FACTORY compression spec 1988 Force 85HP

TexasBayFisher

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What's the FACTORY spec for compression on a 1988 Force 85HP?
All three measured 120 so it's been well cared for.
Is factory spec up around 140 or what, does anyone have the SPECIFIC original spec for the 85HP Force?
 

JB

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Re: FACTORY compression spec 1988 Force 85HP

There are no factory specs on compression psi. Not from anyone.

Why?

Because differences in guages, technique, temperature, conditions and psychological factors can make as much as a 50psi difference in one cylinder.

What psychological factors? Outboard hypochondria, which is rampant this time of year, drives cabin fever sufferers to want to fix something. . . even if it ain't broke.

It ain't broke. If you have to "fix" something change the stale air in the trailer tires.
 

roscoe

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Re: FACTORY compression spec 1988 Force 85HP

most were 145-160 from the factory, some more, some less.

120 is fine for that engine.
 

GrindKore

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Re: FACTORY compression spec 1988 Force 85HP

As long as all cylinders are with in +/- 10% deviation from each other, you are in good shape.
 

TexasBayFisher

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Re: FACTORY compression spec 1988 Force 85HP

JB, looks like roscoe provided a range when new.
I'm a mechanic, I know how to use my tools, so psychological factors do-not apply. Variance between gauges is minimal if any.


An approximate number for what the compression was new for this particular 85HP line of engine was all I was after. 120psi in all 3 ain't too bad, its got some hours on it but the rings have worn very even. In fact I think they were all closer to 122-125
Thanks Roscoe


JB, In this area of Texas you can fish all year long. When people further north are in the 40's.. I enjoy temps of low 70's and can go fishing in a t-shirt.
 

reelfishin

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Re: FACTORY compression spec 1988 Force 85HP

The last 85 that I tested had a bad hole, but the the good ones were around 140, the bad one barely pumped up 80. I think that was an 87 or 88.
 

GrindKore

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Re: FACTORY compression spec 1988 Force 85HP

My 1990 Force 90 had blown rings on all three cylinders, yet somehow it managed to retain compression above 100 psi on two cylinders. Read my thread from last year here http://forums.iboats.com/showthread.php?t=266436 I have documented the rebuilding process with pictures for future reference.
 

TexasBayFisher

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Re: FACTORY compression spec 1988 Force 85HP

GrindKore, I read the entire thred, good stuff.
I noticed your readings were 118,122,125 with fresh rings
Once it was very well broken in did you re-test?
What are your readings now?
 

GrindKore

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Re: FACTORY compression spec 1988 Force 85HP

I have put about 50 hours on this motor since than. When I winterized it last November the compression was in mid 120 on all three cylinders. From what I understand the #1 cylinder compression will always be few psi lower due to it being used to create vacuum in the block for the fuel pump.
 

JB

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Re: FACTORY compression spec 1988 Force 85HP

Well, as usual, if you leave out pertinent info you modify your question and modify any useful answers.

You did not ask for a general range. You asked for a FACTORY Spec, a SPECIFIC factory spec. There isn't one. I said so. I even explained why.

I would expect an experienced mechanic to know that stuff. . . maybe even point out that I didn't list battery condition in the variables that make a specification useless anywhere but a QC checkstand.
 

Matthew A.

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Re: FACTORY compression spec 1988 Force 85HP

Another factor that can effect the psi reading per cylinder is ring gap. The machine shop that bored the cylinders for my 88' 85, bored one of the cylinders slightly larger the the other two. The resulting overbore increased the ring gaps on that cylinder of around .008" larger then the other two cylinders. Putting the gaps for that cylinder at or slightly bigger then the max. factory spec. I complained about the work but got nowhere with it. It may or may not shorten the time between the next rebuild. Only time will tell. For now the resultant cylinder reads 5 psi less then the other two.
 

reelfishin

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Re: FACTORY compression spec 1988 Force 85HP

Another factor that can effect the psi reading per cylinder is ring gap. The machine shop that bored the cylinders for my 88' 85, bored one of the cylinders slightly larger the the other two. The resulting overbore increased the ring gaps on that cylinder of around .008" larger then the other two cylinders. Putting the gaps for that cylinder at or slightly bigger then the max. factory spec. I complained about the work but got nowhere with it. It may or may not shorten the time between the next rebuild. Only time will tell. For now the resultant cylinder reads 5 psi less then the other two.

