tony27

Replace the bushes, when worn they put more pressure on the seals by allowing oil past. With good bushes in the forks the seals should last for years, make sure you release the air pressure that builds up during riding at the end of the day to help prolong the life of the seals

I had a 350, lots of chrome missing from around the exhaust port mainly so it rattled a fair amount but still ran well. Felt like being halfway up a ladder until I lowered the pegs, did my own disc brake conversion for the front wheel. Disappeared from sight after I sold it to upgrade to a 95 Gas Gas JT350, as far as I know it hasn't reappeared in the last 20 years

Shouldn't make much difference, 1 size bigger if anything but I would be surprised if the standard jetting wasn't a little rich

That's the gearbox mainshaft bearing, no seal needed there. Crankseal is behind the gear that the clutch case is covering on the right, water needs draining to remove the casing & if you pull the waterpump cover with the hose still connected it helps with getting the waterpump gear meshed correctly when reassembling. Probably a good idea to do the waterpump seals while the case is off & check the condition of the shaft at the same time

Something strange with your cases from the sound of it, should have the sealed main bearings by now like you describe & a viton crankseal between the bearing & primary drive gear

According to TY Trials the bearing is the same for all twinshock 250s, their wossner kit includes the bearing so it may be possible that when you get your kit there will be 1 included Not hard to get the dimensions of the bearing to cross reference sizes, measure inside of conrod, outside of pin & width of the bearing cage. Partzilla is normally good for seeing what other Yamahas use the same part, would be surprised if it's not a common part

Are the marks on the bore able to be felt with a fingernail? If they can be then you will probably need to go to the next size piston. Any of the quality aftermarket piston kits will be .5mm larger for each oversize where Yamaha had steps of .25mm From memory & looking at listings on eBay the 311 pistons are made by Izumi, the 438 were made by ART which are cast into the inside of the skirt. If you can stay with standard bore then here is the piston you want, the same seller has rings as well but it looks like it will end up costing more than either a Wiseco or Wossner kit cost - the kits from both are actually for early YZs so you should be able to get both makes fairly easily https://www.ebay.com/itm/YAMAHA-RT2-DT2-RT3-DT3-TY250-STANDARD-PISTON-96MM-OEM-NOS-438-11631-01-96/263601323881?_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D52885%26meid%3Da91bf7daaf9446aaa81b98d5252b7047%26pid%3D100005%26rk%3D1%26rkt%3D2%26sd%3D201687499348%26itm%3D263601323881&_trksid=p2047675.c100005.m1851 Here is a listing for the Wossner kit, you should be able to read the part number off the box & ask your local dealer to see if they can source you 1 https://www.ebay.com/itm/NEW-WOSSNER-PISTON-KIT-1974-1979-YAMAHA-YZ250-IT250-TY250-MC250-DT250-69-64-mm/192451397914?hash=item2cceff491a%3Ag%3AX4UAAOSw9vlaexpX&_sacat=0&_nkw=ty250+piston&LH_ItemCondition=3&_from=R40&rt=nc

Seeing those photos of the inside I definitely wouldn't buy that tank, if that is the state of the tunnel at it's highest point then the base will be a lot worse

TY Trials sell Wossner piston kits, not specifically made for the TY but if they're selling them you will be safe to use them. The price is good as well, think my 70.25mm genuine Yamaha kit cost me more than that from a seller in France on eBay https://www.tytrials.co.uk/trailandtrialsuk/prod_2768676-812-Full-Piston-Kit-TY250-Twinshock.html The 311 pistons are missing the taper on the skirt where the transfer ports are, the piston hits the crank without the taper & prevents the crank getting a full rotation. Piston is also heavier due to thicker skirt, all other dimensions are the same This is a photo I took back when I bought a 311 piston kit showing the differences from the underside, 311 piston on the left, 438 piston from my TY on the right. I have a couple more photos but this shows the differences best

DT250b/c, I did the same to mine after seeing these photos. Gives a different look to the motor but weighs quite a bit more than the original. Compression may be slightly higher but not by a huge amount if it is A small amount of trimming of the fins is needed for the exhaust to clear the head, I also took about 10mm off the rear to blend the head to the cylinder fins properly

Agreed, thermoswitch has stuck closed. Better that than jammed in the open position & the fan not working at all

I have my own story of small end failure. I was swapping the piston port top end on my CZ to a reed valve converted top end & got greeted by all this when I pulled the pin out, who knows how much longer it would have run before making a big mess. What probably saved me from a big blow up is the washers that centre the rod inside the piston kept the bits of collapsed bearing cage contained Sometimes we get lucky & avoid the big mess

Getting rid of the high spots on either side of the scratch is really important, if you fill the scratch without doing that you'll still cut the seals & wear out the fork bushes quicker

Top ring has a L shaped section & the piston is stepped in above the ring land, L shaped ring goes right to the top of the piston. Sounds like that's what you have & bought The piston kit you have shown doesn't have a good reputation, can be noisy very quickly even when bored to correct size

Bearings & seals are fairly common type, I don't see a C3 on the bearings so any bearing retailer should be able to supply them. Unlike the Japanese manufacturers the European companies used common sized bearings instead of having specials made that were only available through their dealer networks, often 1 dimension would be .5mm larger on the outside in Japanese bikes Seals should also be available from the same source, ask for 2 lip Viton seals if possible as these cope with modern fuel better the Nitrile seals which are what the originals appear to be

70mm is nominal standard bore size, if there is no step at the top of the bore or damage to the liner you are lucky. Lots of standard size piston kits around still, just make sure you don't get a 311 piston as they don't fit without modification. Easiest way to tell if the piston is correct is that the 434 piston has a taper on the skirt out to the transfer cut outs, the 311 doesn't & is thicker as well. The 311 piston hits the crank where the taper should be & will not allow the piston to get full travel

Not out of my Sherco Peter, that's still going strong. Big end came out of a crank from the states that I bought to build a spare motor for my CZ380

Pretty sure you should be okay fitting it as it is at the moment, if you were you move the gearlever while looking through the hole you would see the cam detail that the plunger pushes into. The gearchange will be more positive after you get the plunger & spring fitted

Lucky you asked about that bolt, the plunger is for keeping the shift drum stopped at each gear rather than just stopping in any random spot. Strange that the spring & plunger were missing

80:1 is fine with a good oil, running at 2%/50:1 just clogs up the exhaust quicker & produces more smoke

Big end can feel fine while being bad. This pin & bearing came out of a crank I didn't know the history of but felt good, would have failed fairly quickly if I has run the motor without fitting new parts

Or less pies but I'm not 1 to speak about that

If you do replace the fork springs then look at getting them from majesty yamaha, they're a possibly a bit firmer than standard at the beginning of the fork travel & gradually firm up through the rest of the travel. I fitted both the springs & damper rods at the same time & the improvement in fork performance is huge

The 3 other options Splatshop have are these, hopefully 1 is correct. If you email the guys they should be able to confirm if there is any functional difference https://www.splatshop.co.uk/leonelli-regulator-thyristor-12v.html 2 wire https://www.splatshop.co.uk/sherco-diode.html 4 wires as fitted to the older generation Shercos & Bultaco badged bike https://www.splatshop.co.uk/sherco-diode-comex-new-fan.html 4 wires as fitted to later model bikes that Scorpa also use

It's a 250, only TY250 clutch covers fit on TY250s unlike the 175s. The clutch side gasket is unique to TY250s as is the crankcase