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BIA bolt hole distance from top of transom.

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  • BIA bolt hole distance from top of transom.

    I'm mounting my 1973 Merc 500 with a universal tilt/trim unit from TH-Marine. The unit is very similar to the CMC PT-130. The tilt/trim unit has the standard BIA bolt pattern.

    The boat the tilt/trim is going on is a 1999 Alumacraft 16LTD Mag that and the transom has never been drilled. I've got the bolt pattern down fine but I am wondering what the standard distance from the top of the transom to the centers of the first row of bolt holes (the ones 12 7/8" on center). I would like the holes to line up with new/newer 40-50hp outboards in case I or a future owner plan to upgrade later on.

    I can hardly find any info online about this. On site states 1 7/8" from the top to the center. I will be stopping by a dealer tomorrow to ask but the more input the better. Thanks!

  • #2
    Re: BIA bolt hole distance from top of transom.

    That top measurement will depend on a number of things

    - Transom design
    - Outboard leg length
    - position of the cavitation plate in relation to the bottom of the boat

    layyour measurements out so that your cavitation plate is about 1/2'' - 1'' above the very bottom of the transom. Don't worry to much about the top measurement being in place for a future upgrade as nearly all outboards have adjustability to them.

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    • #3
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      Re: BIA bolt hole distance from top of transom.

      According to the standard mounting hole drawing, the upper holes are 2" down from the top of the transom.