Last year I bought a 125cc bike, Modenas brand, most likely unknown to you. It is a moped.


It can go as fast as 110 km/h enough for the city,

However, I gave it to a family person for a while and nearly destroyed it! Luckily the frame is ok and the bike goes straight.

He destroyed the exhaust valve, and 1.5 L oil consumption per 2000km! 1L capacity.

When I noticed it after 200 km running on low oil about 600 ml were missing, believing that it was serviced and ok. He had removed the air filter, and put a noisy exhaust.

I don't want to say much but I fixed everything I had the tools to do. Professional technicians told me that there is a problem with the piston/ring/cylinder.. and that it needs to be fixed... So I did... Cylinder got larger by 0.5mm ...

Here are some photos of the old piston... Do you see any problem with this piston?

Because i find that is has little problem an cant explain how this condition can cause so serous oil consumption problem? piston picture 1 http://dl.dropbox.com/u/4499990/sites/IMG_3760.JPG
piston picture 2 http://dl.dropbox.com/u/4499990/sites/IMG_3761.JPG piston picture 3

Until now, after the service it seems to be running well... Oil level seems stable for the first 200 km.

  • Welcome to the site. Please post the question about the battery as a separate question. Please clarify the question about the piston. Are you asking if that piston should have been making the bike use that much oil? You said "...piston/ring/cylinder and that it needs to be fixed so I did" What did you fix, did you replace the piston and rings? The cylinder got larger? How? Did it wear, did you hone or bore it? Commented Apr 6, 2011 at 0:38
  • Yes i am asking if the piston wear is visible. It has been honed. I am not sure if this word means that it is widened mechanically on purpose.Piston and rings were replaced.
    – GorillaApe
    Commented Apr 6, 2011 at 8:00

2 Answers 2


To make sure I answer correctly let me make sure I understand the question.

The engine was using 1.5 L of oil per 2000km, you did the following repairs to the engine:

Valve job including replacing the exhaust valve. Bored the engine 0.5mm over New 0.5 mm over piston and rings

Now that the repairs have been made it seems to be fixed. It's running well and has not used any oil after the first 200km

You are curious if the piston was the cause of the oil usage because it doesn't look that bad to you.

Visual inspection while important won't always tell you if it should have been smoking. It looks like there is deep scoring on the skirt, I would need to feel it to tell for sure. If the machine shop needed to bore the engine 0.5mm (0.20") over it was most likely worn close to that already or it took that much to get the scratches out of the cylinder wall.

If there was that much wear on the engine it most certainly would have been using oil, and visibly smoking. If there is too much play between the two then oil gets past the rings and ends up going out the tail pipe.

  • There was no visible smoke but everyone that has put their hard at the exhaust told me that it smelled burned oil...
    – GorillaApe
    Commented Apr 10, 2011 at 18:46
  • @parhs don't forget to post your other question about the battery and hooking up accessories. Commented Apr 10, 2011 at 21:03

If the bore has been honed and rings replaced then perhaps the issue is with the valve stems. I once had a CBX250 bike that blew a lot of smoke. It was a 4-stroke, like yours. In the end, it turned out the value stems were leaking oil into the cylinder.

Does your bike produce a lot of smoke?

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