Engine part compatibility question.

Discussion in 'General Maintenance/Repair Questions' started by Hoplite808, Nov 29, 2022.

  1. Hoplite808

    Hoplite808 Well-Known Member

    345
    Jan 11, 2022
    Honolulu, HI
    1996 Sea Ray 270 Sundancer
    Mercruiser 7.4 LX MPI EFI (Gen. V)
    Been having a problem with the valves seating properly even after a full rebuild on cylinder #7. The price of a replacement rebuilt cylinder head for a 7.4 mercruiser is about $1200.00 plus $300.00 shipping. However the head looks exactly like the Chevy 7.4 head of the same year which I can get from the auto parts store for $323.00. Does anyone know if there is a difference between these two parts?
     
  2. PlayDate

    PlayDate Well-Known Member

    Dec 25, 2006
    Washington DC
    1994 370 Express Cruiser
    454 Mercruisers
    Just so I understand…..the engine was rebuilt and the valves aren’t sealing?

    In regards to the heads it comes down to what the base engine is. A part number off the head or the block serial number would be helpful.

    The differences to understand are the intake port (oval versus rectangular) and combustion chamber size.
     
  3. Hoplite808

    Hoplite808 Well-Known Member

    345
    Jan 11, 2022
    Honolulu, HI
    1996 Sea Ray 270 Sundancer
    Mercruiser 7.4 LX MPI EFI (Gen. V)
    I had the head rebuilt and I think the machinist I used may have over done the grinding of the valves to get them to seat again. It changed the geometry of the whole thing and now the rocker arm is holding the valves up from seating entirely I think. I tried shimming it but it didn't work for long. Since I don't have another machinist to go to locally I need to source a replacement head to get everything back into spec. I'd rather not spend nearly $2000 as I've spent way too much as it is already on the boat.
     
  4. ttmott

    ttmott PhD in OCD TECHNICAL Contributor GOLD Sponsor

    Apr 3, 2012
    Space Coast Florida
    2006 52 Sedan Bridge
    Cummins QSM11
    Like Playdate said there are a half a dozen different cylinder head arrangements for the BBC, so it's essential to match.
    All valve installations will have differing installed heights. The machinist measures the installed height then measures the spring tension and installs shimming to make all valves the same open and closed spring tension. The valve stem heights will vary to some extent.
    The rocker arm / pushrod / hydraulic lifter are adjusted on the heel of the camshaft where the lash is zeroed then another 3/4 turn on the adjusting nut which sets the valve train mid-way in the lifter's range. It almost sounds like the valve lash wasn't done correctly.
     
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  5. Hoplite808

    Hoplite808 Well-Known Member

    345
    Jan 11, 2022
    Honolulu, HI
    1996 Sea Ray 270 Sundancer
    Mercruiser 7.4 LX MPI EFI (Gen. V)

    The rocker arms on the 1996 7.4 do not have an adjustable nut. They torque down to a predetermined height only with a bolt. Since the machinist on mine overground the one cylinder to get it to seat it changed the geometry and I had to try and shim the rocker arm bolt to get it to raise the adjustment on the rocker arm so it wouldn't hold the valves open. That didn't hold. So now I'm thinking of just ordering a replacement cylinder head but I'm curious if I can save some money with this part:

    https://www.oreillyauto.com/detail/...vrolet-c3500-pickup-rwd?q=cylinder+head&pos=0

    instead of this part:

    https://www.mercruiserparts.com/883492r1-head-cyl-rm-454
     
  6. SKybolt

    SKybolt Well-Known Member SILVER Sponsor

    Nov 11, 2014
    Kent Narrows, MD
    460 EC
    Detroit 6v92TA
    (Low profile's)
    Alison Gears
    Westerbeke
    12.5kw Genset
    So you also say you only rebuilt #7 and not the rest?

    To answer your question the biggest difference is brass or stainless freeze out plugs. Also marine heads are typically not a triple face valve. Assuming the castings are the right ones that's basically it.

    To adjust the valves there isn't an adjusting nut, it's done by the rocker center nut. The old school way is to loosen until clap and tighten until basically no clap and then 1/4-1/2 turn.
     
  7. ttmott

    ttmott PhD in OCD TECHNICAL Contributor GOLD Sponsor

    Apr 3, 2012
    Space Coast Florida
    2006 52 Sedan Bridge
    Cummins QSM11
    I think you will find that nut in the rocker arm is the adjustment for the valve lash. All Gen 3 Big Block Chevy's (7.4/454) are the same for hydraulic camshaft lifter adjustment. Mercruiser configuration is the same as any Chevy Big Block. Where did you hear it needs to be "torqued down"?
    If you tighten that nut all the way down there is a good chance the piston will hit the valve.... Or if you get off easy spring coil bind and a bent push rod.
     
  8. Hoplite808

    Hoplite808 Well-Known Member

    345
    Jan 11, 2022
    Honolulu, HI
    1996 Sea Ray 270 Sundancer
    Mercruiser 7.4 LX MPI EFI (Gen. V)
    @SKybolt and @ttmott There are no studs on this setup for a nut to go into. It is a special bolt that per the mercruiser repair manual must be torqued down to 40 ft/lbs. This is a different setup than you are thinking of.
     
