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Is a top end rebuild needed if the bike has high hours but has no issues?

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I have a 2015 kx250f with around 65-70 hours on it and I’m not sure whether I should rebuild the top end or not. It has been maintained very well, only ridden on trails, the valves were just adjusted, and there are no signs that indicate a worn top end. Is it ok to keep riding until it shows signs of needing a rebuild, or should I just rebuild it now because of the high number of hours? Apologies if this is a stupid question, I couldn’t find anything on the internet about it.

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i would just compression test it. if is below spec by alot it would probable be good to do a rebuild. test it cold and hot and remove the auto decompression lever if you have one before testing.

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My kx250f 2013 made above 250 hours from full rebuild since I faced issue with the connection rod.

 

Just check if everything is as per the spec, take a closer look in the oil you drain and the oil filter and .....just have FUN 🙂

Edited by phatsmall

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If valves needed an actual shim change then reduce the amount of time between checking from 10 down to probably 5 hrs. If they need another shim in 5-10hrs then I would say start thinking about pulling it apart..

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On 8/25/2019 at 11:35 PM, speedycorndog said:

around 65-70 hours on it and I’m not sure whether I should rebuild the top end or not. It has been maintained very well, only ridden on trails, the valves were just adjusted, and there are no signs that indicate a worn top end

In my non-expert opinion... Wear is exponentially related to RPMs. So if the revs have been kept down, and the throttle not often held wide open, then your piston, rod and crank should all be fine after "just" 70 run hours.  If an adjusted valve is still pulling up, then it'll need to be replaced. When replacing a valve you should get the seat recut. But if you choose to leave the seat rough, then the new valve probably wont last as long as the replaced one.   Beryllium copper valve seats are not super expensive and make the Kawi 250F heads much more durable than stock.

On 8/30/2019 at 8:24 PM, phatsmall said:

My kx250f 2013 made above 250 hours

Wow. What type of use?  Were no parts replaced during those 250 run hours?

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On 8/31/2019 at 5:28 PM, numroe said:

In my non-expert opinion... Wear is exponentially related to RPMs. So if the revs have been kept down, and the throttle not often held wide open, then your piston, rod and crank should all be fine after "just" 70 run hours.  If an adjusted valve is still pulling up, then it'll need to be replaced. When replacing a valve you should get the seat recut. But if you choose to leave the seat rough, then the new valve probably wont last as long as the replaced one.   Beryllium copper valve seats are not super expensive and make the Kawi 250F heads much more durable than stock.

Wow. What type of use?  Were no parts replaced during those 250 run hours?

Only once valves clearance adjust (but no issues), riding on motocross tracks as hobby.

Oil change every 5 to 10 hours.

After 263 hours (info from the hour metter) I got noise from the engine - like noise from the top end, but it wasn't 🙂

Just few hours later, the connecting rod failed and I did full rebuld.

But I believe that for non-profesional rider like, that is not a surprise as I can probably use only 35-40% of the engine capabilities.

Edited by phatsmall

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Did the valves actually need reshimming or did the shop just check them and they were fine? Were you having a problem with starting or did you just check them because you thought it was time? If the bike was running perfect and there was no significant valve movement, I'd run a trail bike up to 150 hours barring any problems. If you were an Mx guy banging it off of the rev limiter, I'd go ahead and replace the piston.

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17 hours ago, phatsmall said:

Just few hours later, the connecting rod failed and I did full rebuld.

But I believe that for non-profesional rider like, that is not a surprise as I can probably use only 35-40% of the engine capabilities.

263 hrs is a lot for one of these motors. Seems you found the limit and avoided a full engine grenade result. Lucky you.

Engine wear rates are dependent on so many things.  A slow/medium pace rider can easily hold a 250F wide open for over 25% of each lap if the track is a high HP track. eg. soft dirt with long straights or uphill sections.

We have to decide for ourselves when to replace parts. Valves and heads are expensive, but easy since they wear out and leak. Clutches grab or slip. Transmissions usually malfunction before major failures.  But pistons, rods and cranks are tricky. 

When buying a used bike, if we don't feel confident in the service/usage history, then we either play it safe and replace parts, or we gamble. 

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Agree with numroe, if we have good feeling about the bike and know it very good, we can estimate some of the intervals for maintanance.

If we check everything if it is as per the specification (on regular base) and in the same time we do good maintanance, then we could have the bike running well for a long time.

Also something very important is that all the above advices I share are actual for non-profesional riders and non-profesional use of the bike (not at the limits)!

If the rider is profesional and use the bike at nearly full capacity, then every compromise with the maintanance will cost a lot more.

 

That is just my opinion and never push someone to accept it 🙂

 

Have fun and be safe!

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