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Archilla: a couple of fun videos for you:

My old 860GT had the right hand selector mechanism. I took it apart once to clean it; heed flynbulldog's advice regarding the spring. Also watch out for the little detent ball and spring. I don't recall having the issues putting the casing cover back on as shown in the YouTube video above. I recall my gear shifts being very positive up and down with no false neutrals. That little eccentric screw is supposed to balance the up and down movement of the shift plate to be the same. I recall tweaking mine a little once I'd cleaned and lubed everything to get a sweet spot.

I highly recommend watching all the Desmowerx videos; I love this guy. Not old bevels but much of his approach/methodology are applicable. Also very pertinent to the Multistrada; mine is new so I don't plan on this stuff just yet. The video above shows a good way to determining shaft end float tolerances. Easier on our older bikes, 0.00mm for the crankshaft and 0.2-0.4mm for the gearbox shafts. Way back in 82, I remember going through the measurements and math to ensure that the crank was centered with the new gasket only to find the original factor shims were spot-on.

Post a pic or two new friend! I'm down to just two these days.

990213
 

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The gearbox problems were caused by a mixture of convoluted cross-over shaft which gave only partial engagement of the dogs and the extra torque of the 900 engine over the 750. It may be possible to prevent it happening on a new box with careful shimming of the gearshafts. Even after Ducati fitted a proper left-side selector mechanism the gearbox problems persisted and were only cured when they moved to a new three-dog gearbox, which was more reliable but gave horrible backlash in the drive train, which was the reason for the six-dog design in the first place.....

The six-dog design is brilliant at removing backlash from the drive train, but because it requires complete engagement and only has three shallow dogs to accomplish it, and because it is only surface hardened, is prone to wearing those dogs, which then causes the gear to jump out. The Norton Cosworth had the same design and was described as a 'very advanced design' and when Cook Neilsen had his gearbox made by Marvin Webster to cure wear problems in racing, Webster remarked that 'There is nothing wrong with the design, it is just that the material's no good.'
 

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Take a pic before you start ! Mark position of anything you plan to change.
 
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Discussion Starter #24
Brilliant...helpful and interesting replies all. Thanks for the tip on leaving the actuator arm attached when opening the cover to prevent the return spring from making things a jack in the box surprise. The selector box is off and I'll open it up this week for a look. Question: are issues with the selector box normally limited to the condition and/or installation/adjustment of the return spring and the fork, or have you seen issues with the detent spring/ball or the selector wheel as well?

@rick81mhr900ss Thanks for the videos, especially the second one is really helpful to understand the arrangement and movement of the forks and gears at each change. No beauty shots of the 900SS yet, as since I've received it I've pretty much had it in various states of disassembly sorting out various issues. Here is basic shot of how it looked the day it arrived. Also a shot of the Multi, which is basically a stock '13 S Touring. The other bikes currently in the stable are not Ducs, so perhaps not of much interest around these parts...

990287

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Hi everyone,
New to me '77 900SS, first real ride on it today and found that it will not stay in 4th gear. Shifts into 4th just fine (either up from 3rd or down from 5th), but pops out of gear shortly after, even at light load or engine braking. All other gears work fine, and it shifts easily and cleanly into every gear. Any ideas or advice?
The problem may be in the shift box and not in the transmission. The shift shaft has a small threaded bolt with a flat screw slot on one end secured by 10 mm nut on the outside of the box. The other end has an eccentric bulge inside the box. The position of the eccentric end controls the fine tuning of the two pronged shift fork in the box. The fork ends move studs on the round shift plate corresponding to the 5 gears. The other side of the plate has divots in it that correspond to the gear selected. There is a spring pressured ball bearing that when the shift shaft is moved, the ball is moved into the divot on the back side of the plate and the gear changes. The two prongs of the shift fork must be equal distant on the inside to the studs in relationship to the two fork prongs. If they are not equal distant, miss shifts are prone to happen along with false neutrals. There also may be damage to the inside of the shift fork where they contact the plate studs. It would be best to start off by loosening the 10 mm securing nut on the outside and turn the bolt either clockwise or counter clockwise so the linkage arm from the shift shaft moves back and forth. You want the shaft to be in middle of that movement. Once there, tighten down the nut while maintaining the screw bolt position. Try shifting manually with the rear wheel off the ground. Shift through each gear and listen for a second engagement sound between each shift. You want to hear that second sound as that tells you the next gear is ready for engagement. If you don't, try changing the position of the adjustment screw a little bit and try again to hear all secondary sounds between shifts.
 

