Replace exterior rusted manifolds?

Discussion in 'General Maintenance/Repair Questions' started by vector4, Feb 16, 2020.

  1. vector4

    vector4 Member

    52
    Jun 24, 2015
    Baton Rouge
    2000 Sundancer 270
    310hp
    First I have a full closed cooling system. This is a 7.4l mpi with 740 hours on it. I yanked my risers and they are not terroble themselves but going to replace them. Now my manifolds have surface rust pretty decent. In fact the screws into the block are rusted and will need to be cut off and drilled out. That's the extent of the surface rust.

    Now the inside of the manifolds and riser spacers looks absolutely fine. Barely any surface rust on the interior walls at all. Is the rust on exterior surface enough that I should replace them now? Or should I continue to use them as they are working fine?
     
  2. joeyleggz

    joeyleggz Active Member

    589
    Jan 15, 2013
    long island
    current boat: 1997 sea ray 330 express cruiser
    twin 454's efi
    Pics?
     
  3. DWABoat

    DWABoat Well-Known Member

    606
    Aug 13, 2019
    S Louisiana
    2001 280 Sundancer
    1989 220CC Cuddy Cabin
    Twin 4.3 w/Alpha Ones
    5.7 Mercruiser Alpha One
    Were there symptoms leading to pulling the manifolds?
    Engine temperature?
    Water seepage?
    Hot spots?
    One side running hotter?

    Or was this just routine maintenance and all was fine operationally?
     
  4. vector4

    vector4 Member

    52
    Jun 24, 2015
    Baton Rouge
    2000 Sundancer 270
    310hp
    Sorry, didn't grab pics when I was there today.

    I'm pulling the motor to have the coupler changed. Thought it's a good time to do manifolds if need be. I've known the bolts are rusted badly for a while. No issues with exhaust otherwise. The shop can do it while the engine is out but I'm looking at a decent cost to have them screw around with extracting the studs after they cut them off.
     
  5. Air O'Nautical

    Air O'Nautical Well-Known Member

    Feb 5, 2008
    Stamford CT "Ponus Yacht Club"
    1995 400EC. Raymarine E80,4kw dome, Garmin 235 gps,
    2013 7.4L Mercruiser Blue water.
    Hurth straight shaft 630's,
    Westerbeke 7.0 BCG
    Often you can hammer on a smaller size socket to remove the studs or drill off the heads and vice grips after the manifold is off. Not a big deal if the motors out of the boat. If at all in question..replace them.
     
  6. DWABoat

    DWABoat Well-Known Member

    606
    Aug 13, 2019
    S Louisiana
    2001 280 Sundancer
    1989 220CC Cuddy Cabin
    Twin 4.3 w/Alpha Ones
    5.7 Mercruiser Alpha One
    Given there were no symptoms I would think your passages are fine. The mating surfaces are the concern.

    Was water leaking from the joints causing excessive rust? If you have surface rust is that degrading the mating surfaces where the gaskets keep water passages separated? Any problems there then manifolds must go.
     
  7. RBB

    RBB Well-Known Member

    527
    Dec 18, 2019
    Kentucky
    2004 390 MY
    2019 Sea Doo GTX 155
    2020 Walker Bay Venture 14
    Pelican Kayak
    8.1’s straight drive
    60 hp Honda
    If you end up having to drill bolts out use a left hand drill bit. Works great and most likely the bolt will back out on its own.
     
  8. Espos4

    Espos4 Well-Known Member

    Jan 1, 2017
    Long Island NY
    2007 240 Sundeck
    350 MAG Bravo 3 W/DTS
    If the bolts are rusted and need to be cut off, do it now with the motor out of the boat.
    It’ll be much easier than having to do it with the motor in the boat.

    Heating them with a torch is much safer to do in a workshop vs. in your bilge.
     
    DWABoat likes this.
  9. techmitch

    techmitch Well-Known Member GOLD Sponsor

    May 1, 2008
    Chesapeake Bay, Middle River/Frog Mortar Creek MD.
    1999 270 DA
    Twin 4.3s W/Alpa I Gen II's
    If they are original/20 years old, replace them without question.
     
