Wrist pin installation

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  • bob-d

    US Member - 2 Years
    Replies: 454
    Topics: 94
    #266482

    Redoing a 1967 33 HP Johnson block, and would like to know if there are any tricks uses regarding installing wrist pins.
    I had the block bored out, and have new .020 over pistons. I don’t want to deform them by just smacking the pin in. The Johnson big red service manual just shows a guy holding the piston in one hand while hitting the pin with the other.
    I did some research, and saw a lot of different methods used, ranging from Sunnen rod heaters, torches, bathing the piston in hot oil, putting the piston in the oven, and at the same time putting the wrist pin in the freezer etc….
    Just wondering what has worked for everyone over the years in their home workshops?
    Thanks,

    Bob D

    DD86160F-0D80-4338-BB0F-857A77813EE7


    frankr

    US Member - 1 Year
    Replies: 6324
    Topics: 50
    #266491

    The deal is, the pin is a press fit in one side of the piston and a slip fit in the other side.  That allows the aluminum piston to expand and contract without being constrained by the steel wrist pin.  The slip fit side is marked LOOSE.

    To remove the piston, if you plan to save it, you must support the tight side over a hole (for the pin), and drive the pin down through the tight side. Since the tight side is supported, the driving pressure will not distort the piston.

    But I assume you are installing new pistons.  Support the tight side and using oil, slide the pin in through the loose side until it meets the tight hole.  Then continue driving it home into the tight side.  Be careful you don’t go too far, or you will need to drive it all the way through and start over.

    Yes, it will help to heat the piston.  No need to get it red hot, but about as hot as you can handle is ok.

    The book shows use of a cradle, but that isn’t really necessary.  I’ve never owned a cradle, but have installed hundreds of pistons.


    bob-d

    US Member - 2 Years
    Replies: 454
    Topics: 94
    #266500

    Ok Frank, I’ll pop the pistons into the oven for a while, and use the chilled technique with some oil on the wrist pins. I’ll put the circle clip on one side so I don’t overshoot my stop point.
    Thanks,
    Bob D


    Mumbles

    Canada Member - 2 Years
    Replies: 5554
    Topics: 294
    #266545

    If you can, it’s better to use a press to install the pins instead of pounding them in.  There’s more control this way and less chance of damaging a piston in case of a miss. I use wood blocks for support which are cut to fit the piston and are then lined with felt to avoid scratching them.

    It’s also a good idea to measure the pistons with a micrometer before and after installing the pins to check for any distortion caused by the installation process.  I’ve seen pistons turn egg shape before caused by a tight fitting pin and then the pin has to be pressed the opposite way a bit to get the piston back to its original shape. This is why one boss on the piston is marked L or Loose so you know which side to remove or install the pin from.


    bobw

    US Member - 1 Year
    Replies: 1954
    Topics: 51
    #266549

    Here’s my simple cradle I made out of a piece of 4×6 blocking.   Works great, but I like Mumbles idea of lining the block with felt.

    01-Piston-Cradle

    Bob

    1937 Champion D2C Deluxe Lite Twin
    1954 Johnson CD-11
    1957 Evinrude Fastwin 18
    1958 Johnson QD-19
    1958 Johnson FD-12
    1959 Johnson QD-20

    “Every 20 minute job is only a broken bolt away from a 3-day project.”


    ROGER SPINDLER

    US Member - 2 Years
    Replies: 2
    Topics: 0
    #266552

    I have found that heating the piston does make difference


    bob-d

    US Member - 2 Years
    Replies: 454
    Topics: 94
    #266554

    I think I will definitely make a wood cradle . Simple to make, and a good holder for the piston after I heat it. I guess I will also dust off the old Harbor Freight press as well.
    Thanks,

    Bob D

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