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  1. #1
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    Default Shop time/cost estimate, replace pistons, Force 90hp??

    I have a 1990 Force 90 hp outboard with low compression in one cylinder.


    The engine still runs, just poorly, but I'm hoping it's just pistons, rings, wristpins, bearings, gaskets, etc, and possibly a bore or honing, then rebuilding and cleaning the carbs.

    I am also toying with doing the job myself...i have never worked on an outboard before, but have done many top end jobs on motorcycles, and lots of auto experince, and have all the tools, i.e. flywheel pullers and torque wrenches.

    My alternative to rebuilding is installing an 1970's 100 hp Johnson I have to replace the Force, but even that would cost me several hundred between help with wiring (which i am not good at), tune up, and whatever else it may need...It's been sleeping for 6-7 years. I'd rather try and save the Force, because it is newer and has power trim.

    So my question is, approx how many hours of shop time would I expect to pay to rebuild the Force and approx how much for parts, provided I don't get into anything too unexpected. It sounds like this is a super common problem.

    Also, a recommendation for a good brand of shop manual for rebuilding would be appreciated.

    Thank you in advance...I'm glad to have found this great site!!

    PS...i just found a used powerhead for my motor for $500, with COMPRESSION READS of 142/147/146, so there's yet another option for me.

  2. #2
    Supreme Mariner Frank Acampora's Avatar
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    Better at this point to go with the used powerhead because shop time on your engine is going to be way more than the 500 you will pay for the used block.

    Force engines are easy to work on and if you do the job yourself, It costs about 125 each for new Wiseco pistons and say 50 each for boring the cylinders. Then say 50 for gaskets. But its less work to put on the used block.

    At any rate, I would not recommend having it done at the marine shop if you are at all mechanically inclined. Figure about 8 hours to remove the old block and re install the new (used) one.

    How low is the compression on the bad cylinder? If it is low because of bad rings or a damaged piston, it is possible to replace a piston without dismantling the engine--that is IF the cylinder is not scored and you don't need a bore job.

  3. #3
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    Thanks Frank...I was thinking the same thing regarding buying the used powerhead, then just keep my old one for parts. I called one marine shop and they quoted me a generic $3,500 to rebuild....ouch, the boat isn't worth that! I have a call in to see if there is any warranty or anything on the used powerhead. It supposedly runs great...The compression he advertised, of 142/147/146 seemed kind of high, and I'm wondering if maybe it has been rebuilt....I'm waiting on his return call.

    My current motor has like 120 lbs in 2 cylinders, and I think it was 60-80 lbs in the bad one. How could you replace a single piston or rings without dismatling?? I'm betting it is probably scored anyways, from what I have read.

    Another question regarding this used block, I believe it was used in saltwater. Does that have any adverse effects on engines, like it does lower units?

    Thanks for the help!!

  4. #4
    Supreme Mariner roscoe's Avatar
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    Before you decide anything, pull the head off and inspect !!

    Could be no damage at all, and only a blown head gasket.

    And these motors are not known to handle saltwater very well.

  5. #5
    Supreme Mariner Frank Acampora's Avatar
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    Agree with roscoe! 80 psi does not usually indicate major piston/cylinder damage. USUALLY when you do enough damage, piston will be melted, cylinder will be heavily scored (At least .005 and primarily on the exhaust side) and compression will be about 20 to 0 psi. Light scoring, which would be a few lines less than .002 deep, will usually not affect compression to any great degree.

    So 80 psi could indicate a damaged head gasket or a slightly damaged piston/rings.

    Still, if it is a piston that needs replacement and the bore is not scored, you can remove the head, then remove the manifold and carbs. You can then access the rod big end bolts through the reed cavity and just push out the piston and rod. If you are real careful and use a lot of oil to hold the grit, you can even hone the cylinder provided you clean it very well. proper honing removes only a couple of .0001 material and will not require an oversized piston.

    Putting the new one in takes more time and patience (account for all 16 big end rollers and be sure you replace all 16; one floating around in the block is a quick way to scrap an engine.) but it still is way quicker than tearing down the engine.

    Example: I bought a Chrysler 4 cylinder 140--cheap. The p.o. used the wrong plugs and melted a 1/4 inch hole in the crown of two pistons--0 psi compression. The other two were melted but not as badly. The cylinders were not scored. I replaced all four pistons through the reed cavities. It is much more difficult on the 4 cyl blocks because the reed cavities are vertical. So, it took me about 4 hours. Still, beat the hell out of 10 hours to disassemble and reassemble the engine.

