1975 Pontoon with OMC120 stringer, replace with Mercruiser 3.0 Alpha one?

[stan2076]

Got 8 years out of my OMC120 stringer before recently lost top end power with loud pinging and water spitting out of exhaust. I assume head gasket + whatever other damage. She gave me 8 good years but I'd like to just modernize a little to a Mercruiser 3.0 with and Alpha One outdrive.

The motor sits in an aluminum box with a vertical back with the OMC stringer hole. I know that the alpha One is designed to push against the transom unlike the stringer that was bolted to the motor. I have good fabrication skills with access to a machine shop and materials to create a strong mount and angle to mount the Alpha One.

What do I not know about converting? Does the Merc 3.0 have the same engine mount points as the OMC120? Will the prop run at the same height as the stringer did? I'm looking for a donor boat so I can get all the parts and pieces. Are there any years to avoid? Please help me learn what I don't know. I redid the pontoon from the deck up when I got it and it's still in great condition and we love it. Thanks!

 

[Scott Danforth]

What do I not know about converting? Does the Merc 3.0 have the same engine mount points as the OMC120? Will the prop run at the same height as the stringer did? I'm looking for a donor boat so I can get all the parts and pieces. Are there any years to avoid? Please help me learn what I don't know. I redid the pontoon from the deck up when I got it and it's still in great condition and we love it. Thanks!

No, not even close

the stringer mount is 4 points. the forward mount with or without electric trim and the aft trunion mounts

the transom on an OMC needs to only be thick enough to hold the screws for the boot.

the mercruiser mounts off the transom. the transom shield clamps to a transom that is 2.000" to 2.250" thick. no more, no less

that transom needs to be at the correct angle

the front mount then is a single mount about 3" in front of the pulley and about 8" below the crank centerline.

you will need to rebuild the transom per the mercruiser requirements

look at the stickies on top of the Mercruiser forum.

MUST HAVE Technical Information for DIYers.

MUST HAVE Technical Information for DIYers. (< An @ Work Safe Search Version Of ^^^) If you have NEVER posted in this forum before, please click on How to ask a question link first. 1. How to Ask a Question .... Please Read This Before Posting A Question... 2. Miscellaneous Mercruiser...

forums.iboats.com

there is a link to the mercruiser installation drawings. https://www.mercurymarine.com/en/us/documents/installation-drawings/

 

[stan2076]

No, not even close

the stringer mount is 4 points. the forward mount with or without electric trim and the aft trunion mounts

the transom on an OMC needs to only be thick enough to hold the screws for the boot.

the mercruiser mounts off the transom. the transom shield clamps to a transom that is 2.000" to 2.250" thick. no more, no less

that transom needs to be at the correct angle

the front mount then is a single mount about 3" in front of the pulley and about 8" below the crank centerline.

you will need to rebuild the transom per the mercruiser requirements

look at the stickies on top of the Mercruiser forum.

MUST HAVE Technical Information for DIYers.

MUST HAVE Technical Information for DIYers. (< An @ Work Safe Search Version Of ^^^) If you have NEVER posted in this forum before, please click on How to ask a question link first. 1. How to Ask a Question .... Please Read This Before Posting A Question... 2. Miscellaneous Mercruiser...

forums.iboats.com

there is a link to the mercruiser installation drawings. https://www.mercurymarine.com/en/us/documents/installation-drawings/

The drawings are a big help, thank you. So the mount for the outdrive and the rear engine mounts are bolted together sandwiching the angled transom. Three mounting points for the engine if I am seeing it correctly.

I went to look at a donor boat yesterday, the guy said the motor and outdrive had been checked by a mechanic and were perfect, but animals got into it and destroyed the interior. The animal story was true but when I pulled the dipstick the "oil" that was in there came out the same color and consistency of egg nog. So the hunt continues.