1999 Mercruiser 454 Horizon Inboard Valve seats

Discussion in 'Mechanic Recommendation Forum' started by West32, Oct 15, 2018.

  1. West32

    West32 Active Member SILVER Sponsor

    May 24, 2017
    1999 Sea Ray 370 EC Hardtop
    Raymarine Axiom Pro system, AP, Radar, VHF, Fox Marine Engine Monitor
    Twin Mercruiser 454 Horizon 380 hp
    So after I got the heads back, I installed them. While looking up Torque information in the Manual I stumbled across this little Tidbit of information in the service manual - "
    Rocker Arm Stud Kit
    These engines use a net lash or non-adjustable rocker arm system. Torquing the rocker arm
    bolt to 40 lb-ft (54 Nm) sets the valve lash. When rebuilding these engines with a net lash,

    machining of the cylinder deck surface or cylinder head surface or grinding the valve seats
    or valves may cause an incorrect valve lash condition. The most common problem is that

    the valves will not close completely after the bolt is torqued.
    The kit can be used to convert the engine from a non-adjustable to an adjustable valve lash
    engine. There are 8 studs and 8 nuts in each kit. The valve lash is 1 turn down from “0” lash.

    No one mentioned this in all my questions to many builders.

    I bought the kit and installed them. I started the motor, Timed it, then adjusted each rocker once it warmed up. The motor has NEVER run better. Full power all the way up to WOT. Compression is 162 to 165 in every cylinder.

    Learn something everyday..
    douglee25 likes this.
  2. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx
    502 mpi Bravo 1
    That improper lash should of been picked up on the previous build. Even though the lifters are non adjustable they can still be checked for proper fit. You tighten the rocker to 0 lash and then it should take about 3/4 of a turn till you hit the 40#'s.

    Glad it's working out for you. So did you do new seats ?
  3. PlayDate

    PlayDate Well-Known Member

    Dec 25, 2006
    Washington DC
    1994 370 Express Cruiser
    454 Mercruisers
    So to summarize:

    The original rebuild was improperly machined and assembled with the wrong rings and valve lash. As a result, performance decreased over time, compression fell into the 120s and the Intake valves stopped sealing.

    In terms of diagnostics, a leak down test would have shown the intake valves were leaking which might have led to sorting out the valve lash sooner.

    The valve lash issue was entirely avoidable if in the process of doing the original rebuild the machinist had installed new valve seats and ground them to specification. By doing that, the setting of valve lash would use the stock procedure. This is normally the way it is done by competent machinist which is why the Mercruiser footnote on net lash is more of an anomaly.

    Using the wrong rings.....is simply inexcusable.

    I'm glad everything is working well again.
    douglee25 and techmitch like this.

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