The story begins with simple installation procedure of a BIG BORE kit that we ordered from USA . The manufacturer
and the dealer send us the correct kit and so far so good !

The bike is a KTM 250 sx-f ,2011 model and bellow there are some photos for you, just to see how careful I am throughout the process .



I change the valve seals and the exhaust titanium valves and springs with the PROX steel kit and make a little polishing on the intake and exhaust


Put the cylinder-piston on and always with guidance from workshop manual and correct clamps...




Until now we where happy !!! We start the engine ,and everything seems to be ok!

Began to make the first ride and…… DISASTER !!!

The engine start bleeding OIL from the hose-tube !!!!!!!!! I thought its normal if it is one-two drops but unfortunately didn’t STOP .

We did one more ride to see what happens and still BLEEDING. Back to the WORKSHOP.


Edited by element9

Ring blow-by. Are the rings gaps positioned correctly on the piston? It's something to check.

Excess oil maybe and are you sure the piston sits correctly in the cylinder? Shouldn't the arrow face direction towards exhaust?

I bet ring gaps are not in right position also

I doubt that ring gap position would cause this much of a problem. Incidentally, what was the installed ring gap?

A common mistake is the improper installation of the oil control rings, especially where the corrugated ring is not properly butted snugly end to end.

Excess oil maybe and are you sure the piston sits correctly in the cylinder? Shouldn't the arrow face direction towards exhaust?

The piston is installed in the correct orientation. On the RC4 engine, the cam drive runs up the right side (as you sit on the bike) compared to the left side if you were looking at an RF4 (450/505 DOHC). You can see the top of the starter behind the intake side of the cylinder with the arrow pointing the other way towards the exhaust.

Hello again and thank you for your interest.

So...the arrow points the exhaust. I use motorex 10-60 oil . The ring gaps are at the right angles as the directions including in the Cylinder Works kit.

Before I took everything apart,we start a small conversation with the company representative (dealer)who insist that we had to check the compression ring GAP.

So we did. I took everything apart and with his suggestion I did the gap from 0.2mm to 0.48mm . In the meantime this is what I found inside the cylinder WITH ONLY a 5 minute RIDE.


We decided to correct the ring gap and put everything back together



After this,I started checking everything. I checked the oil passages ,the valve seals ,the oil pumps, the oil valve,even clean the injection and put a new air filter etc

check the right site crank oil seal . if it is out you have a uncontroled breather presure in the camchain room .

Edited by speedboot

To be honest , I thought if the damaged seal is responsible of all this and I checked it by visual (you can see it if you open the right engine cover ) . Although I cant understand why the crank seal has anything to do with the scratches on the piston and cylinder.

was the seal damaged ?

I checked it by the following way . With the cylinder-piston and head on ,and with the oils left inside the crankshaft room , I turned the crankshaft from the left site by hand and when the piston was getting downwards althought a sound of pressing air was heared , I didnt see any bubbles or hear something coming through the seal. Everything seems to be OK.

Any suggestions ???

Yes your expander ring was overlapped and not butted. This is why you are seeing a scrapped cylinder wall. When you overlap the expander you force the ring into the cylinder wall. This is a common mistake.

No my friend, I am very careful with this prosidures.

If I done that, only the cylinder would be scratched,not also the piston

No one ever had the same issue ?

Was the ring gap too small before you corrected it?

or, dirt.........looking at how dirty the bike is, covered in dust, before you stripped it down, there is every chance that dirt dropped into that engine, bad idea.

The bike was carefully cleaned before the surgery and the engine was rinsed out two times with clean oil before the final refill . The gap at the beginning was 0,25 . I don't think that this was the problem .

Is the piston or cylinder round?

Nope. But there is a kind of an issue to those two. I think so

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