1990 force piston size

[jrm]

Hello, I have a 1990 force 150 outboard motor. I am rebuilding it myself. I put the pistons in and bolted the head back together. After I bolted the power head back to the motor leg, I realized the three pistons I replaced are not the same diameter. Will they still work? The pistons appears to be about 1/8" in diameter smaller than the stock pistons. The rings seem to be against the cylyinder walls but I fear the rings may break because the are not compressed far enough. This is my first rebuild and I trusted they were the right size when I purchased them. I didn't think to size them up. A boat shop peressed the rods on them. Thanks

 

[JB]

Re: 1990 force piston size

Moving to Force Troubles.

 

[kenneths]

Re: 1990 force piston size

I'd say ya gota blow-up waiting to happen....<br />You need to know, first of all, what is factory STD. bore size, measure cylinders in 2 directions with a bore gauge-parallel to pin, and 90 degrees to pin. See how this measurement compares to STD bore.<br />Part of an O/H also cosists of checking ring end-gap clearance which should have clued you in to piston difference provided the rings were for the pistons you received.<br />If they are truely 1/8" smaller, that would be .067 smaller in piston size.<br /> Unfortunatly, this sounds about right for an engine that has been bored .060 over (max), and received a STD piston. .007 would be roughly the piston/cyl clearance.<br /> Was this engine bought new by you??? If not, there could have been one or more cylinders that had been bored and re-pistoned accordingly, and you may have a multi-sized piston mess-unbalanced to say the least.<br /> You've GOT to measure the bore of every cylinder to know what to do first.<br /> I would strongly advise you NOT to run this until you find out what your measurements are.....<br />p.s....the piston skirts will be the first thing to break. If anymore damage is done, you will be needing to re-sleeve if thats even possible on that engine, and you will quickly have more in repairs than the engine is worth.

 

[jrm]

Re: 1990 force piston size

I purchased the boat new in 1990. I measured the piston itself this morning. It is only 1/16" in diameter smaller than the originals. When the pistons were in they looked much smaller. The old pistons that came out fit the same as the two originals that were reuseable. This is in all five cylinders. This motor has a 3 3/8" bore Shouldn't the replacement pistons be the exact same size as the original pistons. The replacement rings are wrong. No need to use a feeler guage, they have about 5/16" gap.I am rebuilding from a Clymer/Force manual. If this is the only piston I can get, should you use over sized rings? Their is also a difference in the top of the piston. The intake baffle on the original pistons is a constant 3/8" wide, the new pistons start at 3/8" on the outside and the immediately change to 5/16" and then to 1/4" where the c shape is formed. Does this area change cuase a difference in the compression? Thanks for your info. The boat shops around here are reluctant to give you info. I don't blame them. They need to make a living; However there probably aren't many people who would attempt to rebuild a motor and I dropped $1700.00 in parts.

 

[jrm]

Re: 1990 force piston size

 

[jrm]

Re: 1990 force piston size

first boat rebuild, learning experience

 

[kenneths]

Re: 1990 force piston size

The pistons would only be the exact demensions of the old if they are direct factory replacements. Each piston manufacturer takes into account the expansion of the piston, and builds it "X" smaller than the bore to maintain proper clearances at temp.<br />The fact that you are saying the rings have 5/16" endgap incylinder tells me you have the WRONG piston assemblies, and by the shape you are describing, I don't think what you have is for this engine at all.<br /> Bad part is...you still need to measure the bore with a bore gauge, and the piston with a micrometer, 90 degrees to pin hole.<br /> If STD bore is 3.375, then the piston should measure around 3.365-3.370.<br /> The "FIT" is what we are looking for here.<br />If you know the STD bore then let's get a measurement on the pistons in question, and the old pistons as well.......