    As far as salt use---internally, it doesn't usually affect them unless abused, HOWEVER-- The head bolts and mating holes tend to corrode very badly, the thermostat cover bolts also and can be very difficult to impossible to remove. Additionally, because of the design of the head gasket, water tends to collect behind it on the head sealing surface. At those points the head corrodes and can get to the point where it enters the combustion chamber sealing surface causing loss of compression and requiring replacement of the head. Sometimes it also requires resurfacing the block("decking") Now remember, these are worst case examples. If possible, just inspect the block carefully.

    90 Hp compression ratio for a new engine would be around 155-165 for the Chrysler 90. I don't have specs for Force but 145 or so sounds reasonable for a well maintained engine.

  6. #6
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    Thank you Roscoe. I will pop the head off and take a look today and see how things look.

    Frank, what a great idea on replacing pistons...Thank you!

    What would be the best shop manual for me to buy, keeping in mid I am mechanically inclined but have never worked on an outboard before? Preferably something with really good step by step details with lots of pictures.

  7. #7
    Supreme Mariner roscoe's Avatar
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    The best manual is the factory manual from Mercury, you may find one on ebay if you watch for a week or two.

    The 2nd best, is the Clymer Force Repair Manual.
    Its most detailed, and has model specific info.

  8. #8
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    Thanks again Roscoe...the sun is shining and it's not too cold out, so I am off to re-test the compression and pop the head off, with any luck.

  9. #9
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    OK, now i am really confused.

    Let me start by saying i just bought this boat, and the info i posted was all based on what the P.O. told me, who i felt I could trust.

    So, first I squirt each cylinder with oil lightly, and do a cold compression test. from front to rear I get 100/70/105.

    So then I decide to try and start it. It fires right up (once i figured out there is a manual choke) and i run it for 10 minutes. It sounds perfect... Idles fine, revs great, runs smooth. I shut her down, and re check the compression. Now it is 100/90/100 with the engine warm.

    My comp. tester is 25 years old, but a decent brand...KD.

    What do you think....drop it in the water and try it? Everything seems to be working perfectly...all the gauges, the power trim, the lower lube looks clean.

    The story from the PO was he traded this boat last spring for 2 jetskis. The guys came back and said it had low compression in 1 cylinder, and wanted to trade back & he let them. He said they told him they put a head gasket on and one of the cylinders were scorched, and I can see the head has been removed. Since it was running so good and I was running low on time, I didn't pop the head off yet...kind of wanted some feedback from someone first, but it looks easy enough to remove...14 bolts. Other than the compression, this motor (and boat) is quite immaculate. The PO said it ran fine and he hadn't had it running since he got it back last year, but I was really assuming it would run worse, if at all, and am quite surprised at how well it does run.

    Any thoughts???

  10. #10
    Supreme Mariner Frank Acampora's Avatar
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    Compression is a little low, but even. Put it in the water and see what kind of speed and rpm you get. If it gives you enough speed to satisfy your needs and idles well, leave it alone.

    Since top speed depends on boat size and load, I can't quote an expected speed but It should be able to get the boat on plane and at a minimum with a hull less than 19 feet long you would expect more than 25 MPH top speed. On any hull, I would expect it to turn in the range of 4500 to 5500 RPM.

  11. #11
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    Thanks Frank, I think i will clean the carbs and drop it in the water for a test run. I used the search engine here yesterday and read dozens of posts regarding compression, and found one post stating that the shop manual for the 1990 90 hp Force called for uneven compression in this motor, although the opposite of what mine is, so mine probably does have a scorched cylinder. Even in mint condition, it really doesn't justify me doing a rebuild that may cost as much as the boat is worth...Interestingly, the NADA average retail of this motor was something rediculious like $275 high retail...I know it's worth more than that for parts!

    A great site...I'll be back !!

  12. #12
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    Encouraged by another post also describing in general how to remove a single piston, I did so today on my 1989 Force 125. Getting to the crankshaft from the intake was easy. BUT, getting the piston out, after removing the two 1/4 " 12 point bolts holding the crankshaft cap, without the rollers going everywhere was impossible. I retrieved all 16 of the rollers but, since you've apparently done this before successfully, please tell me how you reinstalled the piston while keeping all those rollers in place in the two bearing cages!

