Howdy all,

Slowly rebuilding my 1989 k100RS. While it has been slow I finally got it running with new tyres and major service items and was preparing for my roadworthy certificate.

Took the bike out for a spin, 1st gear all went well, 2nd gear nothing, lunchbox gear selector went blank > couldn't even get to third.

The gearbox was the only fluid I hadn't changed, was an oversight on my behalf. Since the bike was neglected I pulled all the oil and it came up very silverish/dark, which from what I have read can be ok, but isn't great. The oil didn't feel metallic but the drain plug was full of gunk and felt slightly metallic.

I replaced the oil and gave it another run, this time i could get into second but it just rev'd like it was in neutral, I made it to third and could ride around. Obviously, this is not ideal. I have read many forums and it seems like this is a pretty common issue, with the result being a replacement gearbox?

If the end result is a replacement gearbox it would have to be from a wrecker. However, in my mind there is no guarantee that gearbox works or isn't about to die. I hear its a 7-hour ordeal to remove.

What do others do here? Rebuild with the following parts? - https://www.munichmotorcycles.com.au/product/gearbox-overhaul-kit-2/

Is a gearbox rebuild complex for a non-mechanic?

Looking for some advice, much appreciated.

You will buy those parts except for the very small seal for around $80 from any bearing place they are very common sizes. It sounds to me like you have more than bearing and seals problem. Quite possible a selector fork is bent and gear dogs damaged or the grub screw in the selector shaft is lose but that requires removal. 7 hours is probably right for a first time removal but most here do it in 4 hours or so.
I have here many gears and internal parts which were given to me so for the cost of postage you can have them if I have good ones.
Dear old Bruce at Munich Motorcycles has the strange idea that he has a right to get rich at the expense of others.

RicK G wrote:You will buy those parts except for the very small seal for around $80 from any bearing place they are very common sizes. It sounds to me like you have more than bearing and seals problem. Quite possible a selector fork is bent and gear dogs damaged or the grub screw in the selector shaft is lose but that requires removal. 7 hours is probably right for a first time removal but most here do it in 4 hours or so.
I have here many gears and internal parts which were given to me so for the cost of postage you can have them if I have good ones.
Dear old Bruce at Munich Motorcycles has the strange idea that he has a right to get rich at the expense of others.

Hi Rick, really appreciate the generous offer on those parts and your help with my query. I guess I have no choice but to remove the gearbox and let you know what I need.

To me, I would feel more comfortable rebuilding than getting a replacement box, what are your thoughts here?

A replacement is just a pig in a poke

Is the gear shifter loose and sloppy?
Regards Martin.

MartinW wrote:Is the gear shifter loose and sloppy?
Regards Martin.

Hey Martin,

Not really, its pretty solid with a small amount of play.

The shift will normally have heaps of slop if it's the grub screw.
Regards Martin.

MartinW wrote:The shift will normally have heaps of slop if it's the grub screw.
Regards Martin.

Yep, I have read that. Think I just need to pull it out and check it out.

The problem could be a few things, as already mentioned. I'd also add that it could be wear on the selector drum, or, possibly slop on the detent wheel. 
Can't see why you shouldn't take it apart to have a look. You'd only be throwing it away if you decided to buy a replacement, so you've nothing to lose and everything to gain or learn.
As for the technical side, it's nothing too complicated, and the parts you might have to replace do not require special tools to remove.

If you find selector fork wear on the second gear (easy to spot, work out which is second gear and look for a bent fork or obvious wear on the side face of the fork) then don't do what I once did and decide that you've 'found' all the problem. The second gear cam on the selector drum is very pointy and the fork requires accurate positioning otherwise it can start to jump out of gear. If this happens under load the selector drum wears very quickly at the critical point and loses its ability to keep the fork and hence the second gear in place. Same thing can result from a sloppy detent wheel. If these terms/functions are a bit unfamiliar to you, then take the thing apart and work out how it functions - it's all straightforward and easy and satisfying to figure out.

