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250sxf piston

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After many hours of riding and racing my Son's 07 250sxf needs a new piston or at least a comp. ring. Although it seems to start and run OK (the valves have not moved) it blows oil out of the crank case breather tube at the rate of about 250 cc per 15 min. moto. This brings me to my question has anyone used a piston other than stock in there 250sxf motor ? I see Wiseco lists a .5 over comp. (13.3-1) piston and titanium valves for the 250sxf.

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Just use a stock piston, why would you wanna use some junky aftermarket one when you can use the KTM stock quality one?

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I should have put an hour meter on but never did. I would guess 60-80 hrs. Quality oil and filters changed every 1-2 rides along with air filter. Bike is stock with the exception of a white bros pipe, 42 pilot,180 main and a 49 tooth rear sprocket. we run pump 91 gas. The bike has been bulletproof and this is our first problem since we bought it in oct. even with low comp. and blowing oil it was still able to holeshot and win both motos of last weeks CMC golden state nats. at competitive edge in the 250f/c class (the only KTM against a full gate of jap bikes). I was just wondering if the after market pistons would work as well as they do in the jap bikes at 1/2 the cost of OEM KTM.

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I have gotten as much as 100 hrs of trail riding, but MX is much more RPM ya know.

If you reach out to Munn Racing you can get the Number 2 piston, with gaskets rings and just replace it yourself.

Very detailed instructions here and in the downloadable service manual.

Have done it 3 times now and its a breeze.

So easy a cave man can do it.

Now if you want to invest a tiny bit more and have Munn go through and check everything, you can trust that they will only replace the ring and piston and nothing more if it does not need it.

You will be amazed what a fresh top end will do for the bike. Although it runs great with worn rings, you loose a lot of bottom end. Just had mine done again and its addictive for the money. Pulls everything a gear higher of coarse. Don't have to be on the pipe all the time any more.

Started and ran great, but slowly lost all the bottom end and was just running up on top like a two stroke with 80 some hrs.

Sound similar?

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Ponie, how often do you check your valves? Just wondering what the relation of valve life to piston life has any similarities.

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Because of all the guys that owned KTMs from 6-7 years back, lots of em check valves every 15 hrs.

It’s the smart thing to do, but after owning an 04 450EXC and two 250SXFs I can honestly state that they have never needed adjusted.

The 05 SXF has 300+ hrs without a valve adjustment and the 06 is up to 120 without any movement since it was purchased.

To answer your question. At first I had dealers check my valves every 90 days like clock work. After writing down the clearance new verses each check I discovered the SXF valves never move.

Although I strongly believe that the new generation KTM motors are bullet proof. If we all stop checking and maintaining the valves, we might end up like the jap bikes.

Change the oil every 5 hrs and run the heck out of it. Check your valves to your comfort level. It takes just minutes to do with the proper gauges.

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Ponie, that is exactly what has been happening to my Son's bike. I am usually more in tune to whats happening with his bike but this series had us traveling from NV to a new track in CA for 5 weekends in a row. I suspected the piston or rings 2 races ago but to be honest the past 5 weeks has been a whirlwind of work and traveling with most of the bike maintenance being done the night before the race at the hotel. I kept on asking my Son hows the bike running ? and just like most 13 year olds who are a wealth of knowledge when asked about powerbands or suspension he said awesome dude. To me the 250 sxf wears out in a very sneaky fashion. It starts right up, does not smoke, seems to run well and as a side benefit the rear disc stays well lubricated with fresh synthetic oil from the crank case breather (maybe thats how he is able to holeshot on a worn out ktm cause he can't stop). Thank you Ponie for the help I have a new OEM piston on the way.:censored:

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Because of all the guys that owned KTMs from 6-7 years back, lots of em check valves every 15 hrs.

It’s the smart thing to do, but after owning an 04 450EXC and two 250SXFs I can honestly state that they have never needed adjusted.

The 05 SXF has 300+ hrs without a valve adjustment and the 06 is up to 120 without any movement since it was purchased.

To answer your question. At first I had dealers check my valves every 90 days like clock work. After writing down the clearance new verses each check I discovered the SXF valves never move.

Although I strongly believe that the new generation KTM motors are bullet proof. If we all stop checking and maintaining the valves, we might end up like the jap bikes.

Change the oil every 5 hrs and run the heck out of it. Check your valves to your comfort level. It takes just minutes to do with the proper gauges.

That's awesome. When I asked my dealer about valve checks he said he has a 07 with 200 hours that haven't moved. I guess it is true :censored:

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Good choice (the OEM piston) !! KTM 4 strokes use a very high quality,forged piston. It may cost more than aftermarket pistons but it also last that much longer.

I also know that KTM has tested with 250 SXF`s reliability by having their test riders just ride two different production bikes, which they just randomly choosed from the prod. line , without any maintenance on them, except oil change every three hours. When i heard this story they had 130 hrs. on those bikes and they were still going strong. And those guys are no slouch on a bike.

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On our teams practice bikes we let the piston and ring go 20 to 30 hours.

Much longer or if a bunch of dirt comes through the air filter you will get the oil coming out the breather.

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Thanks to all for the help, I just put in the new OEM piston and just as Ponie predicted the bike runs much better from bottom through mid, starts easier and does not blow any oil out of the crank case breather. I am very impressed with the quality and how easy it comes apart and goes back together. By far the highest quality, most well thought out and easiest four stroke top-end I have ever worked on. Unlike it's Asian counterparts you don't have to be a three armed Ukrainian gymnest to do the top-end on a KTM. Austrian/German engineering in the house yo.

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How hard is it to do a top end on the 250sxf?

I've torn 2stks down completely to change broken tranny gears and replace cranks. So mechanically I know I can do a top end on the 4stk.

I have read the shop manual several times and just seem to lack the confidence in removing the head to change the piston and rings.

I've never torn down a 4 stk. Worried about getting the valve timing set correctly when I put it back together.

Any help or advise would be appreciated.

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kdr, I know exactly what you mean. Here's a guide for checking valves (its a CRF250, but same concept, just 2 cams on KTM). Pictures are the key. Also, here's a link to some pics I found of the 250sxf engine, they might help.

http://www.thumpertalk.com/forum/showthread.php?t=179775

http://www.ktmcyclehutt.com/KTM_250SXF.htm

I've got the valve checking down.

What I would really like is a video of someone tearing one down and putting it back together. Would be nice to just see the whole proceedure.

Just don't want to get putting it back together and mess something up.

Oh hell, what's the worse that could happen. Just have to take it to the dealer to do it right.

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