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  1. #1
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    Default Johnson 90 HP piston failure

    Well boys - looks like my 1988 Johnson 90 just ate a piston.
    Funny thing was it acted up for a bit on the lake, and then ran normal again.
    Anyhow, I decided to pull the spark plugs, and discovered that all were at about 0.035 except for what I believe is cyl #4 (port side lower). It was at less than 0.020.
    I checked for rod play using a dowel and there was none. So I thought it must have sucked in a washer or something, and decided to pull the port head.
    Yep, edge of the port side lower piston has come apart, and the glorious little dents all over on the inside of the cylinder head.
    It still had good compression despite all this (120 PSI on all, except for the damaged one which has 115) so now I am rather actually thankful that my old Johnny got me home instead of leaving me stranded. Still started and ran perfect when I flushed it out on muffs yesterday evening.
    Was wondering how feasible it would be to try and slap another piston in there and call it fixed.
    Any comments or advice on this matter is welcome.
    Thanks for reading!
    CarMech1969

  2. #2
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    Default Re: Johnson 90 HP piston failure

    In my opinion everyone of these V-4 crossflow engine will suffer the same fate.----Broken rings.-----Simply due to the design / wear characteristic of the top piston rings.----You need 4 new pistons either standard or O/S to make it live again.-----I do not have enough fingers and toes to count all of these in the shed / on the shop floor with the same failure.

  3. #3
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    Default Re: Johnson 90 HP piston failure

    Thank you racerone for chiming in!
    Seems to me that this is a very common issue for these motors as you say.
    Any recommendation on piston brand?
    CarMech1969

  4. #4
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    Default Re: Johnson 90 HP piston failure

    Need to "mike" the cylinder/s to see if out of round (90% are)at ports, and bore to size that cleans up. If cylinder is scuffed/grooved it will need boring anyway. I use WSM pistons only . I only use Seizecos as last resort.
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  5. #5
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    Default Re: Johnson 90 HP piston failure

    Thank you faztbullet for your reply.
    I have seen on numerous posts here and on other forums that it is ok to punch out only the affected cylinder, and leave the other ones alone.
    Wouldn't that affect balance? The OS piston will be heavier.
    How critical is balancing for these outboards?
    Just trying to get an idea of what it will take to fix it properly - and also what I could get away with if I wanted to do a cheap job.
    Thanks again to everyone that dropped a line
    CarMech1969
    Last edited by CarMech1969; 10-19-2020 at 04:03 PM.

  6. #6
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    Default Re: Johnson 90 HP piston failure

    Why would you say the O/S piston would be heavier.-----Have you weighed any ???------The Wiseco----(AKA Seizecos according to some ) catalogue clearly states pistons are matched for individual replacement.-----I have installed numerous Wiseco pistons.------Picked up in Woodstock, Ontario with no problems !!!-------You take your motor apart.-----Measure and evaluate all parts.----Then decide on the path forward.----You are fixing your own motor so you can do as you please.----No need to offer any warranty to yourself !!

  7. #7
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    Default Re: Johnson 90 HP piston failure

    Thanks again racerone for the additional information.
    To answer your question - no I have never weighed one, and have never delved too deeply into two-stroke motors.
    Well now I have some idea of what I am looking at.
    Appreciate all the help everyone!
    If there is anything else that I should be mindful of (as I am a novice when it comes to outboards), please don't hesitate to let me know.
    CarMech1969

  8. #8
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    Default Re: Johnson 90 HP piston failure

    Tear it apart, mike bores, replace ALL pistons as one has failed and the others are not far behind. Usually all 4 will need bored due to wear around ports. A quick and easy way to test is to take a ridged hone and run thru the bores. The out of round areas will not be touched by hone and will stand out big time. If you use Siezcos make sure to run them through several heat cycles to temper them up as they as soft for a forged piston. I also never mix cast and forged pistons.
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  9. #9
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    Default Re: Johnson 90 HP piston failure

    Thanks again faztbullet!
    Appreciate all the help.
    CarMech1969

  10. #10
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    Default Re: Johnson 90 HP piston failure

    Wiseco has been making pistons for how many years now ??

  11. #11
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    Default Re: Johnson 90 HP piston failure

    Quote Originally Posted by racerone View Post
    Wiseco has been making pistons for how many years now ??
    I have no idea, but if people are calling them "Seizcos" that is a cause for concern (in my mind at least anyway).
    I am going to start the teardown, and take plenty of pictures...and see what else I might find.
    CarMech1969

  12. #12
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    Default Re: Johnson 90 HP piston failure

    If there was something wrong with Wiseco pistons , they would have corrected that 30 years ago !!----I install them.----Pick them up in Woodstock.----No issues at all.

  13. #13
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    Default Re: Johnson 90 HP piston failure

    An update on the current situation:
    When I pulled the carbs and had a look down into the intake, I had found one of the mounting bolts that was missing from the air silencer base that mounts to the top of the carbs. Son of a gun. The washer did not stay with the bolt. Guess which part of the intake i found it in? Yep...port side lower. So it looks like my engine ate the washer....and it went jumping around in there and hammered the piston up pretty good. Cylinder is not seized - and still makes 115 PSI compression. Lol. Just my luck that the bolt loosened up. Should have checked them but didn't think of it. And I had rebuilt the carbs recently - so you know what that means. The gaskets were still fresh and were still squeezing down, allowing the bolts to get loose. I forgot to retorque 'em. As they say, hindsight is always 20/20.
    CarMech1969

  14. #14
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    Default Re: Johnson 90 HP piston failure

    Here's a video of the powerhead up on the bench.
    https://vimeo.com/470799893
    Last edited by CarMech1969; 10-21-2020 at 07:50 PM.

