Second gear slipping - cause and options

[Motorhobo]

For about a month I've had the dreaded 'second gear slip'. As to the cause I can't say for sure, but it started happening soon after I ran at +7000rpm in 3rd gear under the testosterone-induced peer pressure of a few madmen here who will have you believe that if you're not redlining, you're not bricking. Brickers -- don't buy into their evil propaganda! Sure, the engine might have a little fun, but the engine isn't the only thing on these bikes that has to deal with all that power and unless someone can say with 100% certainty that sustained high revs will cause absolutely under no conditions no excessive wear or damage to the transmission then there's no reason to wind it out just because you can and a classic k bike can run its whole life at 5500 - 6500 rpms and be perfectly happy...so there, and you know who I mean...

Now that I've gotten that rant off my chest, the second gear slippage is well-described here:

http://www.motobrick.com/index.php/topic,8257.msg62325.html#msg62325

And possibly here:

http://www.motobrick.com/index.php/topic,4840.msg30296.html#msg30296

What I have at my disposal is a spare transmission whose input shaft is pretty much toast -- splines chewed up because of a misaligned clutch that ended in clutch disk spline failure. What I'd like to do is take swap the input shaft from the transmission with the slippy second gear into the one with the good second gear and bad input shaft, or even better, find a cheap good input shaft I can put into the spare transmission so I can just swap out trannys one day without too much downtime.

So, questions are:

Is there any other cause of this I could be missing? The clutch is properly adjusted (but probably was out of adjustment for a couple thousand miles over the winter...lazy me) but I can't think of any reason a clutch would only slip in second gear in a certain torque range and not in other gears in the same torque range. Oil level in transmission is fine and it looks clean -- I didn't drain the oil and look for metal but that's moot at the moment since I'm not looking to fix the transmission but rather to swap it. I swapped out final drives yesterday while I was swapping tires -- no change, so it's not that. Driveshaft is new OEM, so it's not that. I'll take audio next time I'm out and post it, but in the meantime some questions about options wrt transmission work:

Has anyone ever swapped input shafts in a transmission, are special tools needed and how easy is it to completely f**k something up in there while doing it? A new OEM input shaft #1 on the fiche diagram  is +$350. I don't need a new one, I need a good one, which I already have, I just need to get it out of one and in the other.

Where can I get a cheap replacement input shaft? There's nothing on ebay and I've never seen any individual transmission parts sold there. Is there a bike breaker I don't know about that might have one?

If I can't find a replacement input shaft I'll have to go the swap-one-into-the-other route and I'm wondering how many days I'm talking here, and whether it might best to just ride it with slippy second and meanwhile bite the bullet and have my spare rebuilt. I'm not particularly interested in buying yet another transmission off ebay or wherever -- my garage is chock full of Kbike parts as it is.

Input appreciated...

MH

[johnny]

greetings...

is it only slipping in second gear...

i have been through 5 clutches... one grenaded downshifting 5th to 3rd pulling a 2up pass... the other 4 happened all the same way... they way it worked for us was i would getts slippage in 5th when whacking... then 5th and 4th when whacking... then 5th 4th 3rd 2nd when whacking... one day we rode 2up 200 miles in 1st and 2nd cause it went full whack... 

j o

[Motorhobo]

greetings...

is it only slipping in second gear...

Yeah, I've had slippage from an oiled or worn clutch plate before, this isn't like that. It only happens in second gear. It's not like a clutch slip I've ever had -- it's like a gear spins through momentarily and then quickly engages again, so what you get is a lurching with an accompanying clacking noise as the gear reengages. I'm not saying it's not the clutch -- I've just never had that before and it sounds like it's coming from further back than where the clutch is, and if it were the clutch I'm not sure where that clacking would be being generated.

It's only in second gear when accelerating, i.e. into a turn. That's when I noticed it first, accelerating of a curve and said 'Whoa Jack!' as the power went away and came back. Not fun...

I can ride in second no problem if I keep steady RPMs with only moderate acceleration. But when I give gas...thunk! Happens regardless of straightaway or lean -- the torque is the trigger.

I'll try to get audio later this evening, and will see what happens with more RPMs. Thanks for chiming in Johnny! I was thinking everyone was either stumped or not wanting to get 'involved' whatever that means...I suspect there aren't too many out there who have opened the transmission casing -- I personally am scared shitless something will jump out and eat my face from in there.

Nobody ever tried to replace a trans input shaft, huh?

[Filmcamera]

From reading the threads you posted it seems it probably is just second and that you cold mix and match your two transmissions to fix the issue.  In fact it sounds like it might be the dog as someone had in an earlier post.