  13. #13
    Chief Petty Officer weimed's Avatar
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    Quote Originally Posted by mano View Post
    Thanks Frank, I think i will clean the carbs and drop it in the water for a test run. I used the search engine here yesterday and read dozens of posts regarding compression, and found one post stating that the shop manual for the 1990 90 hp Force called for uneven compression in this motor, although the opposite of what mine is, so mine probably does have a scorched cylinder. Even in mint condition, it really doesn't justify me doing a rebuild that may cost as much as the boat is worth...Interestingly, the NADA average retail of this motor was something rediculious like $275 high retail...I know it's worth more than that for parts!

    A great site...I'll be back !!
    nada are a bunch of low ballers, I bought a 1996 6hp long shaft for 610 bux a year ago. I see olders ones going on ebay for over 700 (nada saying its worth 400)
    Thank You.
    Zodiac Zoom 350s with 2006 Mercury 15HP

  14. #14
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    With 100/90/100 comp, I'm still inclined to at least remove the head and check the integrity of the gasket. If you are really careful and the gasket is not damaged, you can re-use the old gasket while at the same time having the opportunity to check the cylinders. Obviously, if the cylinders are all smooth, then at least you can properly torque the head also. And then you can re-check the comp again.

  15. #15

    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    Quote Originally Posted by dancewithme951 View Post
    Encouraged by another post also describing in general how to remove a single piston, I did so today on my 1989 Force 125. Getting to the crankshaft from the intake was easy. BUT, getting the piston out, after removing the two 1/4 " 12 point bolts holding the crankshaft cap, without the rollers going everywhere was impossible. I retrieved all 16 of the rollers but, since you've apparently done this before successfully, please tell me how you reinstalled the piston while keeping all those rollers in place in the two bearing cages!
    Hi there dancewithme951, welcome to iboats! As we realize it may have been easier for you to tag your question onto the end of this old thread since your question may pertain to some of the conversation referenced above. Unfortunately it may not make it easier for all of our helpful members that like to log on from time to time and answer a few quick questions when possible before moving onto something else in their life. As their time may be short to try and help, they may not have notice the original posters first date posted and answer the first question at the top before reading any of the other post only to come to find out later that the OP has typically already been helped to the best of our abilities and they may be long gone by now and in turn your question may get skipped altogether like it was done in here by two members already due to your question being near the bottom. So it is iboats recommendation to always start a new thread of your own in order to achieve the best over all results for your question. Thank you and good luck with you getting your engine running back to normal. Also for future reference, Please see our announcement linked right here, Announcements - Mercruiser I/O & Inboard Engines & Outdrives

  16. #16
    Fleet Admiral jerryjerry05's Avatar
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    Yup start your own thread .
    The installing of the bearings and cages should be done with OMC's assembly grease.
    Some use Triple Guard Grease.
    The manual say's the TGG is too thick and won't "melt" away as quick as the assy. grease.
    They say it can actually cause damage.
    Try not to use a magnet either.
    Make sure you use Locktite and tourqe the rod.
    IF YOUR OUTBOARD DOESN"T START WITHING A FEW SECONDS THEN SOMETHING"S WRONG!!!!

    Other websites are NOT the place to buy parts.They managed to screw up my last 4 orders.
    NO communication,NO follow through. Incompetent staff.Nuff said!

    Buy American made Cars and Trucks!!!Towing insurance for a boat,a GREAT thing.

    If you fix your problem,do the rest of the posters a favor and send in one more post telling who,what, how it was fixed.J

    88 Bayliner 21CC Twin 85hp Forces.

  17. #17
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    Default Re: Shop time/cost estimate, replace pistons, Force 90hp??

    Quote Originally Posted by Jiggz View Post
    With 100/90/100 comp, I'm still inclined to at least remove the head and check the integrity of the gasket. If you are really careful and the gasket is not damaged, you can re-use the old gasket while at the same time having the opportunity to check the cylinders. Obviously, if the cylinders are all smooth, then at least you can properly torque the head also. And then you can re-check the comp again.
    Another great tactic is to "untorque" all the head bolts, loosen bolts a turn, not removing anything, and then retorque them in the right order, amazing how this simple trick fixes a multitude of gasket type problems. My son's Caddy was leaking tranny fluid, dealer wanted $850 to fix the problem. I simply loosened all the pan screws and retorqued them all, problem fixed.
    It is better to be silent and thought a fool,
    Than to speak and remove all doubt...

    If you can't find the time to do it right,
    Where will you find the time to do it over?

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    Because he doesn't know any other way....

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