How Many Hours On This Piston?

[Kx_rider]

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I recently aquire this bike, 07 crf250r, previous owner didn't have it long, and wasn't aware of how many hours are on it, I pulled the head to have a look, and this only being my 2nd time replacing top end, to me this piston looks like maybe 25 hours, The cross hatching looks fresh to me, ill probably change it anyway well its apart, how many hours you guys think its got, looking for info from people who have done lots of top end jobs,

 

[Ol'89r]

Nobody is going to be able to tell you how many hours is on your piston by simply looking at the top of it. The correct way to check your piston is with a micrometer and an inside bore gauge. You mic the piston at the skirt and compare the clearance with the bore. Your service manual will tell you the correct clearance. If you don't have a service manual, you should get one. If you don't have the proper tools to check your bore, look at the skirts of the piston. If they are excessivly worn and shiny and look like they may be rounded off a little, replace the piston. If those skirts break, the piston can turn inside the bore and cause some very expensive damage. If you have any question as to the time on your piston, it's better to replace it while you still can.

 

[Rich Rohrich]

Agreed. Late model 250f pistons with single compression rings start losing power at the 8 hour mark. Changing it while it's still in one piece is always a wise move.

 

[Kx_rider]

I have a micrometer just need the bore gauge, hows the cross hatching look? AND what about the discolored ring at the top of cylinder about the rings tdc, that normal as well?

 

[Ol'89r]

You can't go by the crosshatch. In a plated cylinder the crosshatching usually doesn't go away. The discolored ring at the top is where the ring stops. It is discolored from the burnt gasses and it is normal. When you re-hone the cylinder, that will go away.