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I can help you with this one, (done heaps of them over the years in our workshop) Remove the Gear Selector Arm spring holding bolt. (In your photo it is the M6 bolt that the curved end of the spring sits against. in a little grooved bush under the bolt head. Part number T305020). Then you can let that Gear selector arm swing to the left. Now you can assemble the drum down onto the shaft. Put it on 180 degrees to the way you are holding it in the second photo. That open cavity faces down, and those two little rollers on the shaft you will probably have to sqeeze in a bit with a little pick or screw driver to get it to go all the way down. Those little rollers then sit in those little cutaways you can see on the inside of the drum. Then rotate the drum until the neutral indent (see the "star point" that has the curved relief in the middle of one of the "points" line up with that Gear selector arm roller you previously looosened). Then put the M6 srew back in so there is that tension on the arm holding that roller in place in that Neutral indent. It is easily achieved using a big phillips head screw driver to pull the spring around. There is a perfect hole under the spring to put the screw driver in , lever the spring around and fit the bush and screw. It is important you set this to Neutral as it makes getting all the gears and forks back onto the shaft possible in the next step.

The part number is 4GG-85540-00 If you internet search by that number you will find them for sale.

The first workshop manual GasGas produced for the TXT trials bikes (2003 until now) was in 2018. Prior to that all you could get was a User Manual and an exploded parts diagram. The 2018 Workshop manual is 95% correct for your bike anyway, so you will be able to figure it out. You can find all the manuals here: GASGAS Legacy manuals

Take the header pipe off and just check the rings and piston skirt through the exhaust port as you rotate the engine. Also make sure the fan is coming on when it should. You can have plenty of coolant but if it gets hot and the fan is not working it will go lean and you could head towards a bind up situation. And I would definitely be swapping oil brands that looks manky!

There is a "faster than fast" option available! TRS sell a modified Domino white tube used on the GOLD TRS models that has a little ramp attached to it, that makes it turn faster than the white tube Fast Domino. It is TRS part number 70129

Main bearing wise make sure LHS bearing hole is aligned with appropriate hole in case when reassembling, (2003 - 2018 bikes) and gear box just take it apart, and put back together with the gear set in Neutral, and it's easy. You'll need a flywheel puller, and a primary drive gear puller (RHS of crank). That's it as far as "special tools".

I google translated that into English and I still have no idea what he is talking about!!!!! "Clutch Lever Flexibility Hello everyone . On my 2004 250 xt I saw that there were no springs but a "plate" spring. I find it a bit harsh. Is there a less tough board? THANKS ."

Just my take on bleeding Trials brakes (and we do a lot of them as I have a bike business) is that you are way better "pulling" the fluid through at the rear bleeder screw, than pushing it through like you are. If you have air behind the caliper pucks pushing fluid in does not clear that. The fluid takes the path of least resistance and goes from the bleeder straight to the reservoir. Much better having the reservoir cap off, fluid level full, and draw the fluid through the system, this then draws out any air bubbles deep in the calliper. I have always found this a much better way to bleed rear brakes. Couple of notes: always make sure you have a little freeplay on the actuating rod, so the piston comes all the way down. That makes a clear path from piston bore to reservoir. Try and use a vacuum bleeder for the best results, but a syringe works also, just not as efficiently. And don't worry about seeing bubbles in your tube from the bleeder. When doing this method you will always get some air draw from around the bleeder. Just pull a full reservoir of fluid through and that should be sufficient.

If the 10th character is M it is a 2021 bike.CHASSIS#EUROPEAN VIN description (GasGas by Torrot).pdf

They are a Torx 100 to remove the bungs.

S3 make a specific one for the Jotagas Hidria Flywheel weight. This is it; https://www.thehellteam.com/s3-flywheel-weights-for-gg-idria-3539

The Braktec Part number is: 853005MO0 The GasGas Part number is: BT460103013CT

Lower of the 3 water pump mount bolts. it has a copper sealing washer on it.

You can actually use a flywheel from a 250/280/300 - it is substantially heavier than the 125 one. Only thing that needs to be done is the spacer removed from the sensor to set it back outboard to accommodate the bigger flywheel, but that is all. Same stator, same cover etc etc. Personally I don't think it "improves" the 125, just makes it doughy, but it's easily done.