    DWABoat and RBB like this.
  10. DWABoat

    DWABoat Well-Known Member

    606
    Aug 13, 2019
    S Louisiana
    2001 280 Sundancer
    1989 220CC Cuddy Cabin
    Twin 4.3 w/Alpha Ones
    5.7 Mercruiser Alpha One
    Espos is totally right. Don’t delay that type of job - much easier standing upright in a workshop than hanging upside down using a mirror to see anything. Also much less likely to cause more damage while doing it.

    My concern is the reason they are so rusted. If your manifolds were leaking so bad, the mating surfaces must be trashed. Check your valves for rust.
     
  11. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx
    502 mpi Bravo 1
    Ferrous bolts rust and there is no water at that gasket.

    Agree r/r when it's out
     
  12. DWABoat

    DWABoat Well-Known Member

    606
    Aug 13, 2019
    S Louisiana
    2001 280 Sundancer
    1989 220CC Cuddy Cabin
    Twin 4.3 w/Alpha Ones
    5.7 Mercruiser Alpha One
    In the photo, the rust around the cylinder to the far right was caused by a manifold leak above the bolt. I almost had to cut the bolt head off it was rusted so badly. The leak also caused surface rust on the manifold. All the other bolt heads were fine. This motor is 19 years old.

    I am curious why your bolt heads are so rusty they have to be cut off?
     

    Attached Files:

  13. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx
    502 mpi Bravo 1
    Big blocks like his attach differently than yours, they're short bolts and hold the manifold on in an area that has no cooling. So they get way hotter than yours and heat promotes rust.
     
  14. DWABoat

    DWABoat Well-Known Member

    606
    Aug 13, 2019
    S Louisiana
    2001 280 Sundancer
    1989 220CC Cuddy Cabin
    Twin 4.3 w/Alpha Ones
    5.7 Mercruiser Alpha One
    So on the 5.7 (my other boat) and this 4.3, the manifolds at the head run cooler than big blocks by design?

    I have dealt with V6’s, small block V8’s, outboards, and diesels but never a gas big block.

    After 20+ plus years, I still learn everyday!
     
  15. Korkie

    Korkie Active Member GOLD Sponsor

    766
    Feb 2, 2016
    Leonardtown, MD Potomac River / Chesapeake Bay
    340 Sundancer 2006, Garmin 7612, xHD Radome
    Merc 496 Mags
    Bravo III Drives
    Change them now. You will be way ahead of the game. Murphys Law - change them now, you know you are good. Wait . . . . and half way through this season you will wish you had. Just did both of my 496s this past fall. No real overheating problem, just a slight rise in temp on one motor when running hard (3800 rpm). Also had no record from PO of when they were last done. Normally cruise @ 3250 / 3400. Was soooo glad I did. After getting them off found all 4 of them were in danger of inner wall breakdown. Could easily push a screwdriver through in more than one place. Merc parts guy said I was just 1 cruise short of a catastrophe. You really cannot tell their true condition from the outside as the inside is what rusts through first and causes the problem. Mine looked great from the outside.
     
  16. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx
    502 mpi Bravo 1
    The manifolds don't run cooler at the head, the bolt head does. That area on the BB where the bolts are, get so hot they wont hold paint. On the small blocks the bolt heads are much further away from the heat.
     
  17. vector4

    vector4 Member

    52
    Jun 24, 2015
    Baton Rouge
    2000 Sundancer 270
    310hp
    I don't have any logs from when they were last changed. I've had this boat 4 years and about 300 hours.

    I guess from the recommendations here most people are saying go ahead and do it, which is what I was leaning towards. It's mostly on the port side bolts and I think it's from drive fluid leaking from the reservoir and getting on them mixed with the heat.
     
  18. techmitch

    techmitch Well-Known Member GOLD Sponsor

    May 1, 2008
    Chesapeake Bay, Middle River/Frog Mortar Creek MD.
    1999 270 DA
    Twin 4.3s W/Alpa I Gen II's
    Drive fluid and heat isn't going to make them rust.
     

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