Post some pics of what you find 

Suzi Q wrote:The problem could be a few things, as already mentioned. I'd also add that it could be wear on the selector drum, or, possibly slop on the detent wheel. 
Can't see why you shouldn't take it apart to have a look. You'd only be throwing it away if you decided to buy a replacement, so you've nothing to lose and everything to gain or learn.
As for the technical side, it's nothing too complicated, and the parts you might have to replace do not require special tools to remove.

If you find selector fork wear on the second gear (easy to spot, work out which is second gear and look for a bent fork or obvious wear on the side face of the fork) then don't do what I once did and decide that you've 'found' all the problem. The second gear cam on the selector drum is very pointy and the fork requires accurate positioning otherwise it can start to jump out of gear. If this happens under load the selector drum wears very quickly at the critical point and loses its ability to keep the fork and hence the second gear in place. Same thing can result from a sloppy detent wheel. If these terms/functions are a bit unfamiliar to you, then take the thing apart and work out how it functions - it's all straightforward and easy and satisfying to figure out.

Post some pics of what you find 

Feeling a little anxious as this will be my first gearbox, but I know I'll get some great help from guys like yourself. Appreciate the post and guidance.

Keep an eye on the shims as you dismantle it: the gear clusters/shafts have large, washer-shaped shims on their ends to control end float and these will either be on the end of the shafts, or remain in position inside the bores in the gearbox cover. They are not immediately obvious and can be overlooked. Later, after you find them on the bench/floor you can end up wondering where the heck they go: they are all different sizes and I defy anyone to have the ambition or dedication to measure things up and work out which shim sizes go where.

So, first job when you've removed the gearbox cover and opened Tutenkamen's tomb, is to locate and identify those shims. Second job is to fish out that ball bearing....oh you'll figure it out!

Suzi Q wrote:Keep an eye on the shims as you dismantle it: the gear clusters/shafts have large, washer-shaped shims on their ends to control end float and these will either be on the end of the shafts, or remain in position inside the bores in the gearbox cover. They are not immediately obvious and can be overlooked. Later, after you find them on the bench/floor you can end up wondering where the heck they go: they are all different sizes and I defy anyone to have the ambition or dedication to measure things up and work out which shim sizes go where.

So, first job when you've removed the gearbox cover and opened Tutenkamen's tomb, is to locate and identify those shims. Second job is to fish out that ball bearing....oh you'll figure it out!

hahaha, think I will start with a heap of photos in a sealed room so nothing can come in or get lost.

....I might as well carry on with the full slacker's guide to K gearbox tinkering.

There's only two tricks you need to know.

1: There's a ball bearing in there which, unless you're weirdly careful on disassembly, will jump out of position and dive into the oily depths where it will swim around and evade recapture. I do not do weirdly careful. I have a magnet on a stick; the ball bearing can swim as fast as it wants, but it cannot outswim its ferrous nature. The ball bearing is only the detent for the selector drum at its neutral position. It is held in place by a spring, which is under the strange and disconcerting-looking 'what happens if I undo this' black allen bolt on the gearbox cover. So, nothing too important there. The ball bearing, spring, and allen bolt can all be dropped back into the hole as the last part of reassembly after the cover is screwed back on. So, find them, put them on one side and leave them there until the end.

2: Along with the allen bolt, the other 'gosh' thing you might notice on the gearbox cover is a small 8mm or so core plug. Very useful this and you'll be glad it's there, although does nothing but fill a hole. This is because it's the hole that's important. On reassembly, you'll find the sprung loaded detent arm inconsiderately fouls the detent wheel as you try to bludgeon the cover back into place. Well, put down the bludgeon. Knock the core plug out with a punch, pick it up off the floor and wipe it on your sleeve, and stick an allen key in the hole-in-the-cover so's it holds the detent arm back. The cover can now be replaced, and you can even retrieve your allen key and replace it in your set. Pat yourself on the back because you're now a real mechanic!

Suzi Q wrote:....I might as well carry on with the full slacker's guide to K gearbox tinkering.

There's only two tricks you need to know.

