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Thread: 300XS bore scuffing
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10-16-2019, 06:44 AM #1Junior Member
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300XS bore scuffing
Just took my heads of a 60 hour 2017 Mercury 300XS and found lots of vertical scuffes along the entire circumference of all 6 bores. Cylinder 1 and 2 are worse then the others. I have attached pictures of cylinder 2 which is the worst one, but all bores visible vertical lines of which some can be felt with fingertips. Motor was was running fine and within 1.5 mph of when it was brand new. However acceleration has become slightly worse over time. I only ever used the new Merc Racing 2 stroke oil (red color) and motor has never overheated.
What could have caused this?
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10-16-2019, 12:49 PM #2
Reassembled without proper cleaning.
Dave1980 Cougar 19 tunnel,90 2.4L Bridgeport EFI in middle of restoration.
1988 BAJA Sunsport 186, 96 225 Pro Max
79 12' Auminum, 95 Merc 9.9
RIP Stu
"So many idiots, so few bullets"
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10-16-2019, 12:54 PM #3Junior Member
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10-16-2019, 01:02 PM #45000 RPM
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I was concerned about the vertical scoring on my cylinders, so I had compression test and leak down done. All six were 155 or 154 lbs. and leak downs were good. I might suggest you have those tests done just to see what they are. If they are within spec, then I wouldn't worry too much about it. Do you give it a good warm up before putting the hammer down?
Whitney Woolard
Oregon/Washington
'96 Allison XB2003 w/Mercury 2.5 260ss
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10-16-2019, 01:13 PM #5Junior Member
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10-16-2019, 01:44 PM #6Screaming And Flying!
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Could it have not been sent into break-in mode when 1st rigged and sold ? Did your smartcraft ga indicate a break-in mode was initiated? I have seen bore scope vids of my 2005 225 opti with 1800 hrs with almost no scratching. I ran prem plus oil it's entire life.
83 V-King, 96 Mariner, 200 hp ff block 2.5 w/a 28p choppa
We gotta clean this liberal mess up, VOTE TRUMP TO MAGA!
Rebuild thread:
http://www.screamandfly.com/showthre...-it&highlight=
http://www.screamandfly.com/showthre...cs.&highlight=
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10-16-2019, 02:08 PM #7Junior Member
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I turned on the motor first time myself and showed 120 mins of break in left. Did the break in myself too and i did consume a lot of oil.
Im thinking motor ingested some debris from the plenum. I’m tearing down the powerhead on Friday and will know more.
I’d expect that type of scuffing on all bores to cause some loss of power.
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10-16-2019, 02:46 PM #8
This was optimax oil?
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10-16-2019, 03:30 PM #9Junior Member
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10-16-2019, 08:25 PM #106000 RPM
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The hash is all wrong to me which trends me to Dave’s statements
it is a mr300xs so?? Looks rich fouled possible carbon scraping. Mostly it looks out of round leading to flame cutting. But dang that some deep hash probably need merc to see these pics if it truly is nib cause that manufacturers defect level bad bore job.
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10-16-2019, 10:53 PM #11
Between digital pictures and my old azz eyes, I'm gonna go with :
Poor wall / port prep. If you look at the transfer side that the streaks are on, the radius's in the corner of the port has little chips missing.
The floor of the exhaust port .. it's hard to tell how much chamfer it has. However, the corner has the ever-popular "machine-finished notch" instead of a nice smooth hand-finished rolled edge.
The sleeves are ductal iron and the rings are steel with a chrome face, so your not likely to see much of a witness mark on the ring, but if you look at your port edges real close .. I'm sure you'll find the source of your foreign objects ..
I spend a good 20 minutes on each sleeve before I even think about using it .
You can see the bottom inside corner of this 300xs sleeve where the machine tool
" J-hooked" before going in the next direction.
The good thing is ... it's the narrowest port. It's at the bottom ( OK - that's two ) And all I can / hafta do is chamfer it back to where the ring will never see it ...
On that one there, 867, I would chamfer it correctly by hand. Hit it a few strokes with a set of 220 stones @ light pressure or 280 stones @ med pressure to make sure the hole is round .. and then finish with a nylon brush in reverse rotation to break away any swarf, wash it @ least twice ... New rings and rod bolts... and enjoy your newly blueprinted, sealed up piece ...
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10-17-2019, 03:44 AM #12Junior Member
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10-17-2019, 06:08 AM #13
U shouldn't need a sleeve , just need fix ones u got then gud hone? Or just send it to Chaz!
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10-17-2019, 07:33 AM #14Screaming And Flying!
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10-17-2019, 08:10 AM #155000 RPM
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Im thinking samething should b under warranty
I would say too poor prep work before assemble i normally run paper towel with cleaner and check for paper towel catching any parts of sleeve. then address it.
On other note i have not had chance to see 300xs ports but exhaust looks small compaired to 225 pro xs 250 pro xs and 300x ports i have seen
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