Why replace scored pistons?

[boatster]

I know this is an engine topic but I like this forum better Anyways why do you have to replace pistons if they have some vertical scoring from overheating? I can understand that the cylinder walls of the powerhead should but if a piston has some scoring like on the lower half of the exhaust side how is that going to effect the performance of the motor? There would still be a significant amount of surface that isn't scored and the piston would remain straight throughout the stroke right? If I rehone and replace the rings and use a new clean block why couldn't I use the same pistons over again? Can someone dispel this ??? for me?<br /><br />I inherited a small Evinrude 9.9 and it was overheated by a lack of water cooling. I took the powerhead off and saw the exhaust side scores. Gonna fully crack into tonight.

 

[LubeDude]

Re: Why replace scored pistons?

There is wear there that you dont see. If you mic them you will see. You dont want them flopping around in there. (Years ago, Been there done that), had to tear it down again.

 

[Boatist]

Re: Why replace scored pistons?

Try Johnson and Evinrude forum.

 

[JB]

Re: Why replace scored pistons?

Like Boatist said.

 

[Dhadley]

Re: Why replace scored pistons?

Its a moot point actually. Since you have the motor apart once you measure the cylinders youll see it needs boring anyway. <br /><br />But to answer your question, once the piston has been hot enough to score like that the hardness is going or gone. Damage will continue. Its possible that at some point the aluminum from the piston will ball up and cause the piston to stick. At that point, if youre lucky, the motor will slow down and lock up. If youre not so lucky the piston will seize and the rest of the pistons will still be trying to move. Something has to give. At that point is usualy the rod or wrist pin. Not good.<br /><br />On a small motor like that it'll just usually seize up.

 

[Paul Moir]

Re: Why replace scored pistons?

Primary (Crankcase) compression suffers as well leading to bad idle, lean running, etc.

 

[boatster]

Re: Why replace scored pistons?

Ok,<br /><br />That sounds good. Thanks guys. BTW I'm supposedly getting a new block. Hopefully it just need a rehoning. We'll see. But I will put new pistons in there.