KTM 125 SX 06 - piston issue

[matej.charvat]

Hey guys,
I've bought a ktm 125 sx 06 from my friend a few months ago. There should be a new (one tank used) piston/con rod at the time when I bought it. Now there is 14hrs on the clock bought by me.
I checked the piston via exhaust port recently and I've seen some weird stuff so I managed to put the cyl down.
Here you can see how does it look inside.
Some object definitely made some **** inside the cyl, whatever it was...it did really bad job. I want to pinpoint that I'm really carrying about maintenance - cleaning filtrer after every ride...
Yes, there are little dings in the cyl's nikasil plating, but all of them seem to be pretty smooth, no sharp edges etc.
Please note that all the pictures are sharpened and it can cause that it looks worse than it really is, lol.











This is my first twostroke and my first engine which I've opened. I think I'm on a really high level manually, but I dont't have any experience with dirtbike engines.
Can you help me out how to solve this issue?
I know I will have to buy a piston and reeds, but what about cyl and head?
Bottom end seems to be ok, all bearings ok, no play anywhere.
Thanks is advance for your replies.

[cnrcpla]

This looks like a case of piston seizure. The marks on the piston definitely indicate that. Check to see if the cylinder head is warped. Those little tiny grooves along the piston crown (top rim of piston) looks like coolant is able to get into the combustion chamber. I had a similar issue, and it turned out my cylinder head was warped. I got it all squared away and no more coolant was getting in. Have you noticed any of the coolant in the radiator disappearing? Anyways, once the seizure sets in and ships away at the edges of the piston, that debris is floating around in the engine scratching up everything and causing those holes in the cylinder head. If this isn't your issue, than another member on here will have the right one.

Oh, and how old are those frayed reeds? Are all the reed screws in the reed block? If not, than there is the problem right there. What are you mixing at by the way?

[matej.charvat]

Hey, thanks for your post.
Well, I don't think that this is my case. my head is not warped and I'm defo not losing any coolant.
I think it had to be some hard object which came (dont know from where) to the engine and did this.
Bottom end is looking good by the way, no scratches...all the bearings seems to be ok, no play anywhere.
What do u think to do with it? Do I need a new nicasil? As it doesnt seem to be that bad to me (im novice).

I dont know how old are those reeds and yes, there are all bolts.

I'm mixing 1:40 Motul 810.

[scotty dog]

That doesn't look too nice, the reeds could have caused the damage too but the little holes look wierd, kinda like a a flaw in the cylinder or water has been sitting. Get a good mechanic to check it, definitely need new piston, reeds, the head would be fine but the cylinders a bit iffy

[matej.charvat]

Yeah, head will be ok i guess...I will clean it a bit with a smooth sandpaper...
But what about the cylinder? Anybody more experienced to help me out?
Cheers

[2T Institute]

Marks on the piston and head were caused by debris, the reed is unlikely to cause that. There is no seizure or detonation. Reeds are fretting because of the lack of oil, to cushion them.

[matej.charvat]

2T Institute:
What kind of debris do u mean?
I started to think that it all happened to previous owner and he just replaced rings (there is a scratch on the piston, but rings are untouched...) What do u think?
And what do u mean by the lack of oil and reeds? U think 1:40 is not enough?

[Jeram]

yeah that sounds about right 2Ti.

Unless a peice of the cylinder, or piston has chipped away and become loose, Im going to put money on there being a damaged bearing in the lower end thats sent fragments of bearing up into the top end.

if theres no chips missing from the piston skirt or cylinder and all bottom end bearings are good then you MAY have been dooped by the previous owner.

[2T Institute]

2T Institute:
What kind of debris do u mean?
I started to think that it all happened to previous owner and he just replaced rings (there is a scratch on the piston, but rings are untouched...) What do u think?
And what do u mean by the lack of oil and reeds? U think 1:40 is not enough?

Could well be that the prior owner did that, 40:1 is not enough due to the fretting of the reed petals. At higher oil ratios the fretting never occurs.

[SachsGS]

I suspect your bottom end is failing - either con rod big end or crank shaft main bearing debris. If you look carefully at the damage to the piston it looks like it was caused by needle bearing fragments.

[matej.charvat]

2Ti: Well, I really don't have any longterm experience with 2smokers so I really dont want to talk like experienced mechanic, but...why does the ktm 125sx has 1:40 in its manual?

SachsGS: Yeah, but everything seems to be ok. I didnt split the cases, but tried for the play on both ends of the crank and also checked the big end on conrond, no play at all. I started to think that the damage is from previous owner...but really dont know.
Nothing weird at all, engine running crisp to the last moment...

[matej.charvat]

Well,
I went through all the stuff today one more time.
Whatever it was (object) what made the **** in my cylinder (especially the big groove in the piston)...I think it cannot pass these rings, they are absolutely untouched, like new.
What do you think?


I also made the head looking much better...I used sandpaper 1200, it doesnt look that bad now...

[2T Institute]

2Ti: Well, I really don't have any longterm experience with 2smokers so I really dont want to talk like experienced mechanic, but...why does the ktm 125sx has 1:40 in its manual?

SachsGS: Yeah, but everything seems to be ok. I didnt split the cases, but tried for the play on both ends of the crank and also checked the big end on conrond, no play at all. I started to think that the damage is from previous owner...but really dont know.
Nothing weird at all, engine running crisp to the last moment...

I don't know either but the reed petal fretting is from the oil not cushioning the reed. The head needs to be cleaned up on the lathe. Just replace the piston, flush the bottom end out and be out riding.

[matej.charvat]

I think 1:40 have to be good all the way...
Well, what should I use for flushing the bottom-end? Gasoline?

[SachsGS]

The rings, as you noted, may be the clue. The crankshaft assy. and rings might have been replaced but not the piston itself (barrel and head not repaired either). If you are certain the bottom end is o.k. then it is probably time to do the job right and recondition the top end as well.