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Small crack on a piston after 53+ hours

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My RM-Z250 2012 has around 53+ hours on it (not more them 65, its 2nd hand and I'm not sure when was the hour meter installed it's now says 53 hours).

 

I have what looks like a small crack on the top of the piston (in the picture), its not all the way through but still looks bad I think so I wanna replace the piston with vertex piston but it has A B C and D sizes.

 

According to Vertex :

"The difference in the sizes is 0.01mm (0.0004") increments with A being the smallest and D the biggest. When a cylinder wears, the customer can choose the next larger piston size to regain the proper factory fitment. As a general rule use the following: A (0 to 50 hours) B (50 to 100 hours) C (100 to 150 hours) D (150-up hours)"

 

So A or B? (Its pretty hot here in Israel) and since I can't measure my cylinder (don't have the tool) what do you think should I do?

 

Also what do you think could have caused this? and could this be the reason the bike wouldn't start when it's hot?

 

 

Thanks.

 

Nativ

 

 

2014-05-20 21.09.36-1.jpg

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B or c is typically used on rebuild. A is for a fresh cylinder. If you want to get precise-call a local machine shop,have them mic the bore,call vortex and they will tell you which one to use. Wiseco customer support is cool like that too. If you want to know if that is indeed a crack-smack it with a hammer. If it fractures at the mark-it was a crack.

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B or c is typically used on rebuild. A is for a fresh cylinder. If you want to get precise-call a local machine shop,have them mic the bore,call vortex and they will tell you which one to use. Wiseco customer support is cool like that too. If you want to know if that is indeed a crack-smack it with a hammer. If it fractures at the mark-it was a crack.

thanks , any thoughts on why has it happen? (the "crack)

 

What aftermarket piston kit would you recommend , Wiseco or Vertex or some other brand maybe?
 
Thanks
Nativ

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I Dont think it is a crack. Typically the valve will bend before the piston cracks. Could be a casting flaw. Hard to say. I like vertex and cp Carrillo pistons.

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How is that thing so clean? What kinda gas you run over there? Mine looks soot black after 25 or so hours. Might be the ethanol?

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Oh I see. That's a lot of hours on one piston in my opinion. I change mine at 30 hours max usually. The metal has probably fatigued after so many times of getting hot and cooling down. It just happens.

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yes I know it's a lot of hours but there are actually a couple of reasons for that. 1st in Israel the riding style in general is not as hard as in the US, that's because the price of the bikes are about 1.7 times more then in the US and so are parts. 2nd it will take for a piston kit to arrive about two weeks since it's not in Stock here. So we ride not as hard and tend to stretch the hours for about 70 to 90...I know of a bike with 140 on the same piston, sure it's not ideal and I know it's kinda of an excuse but that's the way we can maintain the bike in a good shape and not piss off the wife , I try to replace piston every 90 hours.

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