1976 Johnson 115hp should I replace rings on one the one piston???

cforgan

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Jun 11, 2013
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I am about to tear into this motor to replace the crankcase seal. I did a compression test on this motor and found I had a 10-15% difference between the 4 cylinders. Three of them were very close with 1 of the 4 making the 10-15% difference. From my understanding this is within specs but since I am tearing the power head apart does it make sense to rehone and replace the rings on this one piston? sorry I am new to this and this is my first outboard rebuild so I don't want do anything that will cause further damage.

Thanks in advance for you help
 
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Boat Doctor1

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Sep 29, 2012
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no= seal is easy & if you hone -you will increase the blowby even with new rings!! Not worth it!
 

racerone

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Facts----The motor is 40 years old and may have many hrs on it.----------The ring grooves in pistons also wear.-------Look at them under a microscope !--------So installing new rings on an old piston is not considered " good practice " and that o-ring material is easy to get at any BRP dealer / boat shop !
 

emdsapmgr

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No reason to reassemble an overhauled engine with only one new ringset. You could have the other 3 old ringsets fail one or two months later. Replace all 4 at once.
 

racerone

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No, no O/S rings like that.-----If I recall those piston rings are way up near the top of the pistons.----A wise man would install 4 new pistons !!
 

ondarvr

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Apr 6, 2005
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On a motor that old "if" I decided to take it apart because I needed to replace a couple of parts, it would be complete rebuild, I can't see going through all that work and money and still having a 90% old tired motor.
 

1983 ercoa 21'

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No, no O/S rings like that.-----If I recall those piston rings are way up near the top of the pistons.----A wise man would install 4 new pistons !!
that's what I'm a thinking might as well replace all the pistons.
 

racerone

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Piston rings are around $32 for each cylinder.---New piston kits with rings are around $105 or so.--Do the math.--But first inspect all the parts after tear down of the powerhead.
 

Thompsoncustom

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Aug 22, 2015
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If the pistons are all in good shape I would call up someone like wiseco, total seal, etc and give them your specs for the rings and ask if they have any that are a couple thousands over. They probably do and at a better price than most, I also would hone all the cylinders and replace all the rings.
 

racerone

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?????---But the WISCO replacement rings only fit WISECO pistons.----WISECO rings will not fit in factory original 1976 pistons !!!!
 

Thompsoncustom

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?????---But the WISCO replacement rings only fit WISECO pistons.----WISECO rings will not fit in factory original 1976 pistons !

Not completely true there is some overlap but for the most part yes wiseco rings are for wiseco pistons, I was only using them as a manufacturer example.
 

emdsapmgr

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Factory oversize pistons (with ringsets) come in these sizes: standard, .020 over, .030 over and .040 over. You can mix any of these sizes on one engine. Whatever you decide, you must be sure that all 4 pistons are from the same manufacturer.
 
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