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KTM piston changes (and specificly 125)

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How often do you change your pistons?

the manual says "check" at hobby use every 40 hours, but all the mechanics ive consulted said that if you already open you should change.

(which I now know is crap because it takes like 10 minutes to open the head...)

so, how often do you change your piston?

and how do you know which one you need? (I or II)?

Ive changed mine after about 100 hours, and it seems to be in very good shape... I could of kept it for a few more hours I geuss... maybe just change rings...

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Most of your question depends on what and how you ride. If your a slower rider it'll last longer but the faster you get the faster you'll need to replace it. Now, why did you remove the head??? You can check your pistion by checking the compression. I never let ours get any less than 125 psi. You can replace the top end without taking the hear off......

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how do I know which size cylinder I have?

I have "2" ingraved on the left side of the spark plug, but the piston I took out said "I" on it (as far as I remember...)

BTW is compression checking tool expensive? because I don't have one, should I buy one just for these testings's sake?

and can anyone backup the clam of installing a II piston in a I cylinder? can't it break it? (because from what I know you cant oversize these engine because of the special metal - nicosyl or somthin)


I ride it very hard, not much of hard-enduro, but mostly motocross like tracks, with full throttle and revs high up 80% of the time...

I do about 60KM to a full tank of gas (that should give you some indication...)

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it is engraved on the cyl, cant remember where

comp tool is cheap, I got mine at Harbor freight

No, it wont hurt to put a II piston in a worn in I cyl

use the comp test on a fresh rebuilt motor after one ride so the rings seat, this way you know what it should be as a base line and then its easy in the future to know where you are.

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can anyone else shed some light on the whereabouts of the engarved number? or its form? (is it just 1 or 2, or more like a serial number?)

if it helps I can take a pic of the block and post it here, there are a few numbers there, but not sure how to read it (if any of them is the pis at all...)

oh, and - thank you

great to have such helpful guys ­čĹŹ

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