How do you know when you need to get bigger pistons when honing?

[hckrphil]

I have a 1994 Mercury Sport Jet 90HP engine.

I made the mistake of buying standard pistons, thinking I'd just do a quick hone and get it back together without checking the current cylinder size.

Anways, I took it to a guy who knows engines, and he thinks its getting pretty close to the point where I'd have to go buy new pistons and have the engine honed / bored to match

At what point do I buy new pistons and have it re-honed / bored..........

Or do I just try to put the engine back together with my standard size pistons.......

Anyone wanna buy (3) standard size pistons and rings for a 94' Mercury Sport Jet 90HP? haha

Phil

 

[Nate3172]

Re: How do you know when you need to get bigger pistons when honing?

Pretty close? There are a couple things to take into account. If the cylinders are no longer round is one. The other is how much you will be able to hone before your piston to side wall clearance is too far out of spec. If the bore is too much bigger than the piston than you will get "piston slap" and it will damage the skirt of the piston severely. Measure the stock pistons that you have, write that number down. Each one may be a little differen't by the way. Then, measure your bores. Check them in numerous locations throughout the bore istelf. Down low, up high, etc. This will tell you if the bore is egg shaped or has a taper. It will also tell you how much bigger the bore is in comparison to the pistons you have. If the bore is already approaching the 8 to 10 thousandths larger than I would suggest having the cylinders bored and honed the first available oversize and go with that. If you are already too large with the bore, obviously it will only be worse with a hone job. Note too that not only will you potentially have "piston slap" but the rings will also sit farther out of the ring lands on the piston and can potentially break easier. Also the ring gap will be larger and can lose compression from it. It's better to take the time and spend the money since you are this far into it anyway and do it right from the get go.

 

[backwater dawg]

Re: How do you know when you need to get bigger pistons when honing?

Phil--don't laugh--I will need a set of pistons for mine over the winter and havent decided if I'm going to bore mine--I'm going to do the same basic thing youre doing-rebuild the block--are you also replacing the bearings or just the pistons and rings??? good luck--Steve

 

[Frank Acampora]

Re: How do you know when you need to get bigger pistons when honing?

Mike the bore just above the exhaust ports and near the top. Mike each at vertical (direction of piston pin) and across the ports. If the cylinder is out of round or tapered by more than .002 inch (two thousands) then factory recommends boring over. You have the option of .020, .030. and .040. (WISECO used to sell pistons .010 over but no longer does. I don't know if Merc does.) At any rate, overbore the smallest amount needed to clean up the cylinder if scored or to bring it back into round.

Note: The bore is perfectly round. The piston is cam ground (oval cross section) because it expands differentially ( Because of the shape and coverage of the water jackets and because exhaust side is hotter) and assumes a perfect circle shape when at running temperature.

Strange that Force recommends no more than .002. Evinrude allows .005 normal wear.