Bore diameters, piston diameters and ring gaps

[cvlngineer]

Here are the dimensions that I just took from the powerhead and pistons on my 1975 Johnson 70 hp headache:<br /><br />

<br /><br />As you can tell, my #2 piston has some scoring on it. I have alread had the cylinders honed. I am getting ready to order some parts.<br /><br />The cylinders look like they are within tolerance for wear.<br /><br />Thanks,<br />cvlngineer

 

[captweed]

Re: Bore diameters, piston diameters and ring gaps

If you rebuild ALWAYS check the new piston ring gap in the hole they will be used in.there should be a spec for this.My Merc I just did was .018 to .025, check yours ,had to file 2 gaps open some to prevent binding ring.Put ring in bore and square up 1 inch below top of cylinder using a piston to push it in.Measure with feeler gauge.Don't hone excessive,ask how.My '76 200 pistons were sloppy as hell in bore,My buddy who has repair shop laughed and said that's how they run.The weak link on older pistons is the closeness of the top ring groove to top of piston,it cracks off and you eat a hole.Also carbon buildup freezing ring tight.Replacement pistons have the top ring groove lower now.