From flex plate to flywheel.
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From flex plate to flywheel.

  1. #1
    Member oldseddie's Avatar
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    Default From flex plate to flywheel.

    I am starting to freshen up the 400 sbc from my old boat. It has a heavy external balance flywheel and my new acquisition has a flex plate with three point mount. Will I have to change the flywheel for a flex plate and have it re-balanced or can I just shorten the driveline.

    Sent from my Jasmine Note 3

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  3. #2
    Senior Member BGMAN's Avatar
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    You shouldn't need to shorten the driveshaft but you will need a pto flange if you have the newer style universal H bar type of drive shaft. If you have the old type of drive shaft where you bolt the flexplate or flywheel and the driveshaft on at the same time then you might have to switch to a new style.

    Hope that helps.

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    Member oldseddie's Avatar
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    Thanks for the reply. It is an 05 Shoal Runner with a Dominator 12s and Vortec 350 so it is probably the newer style. I need to tighten up the pump and that would be a good time to pull the bellhousing/mount and see what's in there. Can you see this from one of the inspection or starter holes?
    Last edited by oldseddie; 04-25-2015 at 07:40 AM.

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    Senior Member jetboatperformance's Avatar
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    Quote Originally Posted by oldseddie View Post
    Thanks for the reply. It is an 05 Shoal Runner with a Dominator 12s and Vortec 350 so it is probably the newer style. I need to tighten up the pump and that would be a good time to pull the bellhousing/mount and see what's in there. Can you see this from one of the inspection or starter holes?
    You should be able to see which (Heavy Flywheel or Flex plate) is used via the access port yes , many production SBC engines come with smaller OD 153 tooth units vs the larger and more common 168 tooth so beware of this factor along with the aspects of internal and external balance parts as well , obviously those features wont interchange and starters wont either, additionally some bellhousings disallow mix and match of some components , call if you need help Tom

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    Member oldseddie's Avatar
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    The 400 is external balanced and the flywheel is off the mercruiser 305 from my old boat. I'll count the teeth on it as it's in my shop. I'm pretty sure indmar used a flex plate but if the rain ever stops I'll look through the inspection hole to verify. Got a used ss or bronze AB impeller in your collection Tom?

    Sent from my Jasmine Note 3

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    Senior Member jetboatperformance's Avatar
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    Quote Originally Posted by oldseddie View Post
    The 400 is external balanced and the flywheel is off the mercruiser 305 from my old boat. I'll count the teeth on it as it's in my shop. I'm pretty sure indmar used a flex plate but if the rain ever stops I'll look through the inspection hole to verify. Got a used ss or bronze AB impeller in your collection Tom?

    Sent from my Jasmine Note 3
    You can send all your spare rain to Cali (4th year of drought) check your Pms please

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