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06 YZ 250 tranny and engine case probs

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ok guys have an 06 yz 250. first time tearing here down because 3rd gear today started clicking then would shift at all.:bonk: thinking shift drum on that. im very experienced in the four stroke world but this my first two stroke so bare with me lol. so i bought is used from a guy that said it had about 4 or 5 hours on the top end. he was a trail guy so i dont think he rode it hard. so i pulled the head and cylinder off and they are good. piston has very little carbon on top. the arrow and numbers are easily readable. the main problem im concerned about is the crank and left side case AKA ignition side. so on one side of the crank there is a blue/purple mark on it that is obviously from heat. i mix 40:1 with 100LL. should i be concerned about this spot? the left case looks like it has two cracks in it:banghead: running from the crank towards the ignition. i have yet to split the cases to see where it goes. What are my options?

sorry for the essay haha

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The blue heat marks are there from the factory so no need to worry. As far as the cracks go, I'm not really grasping where they're located and what they look like. Could you possibly take a picture?

A lot of times those cracks end up being casting marks.

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ok awesome about the crank. is there a measurement for the connecting rod play? im thinking if i have it all opened up i may put a new rod kit in it. yeah ill post a pic

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Second Rusky, without a picture it's hard to tell. I'd be worried if they're in the area that the jug would cover or running across the mating surfaces, if it's on an area that's normally "outside," like the housing for the stator, it's probably going to live. I've had bikes with chips and cracks along the casting marks on parts that are extruded like that; In that case, waterproof it and just keep an eye on it, start saving for a new case, and hope it gets there before it gives out. But that's me, if it's not on a mating surface, causing a leak or letting elements into the crank or trans, I'd run it out.

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there a measurement for the connecting rod play?

I don't know the exact measurement for an 06, but it's a very small margin. Hit up a site for a .pdf of your owners manual. Foreign Yamaha websites usually have them available for download. As a rule of thumb, if you can put one hand holding the crank at TDC and move the connecting rod up and down with the other hand to where it makes a slapping sound it's time to be replaced. Unless your RPMs are totally out of control and everything else seems to be okay it's probably not an issue.

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ok guys heres the pic. i must say you guys are fast! haha. so side to side play isnt as important as up and down? you can see the cracks im talking about running towards the left from the crank....how do i post a pic on here?

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I think you have to put it on photobucket or something first. I remember someone saying a little bit of side to side play isn't that important since ultimately the piston installed in the cylinder will hold the connecting rod centered. Vertical play on your low end bearing is what you're looking at. Maybe someone can back me up on that.

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Every yz250 bottom end has the things that look like cracks but are casting flaws. I have 4 sets of 250 cases and they all have them. I polished them out on my race engine. I would not worry one bit about the things that look like cracks.

Id worry more about your transmission. Look for wore shift forks. Any shift fork with ware maks in the center of the fork between the the 2 pads on each side should be replaced. Check all the transmission bearings also that they spin freely and arnt nochy even after you clean them out. Gears rarely go bad but when they do they will have extremely rounded enguagment dogs on the gears were they mesh together. You could just have a piece of crap stuck in the groove on the drum proventing it from shifting into that gear.

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ok thanks guys!!! ill just keep running that case. anything i should do while i have it apart? whats the best high compression piston to put in these bikes?

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ok thanks guys!!! ill just keep running that case. anything i should do while i have it apart? whats the best high compression piston to put in these bikes?

there is not one... unlike a 4t you don't change compression with the piston, it's done by machining the head.

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If your main bearings are peachy theres not much else to look at, just be sure to clean up the cases edges before you button it up. I use a razor blade to clean rtv and lightly hit up the rest of the seams with a scotch brite pad. You could do gaskets I guess, some guys are finicky about reusing them but I guess Im just lucky. Although if theres noticable damage or expansion they should be replaced. Also I take a second to throw some grease on the shift cam, starter gear, etc, check the seals on the crank, and clean it really well. Another thing to be mindful of is wear on the clutch basket fingers, little indentations from the friction plates. If theyre not awful you can file them out, that trick works once or twice. Hope that helps.

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ok. i see. even casting cracks are common on the inside of the block?whats the best piston for MX on these things? As long as my main bearings are nice and smooth they are good to go correct? what would make my clutch pull pretty stiff? cable is clean and moves freely

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Right, IMHO in a two stroke its not worth raising compression unless your using it solely for racing. Usually raising compression equates to needing to run race fuel. Its hard enough to get these things not to detonate without bumping compression as I know a few guys that have to mix race fuel with pump to bump compression to eliminate detonation, although they could have had thier squish corrected 100x to fix that problem with the money they spent on race fuel over the years.

While the bottom end is apart I would closely inspect seals and all bearings. I believe in replacing the main bearings every time Im in there. No rhyme or reason behind it but it could save you from having to split the cases again a week from now.

As far as your clutch goes, if your want an easier pull go with some HD springs... they will make a smoother pull. Also make sure your basket and hub ar not notched... they can make clutch pull rather difficult.

Forget about those marks that look like cracks... they are casting flaws, every Yz Ive ever been into has them. If you take a Scotch brite pad and clean them up they will disappear, which isnt necessary.

Your going to start a war getting into a piston discussion... some will say forged pistons are the best (ie. Wiseco) and others will swear by cast (ie. OEM). Just go with whatever you feel more comfortable with. Im not going to get into the specifics of each... as I believe each has its flaws, and each has its strong points. Just do a search and you can find as much info as your willing to spend time reading, then multiply that by a million!

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ok thanks. i've been running 100LL in the bike because i live on an airport and i have easy access to it. The guy i bought it from said it had a high compression piston in it so thats why i've been running that fuel.

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ive got the tranny apart, how do i tell if the forks are worn? im missing a whole tooth off of third gear which got ground up and bound up the shift drum.

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If you think there is a question about whether they are worn replace them... that is my rule of thumb. You can usually find a complete set in excellent condition on ebay for next to nothing.

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I run the 100LL, it works great in the YZ.

If there's contact scoring on the web of the shift forks from the gear teeth, it's bent. Also, if the plating is worn off the sides of the tips of the forks, they are worn.

Check the edges of the gear dogs where they engage. If the corners of the dogs are rounded off, you should replace those specific gears. If you replace the forks and leave in a bad gear or two, it could easily pop out and bend your brand new fork. Also, if you change out the gears and leave a bent fork, it can take out those new gears.

Use your best judgement so you only have to fix it once.

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ok thanks guys. the new shift for is 40 bucks from yamaha! along with a third gear which is like 50. what originally happened is it felt like it got stuck in third so i forced the shifter down and i believe thats what broke the tooth off.

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by the web do you mean where it bent in the "U" shape? and the plating on the tips is the raised area? what years of shift forks fit an 06? sorry for all of the questions!

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Dont buy used forks they will be as bad or worse then yours. Do yourself a favor and just get the new fork. The fork get loose on the shaft and slightly bends. It starts to ware in the center and on the raised ends becuase the fork isnt 100% streight in the slot in the gear. 5th gear fork witch is the bent looking one is the most comin one to go bad. I carry a brand new 5th fork for a spare and replace it when ever it looks bad when I split my lower end to do other things. Yamaha recomends replacing mains at 80-100 hours I believe. So if it were me id put a set in while your there. Ive seen oe cranks go in excess of 300 hours and still have acceptable play.

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