1: There's a ball bearing in there which, unless you're weirdly careful on disassembly, will jump out of position and dive into the oily depths where it will swim around and evade recapture. I do not do weirdly careful. I have a magnet on a stick; the ball bearing can swim as fast as it wants, but it cannot outswim its ferrous nature. The ball bearing is only the detent for the selector drum at its neutral position. It is held in place by a spring, which is under the strange and disconcerting-looking 'what happens if I undo this' black allen bolt on the gearbox cover. So, nothing too important there. The ball bearing, spring, and allen bolt can all be dropped back into the hole as the last part of reassembly after the cover is screwed back on. So, find them, put them on one side and leave them there until the end.

2: Along with the allen bolt, the other 'gosh' thing you might notice on the gearbox cover is a small 8mm or so core plug. Very useful this and you'll be glad it's there, although does nothing but fill a hole. This is because it's the hole that's important. On reassembly, you'll find the sprung loaded detent arm inconsiderately fouls the detent wheel as you try to bludgeon the cover back into place. Well, put down the bludgeon. Knock the core plug out with a punch, pick it up off the floor and wipe it on your sleeve, and stick an allen key in the hole-in-the-cover so's it holds the detent arm back. The cover can now be replaced, and you can even retrieve your allen key and replace it in your set. Pat yourself on the back because you're now a real mechanic!

Beat me to it, what are the gotchas! Great information, although this scares me a little more.

Scared?
Scared is just nature's way of getting you to pay attention, so nothing wrong there at all.
Honestly, the shims, ball bearing, and detent arm the only tricks - if you can even call them that. Dismantling the clusters/shafts can require a bit more attention, and tools, but you're not likely to be going there.

Don't mix up the shims (like I once did  ) - there can be more than one stuck together with oil, this can catch the unwary (i.e. me) out.

The clusters/shafts might look like they'll never come out, but if you remove them as a clump along with the selector shafts and forks, and allow things to wiggle around a bit, it'll all come away. Of course, the forks and selector shafts will all slide apart in your oily, desperate, clutching hands and you'll think your short career as a gearbox specialist will have come to an abrupt end, but it's really straightforward to figure out how it all fits and you'll soon be awarding yourself your hero's badge. The forks have small round bearings on the end that locate in the selector drum. These will head for freedom like the ball bearing, but the magnet always wins.

Well put, Suzi!  If I may add, if someone has been in there before you, the chances are good that the small bearing on the shift fork for 2nd gear has escaped and is in the bottom of the gear box.  

I would suggest that after getting the cases split you spend a few minutes looking at the internals.  Look at the shift forks, are they bent or broken?  Are the dogs on the gears chewed up or broken?  Are the bearings on the ends of the forks on the forks and in the grooves on the shift drum?  Is the shifting mechanism bent or broken?

Run your magnet around the bottom of the gearbox and see what it picks up.  With any luck it will come up clean.  

About the 7 hour number.  If you have spent any quality spanner time with this bike, 7 hours is what I would figure a round trip in and out of the transmission to be with some minor fiddling about with the internals.  It's how long it took me to do a complete repair for a loose grub screw on the first day I ever owned a K bike so I could ride it 500 miles home the next day.  I started in around 9:30 after a leisurely breakfast, finishing a test ride and getting everything cleaned up nicely for supper that evening at 5pm.  That 7 hours included messing with the bearing that allows the shift fork to follow the groove in the shift drum.

Thank you both for the information, really appreciate it and the vote of confidence. I have definitely got some work to do and I will be in touch with some pictures soon (if permits).

Thanks

I'm surprised no one has mentioned this yet but why not just swap out with a second hand gearbox instead of putting yourself through the anguish of going through repairing one, especially if you don't have the confidence to do so?  

The BM Workshop here in Brisbane currently has 2nd hand gearboxes for $110.  I am sure that Discount Motorcycle Wreckers in North Coburg would have something for a similar price.  From memory a g/box weighs around 20Kg, in case you need to have it shipped.  

You should have the standard guarantee of three (?) months for anything second hand and both businesses rely on their reputations.  Private sellers would likely be more expensive with no guarantee.  You could then rebuild the current gearbox at your leisure if you felt the need.  

Just a thought.  

Dave.

TacKler wrote:I'm surprised no one has mentioned this yet but why not just swap out with a second hand gearbox instead of putting yourself through the anguish of going through repairing one, especially if you don't have the confidence to do so?  

