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Thread: M51 v M55

  1. #1
    Join Date
    Sep 2001
    Palmerston North, New Zealand

    Default M51 v M55

    A bit of quick research in Bruce Palmers book shows that internally both carbs are identical.
    There are two external differences one minor but one is a major difference.
    The minor one is the throttle lever 1238-36 / 37 / 40 depending on application.
    The major one is the carb mounting flange with one being 3 bolt and the other being 4 bolt.
    I'll hold my hand up and say knuckles are way out of my league and what follows could prove to be wrong but here goes.
    The M5 fitted to 1936-39 knuckles I suspect was a 3 bolt mount and as most part numbers for the inlet manifold change to -40 part number in my late model parts book I'm led to suspect that that is when the 4 bolt manifold started.
    Robbie or some knuckle guru help me here.
    The main point here with regard to Steve's bike is that I'm at a bit of a loss to understand how a 4 bolt mount carb is fitted to a 3 bolt manifold. Who knows, anything's possible
    Does that answer your question kitabel
    Last edited by Tommo; 11-30-2018 at 11:00 PM. Reason: additional text
    Peter Thomson, a.k.a. Tommo
    A.M.C.A. # 2777
    Palmerston North, New Zealand.

  2. #2
    Join Date
    Aug 2004
    Beautiful Northern New Mexico


    Tommo, an M55 is a three bolt carb.
    Robbie Knight Amca #2736

  3. #3
    Join Date
    Jul 2002
    North Hills, CA and Pine Grove, CA


    The M55 was the factory replacement for the M5.
    Be sure to visit;
    Be sure to register at the site so you can see large images.
    Also be sure to visit

  4. #4


    A bit of quick research in Bruce Palmers book shows that internally both carbs are identical.
    Is that what he actually said? Or the conclusion that you draw from his commentary?
    No, they're not.

  5. #5
    Join Date
    Jun 2001
    Central Illinois, USA


    From those that crossed my benches, Folks,...

    Other than a different main nozzle,
    The two differences that come to mind are that the M55s did not have the 7-64 bowl, nor were the bodies nickel, as were pre-War M51s.
    Post-War differences escape me without an M55 currently on the bench.

    Some of you may have noted that the Pre-War M51s were milled and drilled to accept the OHV bowl.

    Just wanted to get those out of the way, thanks.

    Last edited by T. Cotten; 12-01-2018 at 09:20 AM.
    AMCA #776
    Dumpster Diver's Motto: Seek,... and Ye Shall Find!

  6. #6
    Join Date
    Sep 2009


    My M55 has the early bowl with the cast #, no drain plug and is 3 bolt.

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