YZ250 Rebuild Help

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11/29/2017 2:39 PM
Edited Date/Time: 11/29/2017 3:16 PM

I grenaded the piston on my wheelie boy rescue YZ250 (2013). The piston pieces went all down in the crankcase and the rest is history. I replaced the main bearings, main seals and crank with all OEM parts. I had the cylinder punched out and ported and started the reassembly process. It has been a long one to say the least.
The first time I put it back together, I didn't have the shift drum rotated far enough back to help the main shaft fall in and resulted in a little binding. I split the cases again and fixed the issue. The second time around I could not get it to shift. The transmission moved great but the shifter would barely move. Tore it down and realized my buddy had let a gear fall off the main drive shaft and put it on backwards. Flipped that gear around and everything dropped in just fine. The shift drum moved and the gears moved just fine before I put the cases back together. Now the shifter moves like it should and there is no binding but it will not shift up and down gears. If I turn the clutch, the crank and transmission spin but if I turn the countershaft, the crank does not spin. Spinning the crankshaft does not spin the transmission either. I am pretty mechanically fit but this has me getting really frustrated. Any help?

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11/29/2017 6:04 PM

Shift forks look okay and straight? No chips or worn gears?

RMATV has a video of a tranny reassembly, albeit with WR gears here:
https://www.rockymountainatvmc.com/p/895/73725/Yamaha-Wide-Ratio-Transmission-Kit

Same concept, same bike, different gears.

Sounds like it is in neutral but will not go into gear.

Ensure all the gears are in the correct spaces as well as shift forks.

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If you're not mixing gas, you're not haulin ass.

11/29/2017 11:28 PM

Best thing to do when rebuilding a bike like this is to put the tranny together and leave the crank out, then put case halves together and check to make sure it shifts good. When it shifts good then install the crank, it make's disassembly easier if u have these kind of issues. Also when shifting through the gears spin the clutch shafts as u shift, it sounds like you may have to start from ground zero making sure everything is together correctly in the tranny and go from there

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11/30/2017 9:18 AM

ktm-5 wrote:

Best thing to do when rebuilding a bike like this is to put the tranny together and leave the crank out, then put case halves together and check to make sure it shifts good. When it shifts good then install the crank, it make's disassembly easier if u have these kind of issues. Also when shifting through the gears spin the clutch shafts as u shift, it sounds like you may have to start from ground zero making sure everything is together correctly in the tranny and go from there

This is the first time I have done a bottom end and after the second tear down, I realized I should have waited and made sure everything in the transmission was correct from the get go. Lesson learned.

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12/1/2017 8:23 AM

Just a tip that's useless now but can help in the future is that you can split cases & replace the crank on the yz's without taking apart the tranny. Just leave the shift drum bolt in and you're good to go. Another tip is to make sure the cutout hole on the shift drum faces down when you install it, this allows the oil to exit it when running. That should get you in the ballpark and from there it's just lining up the star on the clutch side (assuming your shift forks, posts and drum are in good shape). The yz trannys are easily the hardest two stroke trannys to get together. Hope ya get it all figured out.

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12/1/2017 10:10 AM

ktm-5 wrote:

Best thing to do when rebuilding a bike like this is to put the tranny together and leave the crank out, then put case halves together and check to make sure it shifts good. When it shifts good then install the crank, it make's disassembly easier if u have these kind of issues. Also when shifting through the gears spin the clutch shafts as u shift, it sounds like you may have to start from ground zero making sure everything is together correctly in the tranny and go from there

Man that is a good tip. Transmissions can be frustrating, but rest assured they are like a puzzle. They go together nice and easy and if they don't, you are doing something wrong. You should never have to fight pieces for them work.

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12/1/2017 10:34 AM

RockyRider wrote:

Just a tip that's useless now but can help in the future is that you can split cases & replace the crank on the yz's without taking apart the tranny. Just leave the shift drum bolt in and you're good to go. Another tip is to make sure the cutout hole on the shift drum faces down when you install it, this allows the oil to exit it when running. That should get you in the ballpark and from there it's just lining up the star on the clutch side (assuming your shift forks, posts and drum are in good shape). The yz trannys are easily the hardest two stroke trannys to get together. Hope ya get it all figured out.

Good info in this thread and glad I'm reading this as I'm about to do a YZ tranny for the first time.
On YZ's; it's not like others where you can put in the tranny shafts, then the drum, then the forks,
and the swing the forks onto the drum and, lastly, insert the shift fork guide pins? I see in that
Motorsports tutorial that they put shafts/forks/drum all in as one unit. Yikes!

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12/1/2017 11:43 AM
Edited Date/Time: 12/1/2017 7:00 PM

FWYT wrote:

Good info in this thread and glad I'm reading this as I'm about to do a YZ tranny for the first time.
On YZ's; it's not like others where you can put in the tranny shafts, then the drum, then the forks,
and the swing the forks onto the drum and, lastly, insert the shift fork guide pins? I see in that
Motorsports tutorial that they put shafts/forks/drum all in as one unit. Yikes!

Yamaha staked the shifting forks to the pins on the YZ250 design. Unlike other gearboxes where the shifting cam actuates the forks to slide across the stationary pins when a gear change is selected, the Yamaha design moves the entire fork and pin assembly simultaneously as a single unit. The pins actually move left to right inside the cases instead of remaining stationary like other makes. This design makes reassembling the transmission more difficult than, for example, a Honda gearbox.
The YZ250 transmission is the bike's achilles heel. Combine a 3-engagement dog gear design with a weakly supported 4th gear, and truly questionable gear material selection/heat treatment and the net result is you can expect to be getting a lot of practice splitting the cases.
The box shifts well, but durability and engineering design is not it's strong point.

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12/1/2017 2:49 PM

Yeah, it’s a real bitch. I’m a lefty which made the process even more of a pain in the ass. Looking at my shift drum and forks I thought I had it all good to go but....I guess I don’t. I too followed the Motosport video but that video leaves out a lot of little details like the orientation of the shift drum. Photo
Photo

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12/1/2017 6:39 PM
Edited Date/Time: 12/1/2017 7:02 PM

Put the cam (drum) in neutral. Assemble the transmission in neutral.
As KTM-5 told you- remove the crank. When you are satisfied with your assembly of the transmission, assemble the cases and tighten a few bolts without the crank installed. Install shift shaft and lever. You'll need to rotate the mainshaft ("clutch shaft" as KTM-5 phrases it) with your hand while operating the shift lever to engage each gear. Make sure each gear 1 thru 5 engages smoothly without the mainshaft or countershaft binding while you turn them. Make sure you can also find a good neutral with the shift lever. Only then re-install the crank and mate the cases together. Yamaha does not use a center case gasket like Honda and other makes do. You must use a thin coat of Yamabond or Hondabond to seal the crankcase mating surfaces at final assembly.

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