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Flathead build question

Discussion in 'The Hokey Ass Message Board' started by AMC360, Sep 2, 2009.

  1. AMC360
    Joined: Jul 24, 2007
    Posts: 76

    AMC360
    Member
    from Canada

    I am in the process of collecting parts for my 51 flathead and I am thinking on buying a crank kit. I am building it for a mild street engine, Offy heads and intake, dual carbs. The question is what crank 4", 41/8", or 41/4". Is there a point in puting the big stroke and bore in it if it's just a mild street engine or is bigger always better?
     
  2. The general consensus is that bigger is always better, but there was a post a while back I read where a flathead expert (Flatdog) had problems with cranks over 4". The problem was the pistons would not keep a good seal and leaked by, this was to to the longer stroke creating side forces in the cylinder. (please correct me if I'm wrong here).

    If you were building a race motor it would be a different story, but for a driver, I like a 4" stroke with a 3 5/16" bore, that should be plenty to keep a grin on your face, if not, supercharge it...
     
  3. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,681

    Bruce Lancaster
    Member Emeritus

    I know Flatdog had oil consumption problems on his 12 second coupe...no functional problems, probably because boost kept the oil downstairs at the strip!
    Theories centered around piston rock from rod angularity. Remember that he was a serious developer, and he tried several types of rings including zero gap type.
    His next engine was to be shorter stroke. I don't remember the stroke he used, but it was one of the very long ones now available. I would stick to 4 or maybe 4 1/8 myself.
    I think his engine was a 307. It did not smoke, at least from what I could see when I got to drive it, but oil just kept going away.
     
  4. I like to stick with 4" Merc (real American Iron) crank and 3 5/16" bore, makes for a good streetable engine. You can go 4 1/8" stroke with aftermarket crank or offset grind the Merc and use early rods.
     

  5. AMC360
    Joined: Jul 24, 2007
    Posts: 76

    AMC360
    Member
    from Canada

    Thank's for the info guys, I will probably buy a 4 1/8 stroker kit. I have given up on finding a used merc crank, gone through 6 supposedly merc engines and all turn up to be Ford cranks.
     
  6. Bruce,
    thanks for straightening me out on the deal, I remember it being discussed before, I just couldn't find the exact post, or remember all the details.
    Todd
     
  7. Flat Ernie
    Joined: Jun 5, 2002
    Posts: 8,406

    Flat Ernie
    Tech Editor

    4-1/8 + .125" will get you a 284 - nice streetable engine w/o any serious issues or custom parts required. You can offset grind a good 4" crank & use early rods, or aftermarket rods. Of course, you can get the entire rotating assembly as well...comes down to how much of a stash you have, or can locate locally vs having to buy all new...
     
  8. Screamin' Metal
    Joined: Feb 1, 2009
    Posts: 506

    Screamin' Metal
    Member
    from Oklahoma

    4 1/8 will be fine.....4 1/4.....don't think so.....getting into the piston rocking chair territory.........NOW, if you were gonna go with quality stroker pistons.....I'd say.....go for it!!!!! Thats what Roddin' is all about..............
     
  9. Screamin' Metal
    Joined: Feb 1, 2009
    Posts: 506

    Screamin' Metal
    Member
    from Oklahoma

    Ernie.....you Da Man!!!!!:cool:
     
  10. AMC360
    Joined: Jul 24, 2007
    Posts: 76

    AMC360
    Member
    from Canada

    This is exactly what I needed to find out, thanks guys. I do not really wont to spend the money on a stroker kit but having everything new is very apealing to me and by the time I find a merc crank and have it machined, the price will be close to a new crank, and I would still have to buy the pistons, rods and everything else. I have spent enough money on flatheads expecting to find merc cranks.
     

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