Well, hell, I can’t figure out what blew up first. A gear tooth, the broken shift fork(s), or the right side bearing of the output shaft. Nothing except the shift drum and the single fork look reusable.
If you had a choice between a ‘99 Monster 900 trans or an ‘04 ST4 which would you choose? The ‘04 is much cheaper and they both look to be in perfect condition.
Just a wild ass guess but I am going with a broken cage on the bearing from one of the rebuilds, this would have lead to a bearing failure and when the transmission shaft started walking around things went south with the rest. I have never seen a broken shift fork and the wear on the gear indicates it was wearing that way for a while. I would think it was making a racket for a while before it stopped functioning as well as the chain tension must have been changing as the shaft was only supported on the one side.
Between the 2 gearboxes i would choose the cheaper one if both are the same mileage, do your homework first and look at the gear ratios of both to verify they are not changing ratios on you.
will they fit? most likely yes.
Will you want the ratios? see if the tooth count is the same and decide from there.
Thanks @ducvet ! Part of the issue is I was stuck up in the mountains and had to make it back to town, which is a 60 mile trek. I found 3rd and 4th gear worked fine, so I elected to nurse it home. That probably took the forks further down the path of destruction. I'd probably still be replacing the case and transmission.
I'm glad I looked up the gear ratios. Thanks for recommending that. I didn't realized the Monsters were geared lower. I do a lot of freeway riding to get to the mountains, and I'd rather not have the engine spinning faster than it needs to. I found an '04 ST4 transmission that pretty much matches the 900SS ratios overall. The main difference being a primary ratio of 1.84 (32 teeth to 59 teeth) where my primary is 2.0 (31 teeth to 62 teeth.) The final drive ratio is 15/38 on the ST4 and mine is 15/37 stock. I can't remember, but I think I'm running a 40 on the back tire.
All well and good since a stock 900ss is 70-something HP and I dyno'ed in the high-90s after the last rebuild.
I got the replacement case today and I don’t think I want to use the bearings that are in it. I think this case sat on the shelf for at least 3 years, maybe more. The bearings felt “crunchy” when I first moved them, but freed up and rolled smoothly after a few back and forth revolutions. I don’t know if it was dried up grease or rust.
Can I transfer the bearings from my old case to this new case? I only had 3500 miles on the old case main bearings.
I would very carefully inspect the mains but with that low miles on them I would hope to swap them as well. to not be too rough on the mains you plan on using if you hammer in a dimple or create damage to a race it will fail prematurely.
I’ve almost got the case ready to go back together. I spent hours shimming the crank, input shaft, output shaft, and drum. I’ve got basically zero float on the crank, 0.1mm on the input shaft, 0.1mm on the drum, and I’ve still got .33mm on the output shaft. Everything spins super smoothly. Feels like a Swiss watch! (Whatever that means.)
So, I still have a little playing around to do on the output shaft shims. The current spacer on the left side (where the sprocket will attach outside of the case) is 1.65mm. I had another spacer that was 2mm and tried that, but it was .33mm short. So, if I go back to the stock 1.65mm spacer, I need to find a .68mm shim. I also had two 1.65mm spacers, but obviously that is too much.
I only see 0670.16.183 at 0.8mm as the smallest on that side.
As an aside, are there standard alternatives to the Ducati parts? Or can I only get these shims as Ducati parts?
I’m focusing on that side as the input/output gears line up better if I shim from the left of the output shaft. I don’t think I have any shims on the right side, but I need to double check. (After disassembling installing, assembling, measuring, disassembling, at least 4 times I got tired and quit for the night.)
It’s looking really good! Can’t wait for the ThreeBond to arrive on Wednesday!
the st4 will have a 5mm longer output shaft, so you'll need to space the front sprocket in 5mm to line the chain up. m900ie 00 - 02 have the 748 close ratio box. carb m900 all have the exact same input and output shafts as the 900ss/ssie
shims from ducati, unless you can find a generic shim supplier with the required shim id and od. i think some of the older gearbox shims are now nla.
I know it has been debated before, but there should be a slight preload on the crank to compensate for case expansion when hot. I've been using approx. .1mm to .2mm for years with no issues. The crank will have some noticeable drag when turned but drop under its own weight when the counterweight is 90 deg. from vertical. I know the spec was much tighter on the later engines but I think it was to comply with noise regulations. Seems like it would be kind of hard on the main bearings.
@wdietz186 Actually, that is right about what I did. Around .1mm. It was stiff to turn when I first put it together. Like the first turn was a bitch to get through. But once I finished that first rotation or two, it’s simply “snug” now with zero play and moves as you describe when the counterweight is at 90. I guess the mains had to seat themselves just a bit more. I‘m going to measure it one more time before I permanently join the cases just to be sure.
Alert! I’m having a WTF moment. See that picture of the output shaft where the sprocket goes? I had a groove at the end of mine where the sprocket keeper (little oval part) slide over the shaft and shifted in to place to keep the sprocket attached.
I looked up the ST4 parts and it uses the same kind of keeper as the 900SS! I did a cursory search and found what looks to be a lock washer and nut setup like on the 999.
I guess the good news is I have a transmission that looks like it was just manufactured yesterday. The bad news is I really have no idea what this transmission actually came from. But hey, it shimmed up really nicely and it shifts though the gears super smoothly. I guess I’ll need to source a 999 (748?) front sprocket, washer, and nut and find the correct torque settings. (And hope the chain lines up!)
st3, m1000, 1000ss, s4rs, sc/gt? you'll need a different output seal as the sprocket pulls up against the seal runner. the 1000ss uses a thinner seal runner than all the others due to the 5mm thing, but the seal runner goes up against the bearing and the lock nut gearbox has a dual row bearing there so i don't know what that does to spacing. if you used that shift drum the neutral light wont work anymore too.
@belter I used my old shift drum as it was in perfect shape. (I had two and picked the better one.)
In the case, the splines stop well short of the seal in the case. Are you saying the sprocket will extend past the end of the splines to rest against the case? Sorry I don’t have a picture of it in the case. I’ll try to get one later.
I found out from my local genius mechanic that the only difference between the output shaft of my vintage bike and the newer ones is the sprocket. So, after much patience and a little cursing at lock washers, I transferred all of the cogs from the newer shaft to my old shaft. Woohoo! We're back in biddness!
Everything was going perfectly until I put the alternator cover on using Threebond instead of the fiber gasket. I pressed in a new alternator cover bearing (end of crankshaft) and made sure it was firmly seated. I don’t think I’m going to be able to get away with using Threebond. When I put everything together and torqued to proper spec, I couldn’t move the crank. If I loosened all of the bolts to finger tight, the crank turned freely. That points to too much preload on the alternator cover bearing. So, I’m guessing I need to use the fiber gasket, at least on the alternator cover side. Am I right?
Do I need a fiber gasket on the clutch housing side? I’m thinking not because there is nothing on that side to get fussy about clearances.