have compression in one cylinder and took it back apart. This is the state that I got the bike in.

On to my question. I figured before I spent the money and time getting new pistons and reboring the cylinders I should make sure everything felt alright in the bottom end and transmission. I was turning it over by the counter shaft sprocket and shifting through the gears tonight, and when I hit fourth gear the countershaft spun freely for about 20-30 degrees before it started moving the crank again. every time I rotated it around to the same spot I got the same result. Would I be correct in assuming there are some missing teeth or could this be something else? If it is broken teeth should I just plan on replacing the whole gearset? Anything else I should check on the bottom end before I get in too deep on it?

 

[angus67]

im not keen on the assembly of these transmissions, but I would assume you could just get a 4th gear from somewhere, but ive seen gear sets on ebay.
Now, I have to ask, What would break the teeth off like that? perhaps po dropped the assembly on the garage floor? or, could be the shift forks are bent, chipping teeth edged.
Gears don't just break by themselves. there is a reason. crack the egg, and do the same rotating to find the dead spot, and look if the shift forks are digging in were they are not supposed to.

 

[JeremiahL]

Alright thanks for the advice Angus. I'm going to try getting the cases split in the next couple of days if I have enough time after work. Getting a look inside should clear things up hopefully and let me see exactly what I'm dealing with. I will report back with pictures when I get there!

 

[JeremiahL]

Also, I hadn't really thought of what would have caused the damage... I just assumed maybe something let go for some reason. It doesn't look like the po split the cases, he was pretty sloppy with the liquid gasket in other places so I think the case seam is too neat for him. There also doesn't seem to be any damage on the outside of the cases as far as I can tell either.

 

[TwoManyXS1Bs]

Haven't heard of this one yet, but I wouldn't be surprized that the 4th sliding gear on the countershaft has shattered near one of its engagement dogs, taking a couple of teeth with it. That gear slides over and engages the large 1st spindle gear, and a harsh shift into 1st gear with a stuck clutch would induce a severe impact on the 4th's engagement dogs. Coupla pics for reference.



If that is the case, best to inspect, maybe replace, the associated shift fork and the large 1st spindle gear...

 

[JeremiahL]

Well I got the cases split this evening, I didn't seem to uncover anything obvious though. To my eyes everything looks to be in great shape. Teeth on the gears and engagement dogs don't really have much sign of wear as far as I can tell. The forks seem to be fine too.
Maybe someone will see something I'm missing from these pics? I got a couple overall shots and some close ups of the dogs on the pinion gear, as well as each side of the middle fork. Let me know if you want to see anything else too

 

[TwoManyXS1Bs]

Take the transmission shafts and place them in the upper case, with the shift forks. Spin the shafts and cycle thru all the gears. See if your syndrome reappears. Also, the fork to examine is the 4th slider, closest to the clutch...

 

[JeremiahL]

Take the transmission shafts and place them in the upper case, with the shift forks. Spin the shafts and cycle thru all the gears. See if your syndrome reappears. Also, the fork to examine is the 4th slider, closest to the clutch...

For some reason I'm not understanding what you mean about checking the fork closest to the clutch. Looking at the picture you posted, when shifting into 4th the middle fork moves the 3rd gear from the left on the shaft that the clutch attaches to moves toward the right (in reference to the picture) and engages with the set of external dogs on the gear next to it. Should something else be engaged at this point too?

I put the shafts in the top case this morning and ran it through the gears...everything seemed to work alright. I'm wondering if maybe I was just an idiot and was thinking too much of the extra play that comes with having the two sets of dogs engage with each other, instead of dogs going into corresponding holes on the gear next to it...if that makes sense. Maybe something else should be engaging at this point too?

Sorry if I sound ignorant on any of this...this is the first time I've gotten into a sequential gearbox.

 

[TwoManyXS1Bs]

In my first pic, above, the gear labeled with the red 4 is the driven 4th gear.
It is splined so that it spins with the countershaft (sprocket shaft).
It is moved by the clutchside (rightside) fork.
It's dogs engage the larger gear to its right when in 1st gear.

You said that the 'skip' occurred in 4th gear.
At the same place for each revolution of the sprocket.
The only gear that could do that is the countershaft's 4th gear.
If it lost teeth from a dog shatter, that would've happened during a shift into 1st.

Hence, checking the rightmost, clutch side, shift fork.

It's best to observe the gears in motion while your spinning the shafts, shifting gears, and watching for that 'skip'...

 

[TwoManyXS1Bs]

Your edit:

...I put the shafts in the top case this morning and ran it through the gears...everything seemed to work alright. I'm wondering if maybe I was just an idiot and was thinking too much of the extra play that comes with having the two sets of dogs engage with each other, instead of dogs going into corresponding holes on the gear next to it...if that makes sense. Maybe something else should be engaging at this point too?

Sorry if I sound ignorant on any of this...this is the first time I've gotten into a sequential gearbox.

Not to worry. The "engagement clearances" between dogs/cogs/slots have upset numerous folks before, including myself, back when they were inventing dirt.

Glad you did the shafts in the top case exercise. Really helps to understand how "constant mesh" transmissions work. About the only thing to check now is to ensure no forks are bent or damaged. When a 'sliding gear' is disengaged, ensure that its dogs have equidistant clearance on both of its sides...

 

[JeremiahL]

Ok thanks again for the help. Sorry about that edit lol, I was trying to get that typed out on my phone right when I got to work and hit the post button halfway through.

If nothing else this was a good experience for my own peace of mind knowing everything looks alright in the bottom end. The po also left the cases sitting on his garage floor with the cylinders off and just a rag laid over the pistons for who knows how long so it'll be good to get everything flushed out.

I should probably replace all the shaft seals now that I have everything apart? And is the seal kit from mikes any good?

 

[TwoManyXS1Bs]

...I should probably replace all the shaft seals now that I have everything apart?

Absolutely.

... And is the seal kit from mikes any good?

Haven't heard any issues. I use a different kit for my XS1B.