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Thread: 300xs rebuild.

  1. #1
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    300xs rebuild.

    Most would have seen my earlier threads on the issues I have had with my 300xs and eventual blow up (piston picked up on no.5).
    I have undressed the power head and removed and stripped internals (I can say these are not motors you want to do this to often, it took me 2 days and lots of pictures as I went to undress and remove power head. A lot more work than any other regular 3l merc.).
    As mentioned no. 5 piston and sleeve are trashed, I also noted no. 6 looked discoloured compared to others, it had a slightly reddish tinge to top of cylinder and just the slightest scuff in hone marks on top side of cylinder, not gouged in any way just enough to take the hone mark off. All other cylinders look a1.

    Rang my local hp merc dealer to organize parts, I was advised merc didn't offer oversize pistons nor did they supply replacement sleeves, a new block would have to be purchased. I couldn't believe this so got on the phone and made some calls. Another good friend of mine who lives 2000km from me (he is also hp dealer) said he had everything I need including replacement sleeve. Have sent down block for the new sleeve to be installed. He also told me that he was yet to see a 300xs get past 220hrs without something picking up . Now this guy does lots of 300xs motors for ski racing here in Australia and I'll note that none of these are modified. He advised I replace all rings as mine had 156hrs on it cause it was pretty much due for freshen up.
    This information blew me away. I had always imagined these motors would run 500-600hrs hassle free. Clearly I was wrong.
    He said he would check all cylinders for size and out of round and hone all just a tiny bit oversize to give the piston a tiny bit more clearance in bore as he now does this to all 300's and has had better success.
    will keep everyone posted.

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  3. #2
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    Forgot to mention all 12 injectors have gone down to be tested and cleaned.
    something else to note with the opti's is the cooling water enters the top of the block and cools top to bottom, hence why no.5 and 6 are first to nip up as they actually run the hottest. Opposite to old 3l's.

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    I have to disagree on the motors only lasting 220 hrs. I have seen m Y fair share of motors on large offshore boats and tournament redfish guys that have well into the 8-900 hrs mark on original power heads. Cooling is key to keep these motors alive and good oil as well. Compressors are a different story.

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    Quote Originally Posted by merctech5591 View Post
    I have to disagree on the motors only lasting 220 hrs. I have seen m Y fair share of motors on large offshore boats and tournament redfish guys that have well into the 8-900 hrs mark on original power heads. Cooling is key to keep these motors alive and good oil as well. Compressors are a different story.

  6. #5
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    I'm not saying your wrong, it's just what has actually been happening over here in our sport (ski racing).
    The majority of these motors my guy works with run at 6000+ rpm for 40min - 1hr at a time in rivers, mine would be one of the exceptions as I run mostly offshore however I am almost constantly at wot and just changing boat speed with tabs and ballast (gives the skier the best ride possible).
    With the rules in our sport we have no issues with cooling as our maximum allowable engine height is prop shaft level with plank (sportsmaster boxes). I actually run 4" below with the sportsmaster (I know this isn't conducive to best speed). I use a Fuchs premium tcw3.
    I guess comparing apples to apples, the older efi 3L motors would run 6-700 hrs ok, these 300xs motors are not making it near as far in our application?

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    Wow show me a 300X that got 600-700hrs!! I never even got close to 10% of that when I was racing. I personally would run the Mercury Premium plus in the 300XS not anything else.

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    I have 455hrs on mine.
    Allison XB21 2+2



  9. #8
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    Thats great but the application in ski racing is 6000-6500rpm+ for 1 hour runs towing 2 skiers.

  10. #9
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    Never said 300x. 3l fishing motors 225 and 250hp. The x motors were renowned for popping as well.
    spoke to Zigs (zigs marine doing my work) today and quizzed him more on why he thought it was happening and asked if it was just the ski racers having problems. He can only put it down to the stroked crank in the 300 causing a harmonics type issue as he said the 250's were doing fine. You could say 1 or 2 not getting many hrs up before problems is just unlucky but this guy hasnt seen one get past 220hrs! He said the 220hr one was on a social boat (just used for towing on slalom skis and general cruising). I personally know of 6 boats that didn't make it through break in before lunching a piston (every one no.5). Zigs had a 30 odd foot boat with 2 300xs motors (used for offshore cruising) one lunched no.5 at 90 something hrs and the other developed a cracked head (ive already been there too) at 50 something hrs and when he pulled it to replace found a cracked sleeve.
    His advise to me was after 150-170hrs your on borrowed time!
    Not discounting that people haven't seen much higher hrs with no problems but you can't seriously turn a blind eye to what is going on with so so many!
    im seriously considering writing mercury a letter. My motor has had constant issues from day one and they just kept throwing parts at it until it ran out of warranty then the next part they sent come with a bill! Now it's stripped getting a $2500 rebuild at 156 hrs (at 100hrs I had merc computer on it and 60 something hrs of that 100 was at idle!!)

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    had over 300hours ski racing on ours and no issues at all chuck plugs at it every 80 odd hours and a set of thermostats and shes all good.just ran the standard tcw3 merc stats and ****y pump gas as ours is some the worst in the world. allso run the in 6l offshore class and other than a compressor pretty good here.
    Your allways going to get sum lemons in anything you buy.
    Good luck with your letter do post it on here

  12. #11
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    Very interesting thread...
    I cant see how harmonics could be causing just no.5 to fry...
    What does the piston look like? Any signs of a lean out? lubrication issue maybe?
    I can understand losing a compressor or coil everyso often... but a piston with so few hours doesnt sit right...

  13. #12
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    I have 175 hrs on mine, going to rebuild, dirt dauber got mine though, **** happens

  14. #13
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    It would be nice to see what all these pistons look like out #5 cylinder to get to the cause of the problem.
    My guess is detention like every optimax I've rebuild lean condition or bad fuel. When I say bad fuel l mean leaving fuel in tank for weeks and running boat gasoline losses octane rating just sitting so motor pays the price. #5 just might be a cylinder that runs a little hotter than others.
    Optimax motors have a great stoichiometric air fuel ratio so little room of errors can be a problem. Example lower octane rating, timing air temp. Efi motors run super rich reason for them been more less likely to blow.
    As far as cooling system on 3.0l the cooling is from the bottom 5-6 cylinder up to 1-2 at the thermostat

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