I think in that case I'd have called one of the custom piston manufacturers to see if they could custom make me a set of slightly longer, grind to fit rings. I've had that done for racing purposes before, I suppose they could to the same for an outboard.

A machine shop that screws up like that should have made good. The only other fix would be to go to the next oversize on that cylinder.
The excess piston clearance is what would concern me most.

While ring end gap will affect compression, your probably also seeing some piston rock as well which will contribute to a poor ring seal.
Something I have seen is variations in piston domes between OEM and aftermarket too. I just went through that with a set of pistons for an Evinrude motor, in which the aftermarket grouped a bunch of years together under one application which were actually slightly different by OEM specs.

The result would have been a point or two less compression using the aftermarket pistons. A measurement of the dome volume showed the difference. The .020 pistons had less dome displacement than did the OEM stock bore pistons. While the actual compression would have been less and the change probably minimal, it still would have been lower than when the motor was brand new.
 

TexasBayFisher

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Re: FACTORY compression spec 1988 Force 85HP

You did not ask for a general range. You asked for a FACTORY Spec

It's more polite to give people benefit of the doubt. :)
I did not expect a response listin a specific PSI, instead I wanted a specified ranged considered as "in interance" when it was brand new. This range varies slightly from each manufacturer. 125PSI would not be considered acceptable for a new one, but for a 1988 it is OK.

I'll do another compression check after I finish the decarb, and will report back.
I don't expect it to change much, might actually lose a PSI or two if gunk is increasing compression readings, but it will be super CLEAN once it's completed.

Most people do a decent decarb, but I am doing a more thorough one.
It's a three stage process to clean it out well, since it's brand new to me.
I'll start by tilting the engine up and filling up the cylinders with seafoam and then screwing the plugs half way in, BARELY bumping the starter w/wires off a couple of times force the seafoam past the rings, then it will sit over-night.
Tomorrow morning I will pull the plugs and turn it over to ensure the engine does not hydrolock. I'll replace the plugs, start it up and spray heavy amounts of seafoam deep creep into each carburetor @ high idle.
Then I'll run it in the water using 3/4 gallon of gas w/2.4 ounces of Amsoil synthetic HPI @ 40:1, along with 1 full 16oz can of Seafoam and run it until it's empty.
Then I'll continue running Amsoil pre-mix @ 50:1 and call it good.

Will create another thread to post results, since this was just to find out what the original tolerance was.
 

Frank Acampora

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Re: FACTORY compression spec 1988 Force 85HP

Matt: You do understand that the tolerance for wear is .002? .008 puts it out without any wear. Hell, that's almost the next oversize.

Or am I confusing myself with OMC? One is .002 the other is .005. I still think Chrysler/Force is .002

At any rate .010 plus 008 is .018 and the next oversize is .020 etc.---that just 'aint right! I wouldn't take another block to them.
 

Matthew A.

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Re: FACTORY compression spec 1988 Force 85HP

Hi Frank,
The .008" measurement that I stated is what the error in machining added to the actual ring gap measurement and not the bore. I believe spec is .008" to .016" for the ring gap. If I remember correctly, the ring gap on the one cylinder ended up being .014" to .016". I could be mistaken. I do remember that it made the ring gap right around max. tolerence. I was quite pissed when I checked the ring gaps and noticed their little screw up. The shop at first tried to tell me that the ring gap would not be an issue and therefore would not cause any problems. Later I got them to admit they screwed up but said they would only be liable at best for the cost of just the one piston and it's rings. And no way would they pay the cost of a complete set of pistons and rings when the other cylinders where fine. I laughed when I ***imagined*** them attempting to explain in small claims court that a three cylinder engine can run just fine without one of it's cylinders. In fact they perform "cylinder conversions" all the time. And why I should be completely satisfied with a new "converted" two cylinder engine. lol.
 

Frank Acampora

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Re: FACTORY compression spec 1988 Force 85HP

Ahhh! I misread you. Chalk it up to old age and late nights. ----And maybe a wee bit of ganja-----I grew up in the 60s and 70s! LOL
 

Matthew A.

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Re: FACTORY compression spec 1988 Force 85HP

Oh sure...that's the excuse I was hoping to use. lol
 

abajek

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85 hp Force rebuild 5 years ago. Check the compression after apr. 30 Hr of running and all three cylinders showing 90psi. What the hell is going on?
 
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