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Discussion Starter #26
The problem may be in the shift box and not in the transmission.
Thanks David. Yeah, I tried adjusting the eccentric on the selector box through its full range (in small increments) on the road. It did make a difference, and in some positions allowed 4th to be selected and remain engaged, however it would cause problems with other gear selections. No position of the eccentric adjuster allowed for proper shifting through all gears. The next step is to open up the selector box and check the condition of the shift fork, return spring, etc. I'm still hopeful the solution will be found in the selector box rather than in the trans itself.
 

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Most likely 4th now will require undercutting and machining - Which means splitting cases, full shimming, blah blah blah - You CAN convert a 860/900 Bevel back to correct shifting and get rid of all those crappy cross shafts and sloppy links, easily -
Search eBay for a shift selector box from a widecase single (which would be WAY easier/cheaper than a correct side shifting twin) and use the splined shaft from it (the one the shifter is supposed to bolt onto)
you remove the one that has no splined stub, and all the jackshafts and bellcranks, and install the one for a shifter, and move the brake back to the correct side.......
I have done this on a few 860/900 GTE and GTS, but they also had the "jumps out of second" problem and needed dogs undercut and 6-dog gears turned back into 3-dog gears.....
 

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Discussion Starter #29
You CAN convert a 860/900 Bevel back to correct shifting and get rid of all those crappy cross shafts and sloppy links, easily -
I have heard of this, and it seems pretty straight forward except for the relocation of the brake to the left side. That said, on an SS, has anyone ever tried leaving the shifter on the left and just finding or fabricating a different shift lever to connect to the actuator arm on the crossover shaft via a link, just like basically every other rearset shift lever on the planet? By far, the weakest point of the whole crossover linkage arrangement on the SS models is the connection between the shift lever and the actuator arm; probably 90% or more of all of the slop and friction of the whole system is in that terrible pin/slot interface...

I opened up, checked, lubed and reassembled my selector box, it's quite easy to do actually. If the spring is not broken and is correctly installed, it holds the spring plate, fork and input shaft together; they can be carefully removed and separated from the rest of the assembly to clean, lube and adjust things. As mentioned elsewhere, the return spring is incorrectly installed in the YouTube video above. When the spring is correctly installed, the spring plate and fork are centered/aligned with each other and the fork centers over the selector wheel, meaning the cover can be easily reinstalled without having to force the fork into a centered position. All I found in mine is that it was basically dry (no grease anywhere) and the selector wheel was not turning smoothly (the shaft was binding a bit in the bushing that it sits in). The fingers of the fork, and the pins on the selector wheel had no nicks or wear at all. I've got the selector wheel turning smoothly now, and after reassembling it selects each detent position precisely and the fork returns quickly after each shift when manually bench testing it.
 

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I don't know how big the splined shaft is on your model, but if it is the same as the later 900SS, it would be easy to fit the later model's footrest, splined arm and linkage.
 

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Thanks David. Yeah, I tried adjusting the eccentric on the selector box through its full range (in small increments) on the road. It did make a difference, and in some positions allowed 4th to be selected and remain engaged, however it would cause problems with other gear selections. No position of the eccentric adjuster allowed for proper shifting through all gears. The next step is to open up the selector box and check the condition of the shift fork, return spring, etc. I'm still hopeful the solution will be found in the selector box rather than in the trans itself.
Looks like you will need to take the selector box apart and inspect. Look closely at the condition of the contact area of the selector forks with the pins. Sometimes those heat treated areas of the fork get buggered up. Careful filing the damaged area(s) can help. Another trick is to stretch or shim the ball pressure spring or find another ball that is slightly larger. Make sure all internal components are there. Hopefully the problem can be solved there and not inside the cases. Good luck.
 

