Using compression to determine rebuild time on 250SX?
most agree once you see a 10% loss time for at least rings, then you can measure the piston clearance and make your decision, to make it easy i just do rings at 80-100 hrs, and a complete top end every other time.
Mike
we dont have anything to measure the piston wear with, so we'll just replace that, the gudgeon pin and small end bearing. hmm getting expensive 
anyone else?
I have always heard that KTM pistons are cast, not forged. Anyway, I think the only people that could provide an "answer" to this question are race teams that have been consistently using the same parts for a long enough time to have a solid data base for when parts fail. They will develop a schedule as to the intervals that engine (and other) parts need replacing. And the answers they provide would not be applicable to regular folks, as their riders are faster, engines are tuned differently, etc.
Compression readings are going to be tricky, and you would have to use the same gauge for all readings. I have 2 gauges, and they do not give the same reading on a given cylinder, and I have no way of knowing which is correct (or maybe neither is correct!). In my opinion, you can use gauges to measure relative changes, but not for absolute values, unless you have some way of verifying accuracy.
I think the best thing that regular guys can do is select a nice and safe interval, and stick to it. I have always stuck to the " 1 top end per year" interval for 250s that are regularly ridden/raced on weekends. I have never found a ring gap out of spec or any other unusual wear when sticking to this interval, and it is cheap insurance for a smoker. And although I have no real data, it seems that forged pistons are more durable when surveying friends about piston failures (e.g., cracked skirts).
Unfortunately, I'm about to break my own rule, as the recession has made a massive dent in our household income, so I think the current top-end in my 250 is going to have to last a while longer this time around.
Mine was 210 new, at 170 your bike will seam to run good until you ride a fresh one, you gradually loose the snap and pull out of corners.
KTM is using Vertex pistons VS Wossner, there half the price, but they also seem to have half the life.
Mine was 210 new, at 170 your bike will seam to run good until you ride a fresh one, you gradually loose the snap and pull out of corners.KTM is using Vertex pistons VS Wossner, there half the price, but they also seem to have half the life.
Very well said......... After haveing the head cut and a very little porting (cleaning) I installed a new Wossner piston which give me 220 psi. Just for fun I just went and checked my bike and it now has 195 psi with about 100 hours of ride time..............YES I'm slow....
My 2014 250sx has about 15 hours on new piston and I haven't noticed any loss of power and it runs fine but I'm getting a reading of 90 psi, something doesn't seem right here...
I did a top end on my '12 250 XC after about two years. I ride about 50 times a year for 1-6 hours and race about 4 enduros per year. Pulled the jug and it looked brand new but went ahead and tossed a new piston and rings in anyways. I religiously clean my filter though and that helps with longevity.
at 15hrs everything should be practicaly new. most likely your gauge is wrong. if you have a fair amount of resistance on your kick lever then certainly your cylinder pressure is far higher than 90psi
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Hey, I found this article on Wikipedia when i did a search for it:
http://wiki.answers.com/Q/How_often_change_top_end_on_KTM_250SX
"It all depends on your riding.racers change 25hrs or so.I ride every other week end 3-4 hrs,its giong on 3yrs since last top end.Buy compression tester 170psi needs top end,my compression 220.KTMs piston are forged like wisco,But austria quality.Usually bike has lost power,runs but not as peppy ,depending on compression"
So, at the start its 220+PSI, and needs a rebuild at less than 170?
How low should I go? 150 or what? Its currently at 192 with no carbon in the exhaust pipe, so i presume there's none in the motor either.
Mike.