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Thread: Announcing the Merc 888

  1. #61
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    Default Crank grinding

    Quote Originally Posted by 89HST View Post
    Why not send the crank out to dave eden, have him machine the crank pin area wider and re heat treat it? That would allow you a little more area for bearing surfaces on the dual rod idea.
    Modifying the crank is certainly possible, along with staggering the blocks 1/4" to negate the offset. But, the objective is to assemble with the least invasive change to learn if the engine will start easily (enough) and behave itself. If it does, further expense is warranted. Remember there is zero cylinder pumping........

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    Not to throw cold water on the approach, but you need to think seriously about some kind of bearing and a much bigger area for the pin on the master rod. The stress in the pin is high and the contact stress is high even for something that isn't moving. No lubrication is going to stand up to that kind of pressure. You are going to bottom out the joint and it's going to gall and wear in no time at all. Cut away part of the counter weights to get some width and you can support proper size pin and bearing. You can make a new rod cap and make it out of high strength material (a high strength steel or nickel alloy like Inco 718). The full complement bearings that are used by Mercury are obsolete in many ways, but they can take really high loading and in this case that's not a bad thing. The stock wrist pins are good for over 9000 rpm in modified motors when proper lubricated so there is the capability to take a lot more load so I'd start with the stock small wrist pins and needle bearings and look at that kind of sizing if you want to run this at any speed over idle.

  3. #63
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    Default Cold water?

    Quote Originally Posted by Yellowjacket View Post
    Not to throw cold water on the approach, but you need to think seriously about some kind of bearing and a much bigger area for the pin on the master rod. The stress in the pin is high and the contact stress is high even for something that isn't moving. No lubrication is going to stand up to that kind of pressure. You are going to bottom out the joint and it's going to gall and wear in no time at all. Cut away part of the counter weights to get some width and you can support proper size pin and bearing. You can make a new rod cap and make it out of high strength material (a high strength steel or nickel alloy like Inco 718). The full complement bearings that are used by Mercury are obsolete in many ways, but they can take really high loading and in this case that's not a bad thing. The stock wrist pins are good for over 9000 rpm in modified motors when proper lubricated so there is the capability to take a lot more load so I'd start with the stock small wrist pins and needle bearings and look at that kind of sizing if you want to run this at any speed over idle.
    Agreed.

    While the radial engine master/link concept is truly an elegant solution, it simply won't work because of excessive loads as stated. If the crank journal were to be widened, balance is adversely affected and worse, crank rigidity is compromised. The only decision, without investing boatloads of cash, is to use two narrow rods running in the same journal. They will be adequate for proof of concept. I'll be very pleased if this engine lasts one hour.

    Tim

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    Let us remember that the downward (towards BDC) force of one piston is being offset by the compression force working on the other (going to TDC). Now, if the master/ slave rod concept is used, these forces will go through the rod, from one piston to the other, instead of through the rod bearing to get there.

    A vote for the master/ slave rod concept!

    Jeff

    PS: Damn but this is an interesting project!
    "We live at the bottom of an ocean of air." - General Marvage Slatington

  5. #65
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    PS: Damn but this is an interesting project![/QUOTE]

    I agree, totally.
    kk

  6. #66
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    Default It's all about swing

    Quote Originally Posted by Krazy Karl View Post
    PS: Damn but this is an interesting project!
    I agree, totally.
    kk[/QUOTE]


    Not to worry, rod options are still open. Saturday I'm mocking-up another master-link combo to determine if it will swing with a 1/2" diameter pin.

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    Tim.......Why couldn't you take 2 mercury rods, toss the end caps and bolt them together on one journal of the crankshaft? Seems that since you already have bearings to fit the rods, it would be a much simpler fix than redesigning an already good part.
    Charley Bradley


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    Default Wishes and horses

    Quote Originally Posted by F-12 View Post
    Tim.......Why couldn't you take 2 mercury rods, toss the end caps and bolt them together on one journal of the crankshaft? Seems that since you already have bearings to fit the rods, it would be a much simpler fix than redesigning an already good part.
    Anything that easy can't possibly work..... or it would have been done before. Back to back rods cannot be made to assemble, let alone rotate.

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    After thinking about what I wrote, that thought entered my mind. Sorry about that.......
    Charley Bradley


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    Default Back to back

    Quote Originally Posted by F-12 View Post
    After thinking about what I wrote, that thought entered my mind. Sorry about that.......
    No problemo...... It was my first thought, too. LOL!

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