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I converted my '77 SS back to right shift soon after I bought it using the 750 GT/Sport selector shaft and lever at the shift box. I did have to fab linkage and weld a tab on the other side of the frame to mount the M/C but it wasn't rocket science and done very cheaply as I was financially challenged at the time. As for the popping out of gear I'm afraid the fears are real, mine took a dump too when I was racing. The problem with the gear is the dogs are machined into the gear and the diameter of the dogs/gear mean that areas of the dog have thin sections where the root grooves for the gear teeth are directly behind the dog engagement faces, meaning there are weak spots on the dog. They chip/break and round off with use. Fixing it isn't super hard but it will take at least two special tools, the flywheel puller and an alternator rotor holding tool. Disassembly isn't hard BUT there are shims everywhere and must be kept track of. I wasn't unheard of to find the shaft shimming off from the factory so checking and re shimming is likely. You'll need a good dial indicator and a micrometer too. If it hasn't already been done, replace the ignition with a Sasche unit because the stock setup will crap out on you and replacements are non existent.
 

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I was just watching Robax's first ride video after his rebuild and one statement caught my attention after reading this thread. In it he states that 3rd or 4th gear on these square case 900s is flawed and failure is common. VeeTwo has a replacement that is much better than the original. In fact VeeTwo has a full transmission if you are so inclined.

So, I still suspect the 4th gear dogs and not the selector box...

I hope Archilla will return here and let us know what's happening.
 

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Discussion Starter #34
Slow progress. I've got other things demanding my time (mostly a newborn baby), so not much time to fiddle with the bike at all. I didn't expect a project when buying this bike, but oh well. Life goes on. I'll make it right eventually.

So far I can say a few things. There were some small issues in the selector box that I rectified. The selector wheel/shaft was not moving smoothly and was binding slightly in the bushing it rides in. I've also tried turning the selector drum directly via the stub end that comes out of the transmission and selecting gears while just turning the back wheel; every gear position, including 4th, can be selected positively and cleanly and remain engaged by directly turning the drum. Finally, I can reiterate that the stock linkage, upstream of the selector box, on these '76/'77 SSs is objectively terrible. The issue is not the crossover shaft itself or the number of connections in the linkage, but rather the specific connection between the shift lever and the actuator arm attached to the crossover rod. It's a sloppy sliding pin/slot interface with a lot of friction and freeplay; it's an inefficient and imprecise design when the parts are new, it only gets worse when the parts are worn. It's hard to appreciate how bad this detail of the linkage is unless you've seen the specific execution on one of these crossover shift Super Sports in person. Anyway, this sloppy connection between the shift lever and the rest of the linkage is such a fundamental part of whether or not a solid and accurate input is made to the selector box (and subsequently the shift drum) that I don't think it is worth proceeding until I address it. Converting to RH shift would be one idea as others have suggested, but I don't believe it needs to be that drastic; the various parts of the linkage just need to be properly connected with real rotating joints (ball end heim joints). So, I'll be doing some fabrication so that I can fit rearset pegs/levers and connect the shift lever to the crossover shaft via a proper linkage rod with balljoints at either end (similar to the stock '78+ SS peg/lever arrangement). Once I'm sure the linkage is precise and I know the transmission is getting good, accurate inputs, then I'll reassess the shifting on the road.

@wdietz186 Your description of the root grooves of the gear teeth being machined right down to the engagement dogs is clear and makes a lot of sense as a weak point, but I only see that happening on 5th gear (on the layshaft). On all of the other gears in the box, it seems there is plenty of radial clearance between the outer diameter of the engagement dogs and the root of the gear teeth. If there is a specific structural weakness with the dogs on 3rd or 4th gears, what is it? Having not seen the parts in person, I certainly could be missing something, but by looking at photos of parts (individual gears and complete gearbox assemblies), I can't see anything that stands out about the design or machining of 3rd/4th that looks uniquely problematic.

-R
 

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I go back to Cook Neilsen, who won the Daytona Super Bike Race on one of these bikes, but during the course of development had to fit new fourth gears for every race, and his bike was right-hand shift. In order to keep racing he had a new, complete gearbox made in the USA, copied exactly from the original, by Marvin Webster, who stated that there was nothing wrong with the design, but the problem lay in the fact that the gears are made of soft gear steel, and then case-hardened, which is OK for a low torque engine, but not for high torque, which is why the 750 Ducati had no problems with the same gearbox, but the 900 did. Basically, the hardening on the dogs wears, allowing the corners of the dogs to round off and then to slip out of gear under load.

To put this in perspective, when Suzuki introduced the bullet-proof GS750, it used a case-hardened gearbox, but when the GS1000 came along it had a gearbox made of steel that was hard all the way through.
 
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