Transom seal repositioning - OMC repower

[GiMLit]

Hi guys

What I am working on:
1977 18.6 ft Cobia Bowrider I/O OMC Stringer
repower from 1977 120 hp 4cyl with electric shift to a 1981 OMC 3.8 l V6 and a 800 series outdrive.

Additional facts ......... transom hole...... 16 screw boot on original (offset to port side on original setup) and 22 screw boot on new setup with centered hole and larger.

Question:
Obviously I have to cut out transom hole to fit larger boot and to bring it to center. I have done a mock setup and beleive I have it correctly positioned. Before I go crazy with the sawzall I have 1 question.

It appears that I will only have to trim out the Starboard side of the transom to accomodate the boot, based on my height being correct. I currently have the
AV plate located ***1/2 inch below the lowest portion of the transom***.

I do have hydraulic trim mounts but I want to start at the best neutral position I can.
Is the 1/2 inch below keel height too low, too high ? or can I go with that?
What would you suggest is my measurement window? 1 inch above to 1 inch below ???


Thanks as always for your opinions.

GiMLit

 

[tashasdaddy]

Re: Transom seal repositioning - OMC repower

outboards they recommend 1 inch above. i'm moving this to the I/O fourm, beleive you will get quicker responce.

 

[GiMLit]

Re: Transom seal repositioning - OMC repower

Thanks, I was going to put it here but since I was asking about the transom hole in particular and prop height secondary I figured it would get moved.

Thanks tashasdaddy for helping me out.

 

[GiMLit]

Re: Transom seal repositioning - OMC repower

Here is maybe an easier idea.

If you don't know the answer outright to my question, look at any I/O and tell me what height the AV (plate above the prop) is at.

I should have done this before I stripped the hull out and hauled it to the dump!
I assume I am at the correct height since the transom boot fits on the left, top and bottom.


Appreciate your help.

 

[whywhyzed]

Re: Transom seal repositioning - OMC repower

If the old electric unit was a Low Profile, it has the same dimensions as the new 800 series as far as pivot pin to vent plate. (A high profile electric is 4" longer/taller so I assume your electric was LOW....) so, you ideally want the new drive to sit exactly where the old drive sat. So rather than measuring, I would say keep the bottom of the cutout the same and only cut side to side (or up if you really have to). It is fairly important with Selectrim to get the intermediate centered in the boot so the boot doesn't get pinched or pulled out of the clamp when trimming.
I have all the installation drawings in the 3.8 factory manual - I could fax them - p.m. me.
What I did, when I recut my 1967 boat was drive to a marina and I looked at about 10 boats with 800 series drives to get a feel for where they should be. Mine's actually an inch too high per the book, but I could not go any lower per my transom design. I'm as low as I could go.
Oddly enough, my engine mounts sit perfect on the stringers even though my old unit was high profile (it had 4" taller engine mounts)

 

[wire2]

Re: Transom seal repositioning - OMC repower

Slightly off topic but related;
What color is the plywood if you file at the opening? If quite dark, you've had the boot screws leaking just as I did on my '84.
Try to push a very small slot screwdriver down into the plywood towards the drain plug/hole. Any more than ?" in means the plywood is rotting.
Mine went in 4", I dug it all out & replaced it with Seacast.

 

[GiMLit]

Re: Transom seal repositioning - OMC repower

Thanks guys.
Transom wood is good.

GiMLit is a dummy <<<<<<<<< ya me.
Why the heck didn't I just measure one of the 2 electric shift drives sitting in my garage on the rack???????? MAN OH MAN I made this way more difficult than it needed to be. Oh well......... how's that saying go ?????
" Can't see the forest through the trees"???
I think one thing that had me sort of question things was the fact that I have to add 2 1/2 inches to the height on one stringer and 1 1/2 on the other.

LOL
thanks again for the help guys.
kjdkjd appreciate the fax offer but I think I can make it without. If I decide otherwise I will let you know.

:redface: