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compression ratios

Jcobb5132

New member
my 1990 evinrude 225 was supposedly bored .060 over. i used a borescope and it has .030 over pistons. can it be bored .060 over with these pistons and what should the compression be or increase by if bored .030 or .060 over?
 
Lots of variables here but. No and no. If you were to bore 60 over, you're present pistons will be a 'loose fit', so to speak. The compression ratio would be the same. Compression would likely be abysmal. Why are we going 60 over? Is there some significant bore damage in there? Liners might bring things back into hand. An actual accurate measure of the bore might be enlightening and more useful than a number stamped on top of a piston. You probably got to ask yourself a bunch of questions about what you want from the engine, what's it going to do, where are you going to use it, how are you going to use it... Probably going to have to prepare a budget (sobering moments here). I am one to lean toward the 'more displacement' theory...
 
Like the other member said, you would not put a +0.030 piston in a 0.060" overbore, way too loose and lots of blowby and piston slap and poor sealing. Chances are if the piston was stamped as a 0.030" then they bored the cylinder 0.030" plus the required piston to wall clearance. Theoretically if you increase the bore size and keep the head cc identical the compression will increase but with 0.030" it would be so slight you would not have to be concerned about it.
 
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