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Technical Plymouth 218 Flathead...How much compression should I have?

Discussion in 'The Hokey Ass Message Board' started by Ratman1953, Jun 17, 2015.

  1. Ratman1953
    Joined: Mar 25, 2014
    Posts: 14

    Ratman1953
    Member

    Hello. I have a 53 Plymouth Cranbrook with a Hydrive 218. I finally got it running and driving decently (its been sitting for 30 years) so now I'm checking over things to make it more dependable. Do you guys know how much psi I should have per cylinder? I did a quick search and wasn't able to find anything. I have yet to check how much I actually have. I was planning on doing it this week. I'm new to flatheads so sorry if this is a dumb question. Thanks in advance for the advice.
     
  2. R Pope
    Joined: Jan 23, 2006
    Posts: 3,309

    R Pope
    Member

    You "should" have somewhere around 80 or 90 pounds, but I have had lots of those old girls that would barely bump a gauge and they ran fine. You want to drive it for a while before doing a comp test, let it loosen everything up if it sat for 30 years.
     
  3. Ratman1953
    Joined: Mar 25, 2014
    Posts: 14

    Ratman1953
    Member

    Thanks. Ive driven it a decent amount since I got it running. Ill check it out this weekend or early next week and see how it is.
     
  4. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,657

    Rusty O'Toole
    Member

    Compression ratio 7:1 so compression pressure should be 100 pounds, more or less. If all cylinders are within 10% of each other and above 80 pounds you are in pretty good shape.

    Oil pressure 45PSI @ 30 MPH

    These engines are foolers. They will continue to start and run with no bad knocks or bangs in an advanced state of wear. The only symptoms are hard starting, bad gas mileage and no power. If you have bad oil pressure and bad compression, try a ring and valve job or in extreme cases rebuild the engine, you will be surprised how much better it runs.
     

  5. Ratman1953
    Joined: Mar 25, 2014
    Posts: 14

    Ratman1953
    Member

    Thanks for the info!
     
  6. plym_46
    Joined: Sep 8, 2005
    Posts: 4,018

    plym_46
    Member
    from central NY

    remember your transmission is piped to and uses engine oil. An oil change requires about 10 quarts give or take.
     

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