  15. #15
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    Default Re: Johnson 90 HP piston failure

    What's the cylinder wall look like?
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  16. #16
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    Default Re: Johnson 90 HP piston failure

    Quote Originally Posted by faztbullet View Post
    What's the cylinder wall look like?
    They look ok just like the other ones - except for a wee bit of scratching in the spot where the suspected washer has landed.
    I dragged my fingernail across that spot and it catches a tiny little bit, almost nothing.
    Knowing what I now know about this motor.... starting to think maybe I should just get the one cylinder bored and honed, fit a new OS piston, and leave the other ones alone and call it good. However, that may not be the best idea. Maybe better to do all 4 and be done with it. Gotta see what this is all gonna take once I get the pistons and crank out. Cost will be the dictating factor here lol. If things start getting ridiculous, might be better to find another outboard.
    Anyhow, will get some pictures of all that up on here soon.
    Thanks for your inquiry!
    CarMech1969

  17. #17
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    Default Re: Johnson 90 HP piston failure

    Well------Looking OK to you and actual measuring to see the numbers are a different thing.

  18. #18
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    Default Re: Johnson 90 HP piston failure

    Quote Originally Posted by racerone View Post
    Well------Looking OK to you and actual measuring to see the numbers are a different thing.
    Of course this is true.
    I have a decent Vernier caliper with a dial gauge that is accurate to 0.001 inch.
    I have no way to measure the bores, so I'm picking up a telescoping gauge set so I can do that.
    Going to start tearing down the powerhead this afternoon if I can get my mojo going good enough.
    It's hard finding energy when doing a half marathon daily.
    Anywhoosers - have a good day, and please keep the good information coming.

    CarMech1969

  19. #19
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    Default Re: Johnson 90 HP piston failure

    And now - time for another video.
    https://vimeo.com/user125564726/revi...258/ccd24d6a99
    Cylinders have some light scoring, pistons are scuffed up pretty good on the sides of the piston skirts.
    Apparently my lower crankshaft bearing is no good, which I believe to be a contributing factor for the unusual piston wear.
    Anyway, will get some pictures and further details up as time allows.
    Thanks everyone for following along.
    CarMech1969

  20. #20
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    Default Re: Johnson 90 HP piston failure

    Looks like I left a bad link.
    Let's try this one for the second video:
    https://vimeo.com/471578258
    "Least amount of damage" - when it comes to the amount of scuffing anyway.

    CarMech1969
    Last edited by CarMech1969; 10-23-2020 at 08:51 PM.

  21. #21
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    Default Re: Johnson 90 HP piston failure

    Bore all 4 .020 OS, replace bearings, install new deflectors,t-stats, impeller,reassemble and go boating
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  22. #22
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    Default Re: Johnson 90 HP piston failure

    Hi Faztbullet,
    That is what I want to do, but am afraid of cost getting too crazy.
    Once I mike the bores, I'll let you know what the numbers are.
    Thanks for stopping by!
    CarMech1969

  23. #23
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    Default Re: Johnson 90 HP piston failure

    Post a video of you measuring the bores.

  24. #24
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    Default Re: Johnson 90 HP piston failure

    Quote Originally Posted by racerone View Post
    Post a video of you measuring the bores.
    I would like to, but I was having trouble getting consistent readings with two hands working the telescopic gauge.
    I swept the gauge around in the bore, and rocked it back and forth to try to get the largest reading possible, and I got a reading of 3.502 inches at the bottom of the bore where the piston ring reaches BDC, and 3.503 inches at the top of the bore. I tried to ensure that I held the handle of the gauge parallel to the bore as best I could.
    This sort of thing I am not very well experienced with. I dragged the gauge lightly around the bore in different positions to see if i could feel any larger diameters, but I couldn't notice anything appreciable. If there is anything wrong with my methods, please let me know. Thanks very much.
    CarMech1969

  25. #25
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    Default Re: Johnson 90 HP piston failure

    If you are seeing 3.503 you need to rebore !!!----Measuring is not a mystery to me.----Worked in a shop where tolerances of +/- 20 millionths of an inch could be held.-----At a job interview I was asked if I knew what the tools on the table where and if I could use them.-----After doing just 1 measurement I asked the interviewer if he would like some more measurements.----His reply----" no that is good enough , no need to show me anything else "---I did get the job !

  26. #26
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    Default Re: Johnson 90 HP piston failure

    Quote Originally Posted by racerone View Post
    If you are seeing 3.503 you need to rebore !!!----Measuring is not a mystery to me.----Worked in a shop where tolerances of +/- 20 millionths of an inch could be held.-----At a job interview I was asked if I knew what the tools on the table where and if I could use them.-----After doing just 1 measurement I asked the interviewer if he would like some more measurements.----His reply----" no that is good enough , no need to show me anything else "---I did get the job !
    That's awesome
    I suspected that this was going to be the verdict coming, but hey - it's an old motor, and not only this issue, but the lower crankshaft bearing is really loose and noisy, and the con rod bearings are not too much better. So basically it was running on borrowed time. I always wondered what that funny rattling noise was for the past 6 years. Well now I know. Lol. Surprised that the crank didn't come flying out of the thing!
    CarMech1969

  27. #27
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    Default Re: Johnson 90 HP piston failure

    I know that I could just slap some new bearings in the engine and toss a used piston at #4 - but that would be a hack job, and probably wouldn't last very long

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