The BM Workshop here in Brisbane currently has 2nd hand gearboxes for $110.  I am sure that Discount Motorcycle Wreckers in North Coburg would have something for a similar price.  From memory a g/box weighs around 20Kg, in case you need to have it shipped.  

You should have the standard guarantee of three (?) months for anything second hand and both businesses rely on their reputations.  Private sellers would likely be more expensive with no guarantee.  You could then rebuild the current gearbox at your leisure if you felt the need.  

Just a thought.  

Dave.

Hi Dave,

We did discuss second hand at the very top and the fact there is no guarantees would conclude possible further work and cost.

I did see that one at BM workshop, it had 186k written on the side, which leads me to believe that it too could be written with problems too.

If I pull the box and have a look, at least I bear the cost of my own time and I could get someone more experienced to finish the job.

Dallas

No worries Dallas.  

Although the photo on the BM shop's website is probably a generic photo I expect he has a few in stock.  I know he can be a bit anal at times but if there was any doubting the gearbox, he would just bin it.  

I don't think the mileage is an issue as the gearboxes are quite robust unless they have been abused.  My spare parts bike has 245k on it and the owner had it maintained by the BM shop.  I would have no hesitation in swapping it out if I needed to.  

just my thoughts.  Good luck.  

Dave.

TacKler wrote:No worries Dallas.  

Although the photo on the BM shop's website is probably a generic photo I expect he has a few in stock.  I know he can be a bit anal at times but if there was any doubting the gearbox, he would just bin it.  

I don't think the mileage is an issue as the gearboxes are quite robust unless they have been abused.  My spare parts bike has 245k on it and the owner had it maintained by the BM shop.  I would have no hesitation in swapping it out if I needed to.  

just my thoughts.  Good luck.  

Dave.

Thanks Dave, appreciate it. You might be right too. Since its a project bike, plenty of time to work through it.

Hi Folks.

I'm experiencing the exact same symptoms described above. Sitting at just about 65K miles (105K km) and second gear is starting to pop out under load. (Currently accelerating to 6000 rpm on flat ground, and 4000 rpm on modest incline will cause the skipping.)

Given that mine is an '89 model of the 8-valve variety, does anyone know if the transmission from a '91 LT will fit my bike? If yes, are there any benefits or challenges to doing so?

The gearbox will most certainly fit. Ratios never changed (ratio variations in the ks were achieved with different final drives) The only differences were the addition of lugs (maybe fully machined, maybe not) for the 16v paralever torque arm front mountings. 
If you can find a cheap replacement gearbox just swap it straight in, Keep the old unit and maybe rebuild it - not that difficult if you rely on the help from this forum. Yes, mistakes can/have been made, but by us (e.g. me) not you!

Suzi Q wrote:The gearbox will most certainly fit. Ratios never changed (ratio variations in the ks were achieved with different final drives) The only differences were the addition of lugs (maybe fully machined, maybe not) for the 16v paralever torque arm front mountings. 
If you can find a cheap replacement gearbox just swap it straight in, Keep the old unit and maybe rebuild it - not that difficult if you rely on the help from this forum. Yes, mistakes can/have been made, but by us (e.g. me) not you!

Beautiful, thank you Suzi Q!

Suzi Q wrote:BTW, your quote could be honed in a bit to what Donald Campbell CBE actually said:


 “Courage is not being fearless. Courage is overcoming and smashing through fear.”


Which basically says a lot about the size of the guy's testicles 

Yeah, screw that FDR guy, whoever he was.

Suzi Q wrote:The gearbox will most certainly fit. Ratios never changed (ratio variations in the ks were achieved with different final drives)

Well actually they did change the 5th gear ratio for the 16v engines from 1.67-1 to 1.61-1 by puting 30 teeth on the helical cut gear on the input shaft where the 2 valve K100 and all the K75s had 29 teeth so there you go add that to the useless info folder of the file.

Never had gearbox issues other than bearings not shimmed correctly. not worth the cost of parts get a 2nd hand from the wreckers. but wouldn't hurt to open it up and have a look to see whats going on first. Where in vic r u