I have never open my transmission but reading posts from those that have it seems fairly daunting but not impossible. I would change the seals while I was at it and tighten the infamous grub screw.  As to which bits so swap you wont really know until you get in there and see why it is slipping in the first place.  As to tools Clymer talks about needing a bearing puller and a hydraulic press to disassemble the shafts (and an egg carton for the bits but I am sure you can lay your hands on one of those!)


The best thing would be if the issue you have is on a different shaft to the assembly with the input shaft so you do not have to disassemble any shafts and can just swap one out.  Ie take the gearshift mechanisms with the gear dogs on it out of the tranny with the bad shafts and put it into the tranny with the slipping 2nd gear.


The amount of time it would take is kind of up to you.  It takes me about three or four hours to remove the transmission, I am guessing another three to five hours to open both transmissions and swap out the relevant parts then three hours to rebuild everything.  Call it a weekend or a seriously long day.


I personally would be comfortable going in and swapping one complete assembly for another but would probably  baulk and taking a shaft apart - but that is just me.

[Motorhobo]

From reading the threads you posted it seems it probably is just second and that you cold mix and match your two transmissions to fix the issue.  In fact it sounds like it might be the dog as someone had in an earlier post.

Well, a dog for me is a creature that rides in a sidecar, see atavar. No that's not a donkey, it's a dog. The point being that the inside of a transmission isn't something I have any familiarity with. I suspect my best course of action would be to put replace the bad input shaft with the good one and leave the one with the bad second gear as a science project. I haven't a clue what tools I'd need for that, how I'd need to brace the open transmissions so they don't flop around when I put torque on something...you know, practical tips of that nature that could prevent my ballsac from exploding from stress and frustration...

If noone has any practical pointers, I will bump in a week or so and see if anyone has remembered anything in the meantime...

[The Mighty Gryphon]

I think the dog Filmcamera was referring to is a protrusion on a transmission gear that makes it stay engaged.

Worn dogs can allow the gears to slip against each other, causing the problems you are seeing.

[Filmcamera]

Here I found this very useful





Also

[Filmcamera]

This is a really good video as well

[Motorhobo]

OK, those videos were super helpful. Even if you don't speak German, this one is awesome -- a master mechanic and professor at the University of Bayreuth completely rebuild a 1982 R100GS transmission -- it says the input shaft just pulls right out.



I'll read the Clymer and contact Drake to see if he's got an input shaft lying around. In the meantime, if anyone here has had the transmission open and could give any tips (i.e. whether you made a jig etc.) that's be appreciated.

MH

[Filmcamera]

I think WMax351 might be the guy to reach out to...


http://www.motobrick.com/index.php/topic,4542.msg28374.html#msg28374

[Motorhobo]

A ha -- missed that thread -- thanks!

MH

[Motorhobo]

Second gear now completely gone -- that didn't take long.

Decided to bite the bullet and get a used transmission off ebay to get the bike back on the road. As soon as I can I'm going to try to rebuild one good transmission out of the two fried boxes I have lying around.

The transmission that just failed had 179k on it, but still, my riding style from now on will reflect that the transmission, not the engine, is the weak link on these bikes and that finding a spare will cost some coin and be a crapshoot.

[Filmcamera]

Sorry to hear your transmission died on you.


When you do open it up please let us know what you find, maybe even with a photo or two.


I think lots of us would find a good transmission tear down thread very useful.


Good luck and I hope you are back on the road soon.

[Inge K.]

I think lots of us would find a good transmission tear down thread very useful.

http://www.motobrick.com/index.php/topic,1808.0.html

[Filmcamera]

Great thread, thanks.  I am left wondering how it all ended! Cliff hanger...

[Motorhobo]

Well, one thing's for sure, I'm NOT doing THAT! I don't have the tools, the skills or desire, at best I may try to swap out the entire input shaft assembly from one box to the other and keep it as a spare.


I'm curious too about how that ended...that's a lot of work. But if it had been me once disassembled I'd probably have taken the whole box to a pro and paid them to put it back together properly.


The transmission I just paid for on eBay is stated to come from a 90 K75 with 75k miles. Cost was about $250 including shipping. There are a number of K100 transmissions on eBay but most are asking +$400 with shipping. Drake has a couple of silver K100 ones but no black ones. I found the one I ordered by chance because the seller has misspelled 'transmission' in the subject line -- normally not a good sign for a seller but he doesn't have to rebuild it, all he has to do is box it up and ship it so orthography isn't too much of an issue for me.


Anyway, hope this one last a while. Bike it on the lift being disassembled now waiting...prime riding season and I have no two-wheeler!