  9. Hoplite808

    Hoplite808 Well-Known Member

    345
    Jan 11, 2022
    Honolulu, HI
    1996 Sea Ray 270 Sundancer
    Mercruiser 7.4 LX MPI EFI (Gen. V)
    @SKybolt and @ttmott This is what the rocker, ball, and bolt look like on this setup. There is no adjustment ability.
    Rocker and ball.JPG Rocker arm bolt.JPG
     
  10. PlayDate

    PlayDate Well-Known Member

    Dec 25, 2006
    Washington DC
    1994 370 Express Cruiser
    454 Mercruisers
    Do you have the original block serial number?
     
  11. ttmott

    ttmott PhD in OCD TECHNICAL Contributor GOLD Sponsor

    Apr 3, 2012
    Space Coast Florida
    2006 52 Sedan Bridge
    Cummins QSM11
    Ah - so this setup requires specific pushrod lengths or possibly shimming under the bolt head. PITA. I didn't know there were any of these in the Merc BBC lineup that year (being Mark V or Mark VI) - learn something every day. So, you are right if the valves were ground and/or the head milled all of the pushrods need to be measured and most likely changed. You will need a pushrod length checker tool and the procedure for that engine/camshaft. Then order the pushrods. My engines require specific length pushrods so the rocker is centered on the valve stem rather than a height requirement. I can say it is a long arduous process.
    You might want to consider installing a more traditional setup.
     
    Nater Potater likes this.
  12. SKybolt

    SKybolt Well-Known Member SILVER Sponsor

    Nov 11, 2014
    Kent Narrows, MD
    460 EC
    Detroit 6v92TA
    (Low profile's)
    Alison Gears
    Westerbeke
    12.5kw Genset
    Sorry, didn't know that setup was available in 96'. Thought that was only way into the 2000'.

    Given that, then you can't use the aftermarket head. The rocker seats are a set height.
     
  13. techmitch

    techmitch Well-Known Member PLATINUM Sponsor

    May 1, 2008
    Chesapeake Bay, Middle River/Frog Mortar Creek MD.
    1999 270 DA
    Twin 4.3s W/Alpa I Gen II's
  14. Hoplite808

    Hoplite808 Well-Known Member

    345
    Jan 11, 2022
    Honolulu, HI
    1996 Sea Ray 270 Sundancer
    Mercruiser 7.4 LX MPI EFI (Gen. V)
    The engine serial number is OF764186.
     
  15. Hoplite808

    Hoplite808 Well-Known Member

    345
    Jan 11, 2022
    Honolulu, HI
    1996 Sea Ray 270 Sundancer
    Mercruiser 7.4 LX MPI EFI (Gen. V)
    Yes, but that photo is from before I removed the head and sent it to the machine shop. Since then I got it back and I know they over ground the valves now on the number 7 cylinder to get it to seat. However shimming the rocker bolts did not work for me for very long on the #7 cylinder, pictured on the far left of this photo, but again this was before the rebuild. Since the shimming to get it right is beyond my ability I'm thinking I should just replace the head but I'd rather not spend 2 grand if I can get the same part for $300.
     
  16. Bill Curtis

    Bill Curtis Well-Known Member

    533
    Mar 24, 2022
    Ahoskie, NC
    '95 220 Overnighter
    454 Mag Bravo3
    I dunno about you, with all this hassle you are having, I would install studs where those bolts go and use the old school rocker, rocker ball and adjustment nut, like 99% of the Chevy's out there.

    May have to have the bolt / stud hole machined to 7/16 / NC to accept the studs.

    Then you would have adjustable rockers and not need to go thru all this crap of shimming and such.

    Side note, where as the valve seat was ground too deep, the seat can be replaced and reground correctly.
     
    SKybolt likes this.
  17. Hoplite808

    Hoplite808 Well-Known Member

    345
    Jan 11, 2022
    Honolulu, HI
    1996 Sea Ray 270 Sundancer
    Mercruiser 7.4 LX MPI EFI (Gen. V)
    The machinist I have access to can't find a source for the oversized valves, guides, and seats in this head. My preference of course would be to not pull the head, again, and spend several hundred dollars on gaskets but I think I've exhausted options for that and the only way to fix it now is a rebuilt head sourced from somewhere else. I can't afford to dump thousands more into the boat right now. Which is why I asked if the Oreilly head is the same as the Mercruiser one. It would save me a small fortune if it were.
     
  18. ttmott

    ttmott PhD in OCD TECHNICAL Contributor GOLD Sponsor

    Apr 3, 2012
    Space Coast Florida
    2006 52 Sedan Bridge
    Cummins QSM11
    Nater Potater likes this.
  19. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx 502 mag
    2005 Baja 242 islander 496
    502 mpi Bravo 1
    496 mag B1
    There's no info on that replacement head. You need a replacement head of the correct generation. What gen. is your engine?
     
  20. Lenny3

    Lenny3 Active Member

    109
    Mar 19, 2021
    Selinsgrove Pa.
    2007 Sea Ray 310 Sundancer
    2006 Sea Ray 220 Sundeck (Previous)
    Twin 5.7 350 MPI
    5.0 Mercruiser Bravo III
    When you change valve stem height in the head 2 things have to change. Rocker arm centerline (fulcrum bolt shims) AND push rod length ( longer). This keeps the geometry, correct.
    I’m not 100% sure but your machine shop should have a chart to calculate both for you
     
    techmitch likes this.

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