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86 hours on stock top end?

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To start this off i'm sorry if this is a repost, i've searched without much luck. If there is a old thread please point me in the right direction. Alright my dad just picked up a 2008 YZ250F, he got it with 82 hours and the previous owner had not replaced the valves, piston or done anything to the motor at all. The valves are in spec and it is maintained well. I don't know when he plans on rebuilding but i know he wants to get another season before a rebuild. I am just worried that it will blow up and our season will be cut short. So basically what i am asking is if it will last another season or if he needs to rebuild it soon.

Sorry for the lack of knowledge and thanks in advance.

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If its only got 86 total hours on it, I'd say the piston and cylinder are probably ok. Sure check would be to do a leak down test at a shop. I've been 125 hours on a motor with unknown hours on it when I got it, but had a failure at that point. Recommend at minimum to replace the cam chain because that is a high wear item. I'd inspect the small gear on the crank also for pointed sharp teeth, sure sign of tooth wear. It will jump time, which is what mine did. It took out the intake valves, so I did a total top end at that point and replaced the crank, chain, valves, had the seats cut, shims, piston, pin, clips, rings and gaskets. Good luck.

Edited by ronbuell

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I'm originally a Buckeye man myself, so I'll throw in a few suggestions also.

Stock cam chains are recommended to be changed out every 50 hours, after all, it is a YZ. I have heard that if you have a MCCT (Manual Cam Chain Tensioner), that one can get away with it lasting up to four times longer. Most guys just go ahead and replace the piston and rings with a new piston pin and clips also, and have the bore re-plated or flex-ball honed depending on the barrel specs at 50 hours.

Now that may be overkill as far as replacing the piston and rings at 50 hours; but, I only ask this, would you rather spend $1200 for a catastrophic failure later or way far less now to keep it in the safe tolerance threshold?

Valves have been known to last into the 200 hour range with no issues, so long as the stock valves are not below a 1.50 shim size on seats that have never been cut to accommodate a second set of new valves. If they are below that shim range, it would be best to verify if the previous owner had the valve seats cut to re-verify running smaller than 1.50 shim sizes. If that is the case, then please disregard the shim size suggestion.

Usually at about 1.50 or lower on the shims, the valve has sunk into the seat pretty good and they start to "tulip" and they are getting close to their service life end because the hardened coating on the valves is pretty much gone at that point. Again, disregard that if the seats are known to of been cut, because it makes my suggestion moot.

A general rule of thumb, is to only shim a valve twice and then replace it after it goes out of specs the second time - this is a good rule, because when you have seats cut on a head, it is very possible to start out in the 1.50 shim range. I started out in the 1.50 shim range after having seats cut, and ended up having my second adjustments leave them at 1.25~1.30 ranges with no issues.

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Like previously stated, spend a little to save a lot. Your looking at around $130 for piston, rings, pin, and new timing chain, depending on where you shop. And check valve shims while your at it. It'll be a big peace of mind this coming season.

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I do a top end and timing chain at least every 50 hrs. I replace the crank and main bearings sometime between 100-120 hours. Call it overkill but I have never blown up a 250F.

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I do a top end and timing chain at least every 50 hrs. I replace the crank and main bearings sometime between 100-120 hours. Call it overkill but I have never blown up a 250F.

This is only a good idea if your a hardcore motocrosser. Someone who lives near the rev limiter when riding. If your a low to mid range rider, then this would be overkill for sure. If your 250F rarely goes past 10K rpms, I wouldnt follow this kind of maintance scheadule unless your someone who is very mechanically inclined, and can do quality work himself. I know guys on modern four strokes that trail ride every weekend, who have over 250 hours on a stock motor. It just depend on how you ride your bike. YZ's have the most robust motors on the market, and will last a really long time if ridden in the mid range. This is the main reason old four strokes lasted so long, they were low rpm bikes. An old XR400 would last forever since it wouldnt rev past 9k rpms. High rpms wear a motor. Theres really no magic hour number, since it depends on so many factors. How do you ride? Do you change your oil every 5 hours, 10 yours, 15 hours? Clean air filter often? I am at about 80 hours on my 08 YZF250, change my oil every five hours, clean air filter often, and I ride the mid range and could count the times I bounced off the rev limiter on my hands. My bike starts on 1st or 2nd kick 95% of the time. I bought my YZf250 brand new, so its a hard call for those who bought it used.

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This is only a good idea if your a hardcore motocrosser. Someone who lives near the rev limiter when riding. If your a low to mid range rider, then this would be overkill for sure. If your 250F rarely goes past 10K rpms, I wouldnt follow this kind of maintance scheadule unless your someone who is very mechanically inclined, and can do quality work himself. I know guys on modern four strokes that trail ride every weekend, who have over 250 hours on a stock motor. It just depend on how you ride your bike. YZ's have the most robust motors on the market, and will last a really long time if ridden in the mid range. This is the main reason old four strokes lasted so long, they were low rpm bikes. An old XR400 would last forever since it wouldnt rev past 9k rpms. High rpms wear a motor. Theres really no magic hour number, since it depends on so many factors. How do you ride? Do you change your oil every 5 hours, 10 yours, 15 hours? Clean air filter often? I am at about 80 hours on my 08 YZF250, change my oil every five hours, clean air filter often, and I ride the mid range and could count the times I bounced off the rev limiter on my hands. My bike starts on 1st or 2nd kick 95% of the time. I bought my YZf250 brand new, so its a hard call for those who bought it used.

I as I said what I do is overkill. I am a little anal rententive when it comes to maintaining my bike and the majority of bikes I do work on are for intermediate to pro racers. I do agree with you though for the moderate rider. I used to just ride trails when I had a 426 and I put 3 years on that bike and only changed the air filter oil filter and oil on a regular basis. I didnt crack the clutch cover open until about the 3rd year. :smirk:

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People just need to recognize who they are, how they ride, and how well they keep up with the basic maintance. Out of fear, to many people spend lots of money doing top ends and valve changes because they heard that it should be done every 50 hours, when that just isnt the case for riders who dont ride the top end and do the basic maintance on a regular basis. People need to identify what kind of rider they are, and design their maintance program around that. Take advantage of the reliability the YZF offers, and who knows, you may be ready for a new bike by the time your motors on its last leg. Especially true for those of us that only ride about 50 hours a year.

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