Late in the game transom problem

I need the help of some experienced boat rebuilders. I am almost 7 years into my complete restoration of a 1979 Mark Twain 240CC with a Merc 260 inboard/outboard. I took it down to the hull as the stringers, deck, and transom were all rotten. All that has been replaced and I am now in the painting stage. As I painted around the keyhole opening for the outdrive, something looked “off.” I put a straight edge across the transom and found the edge of the keyhole was sunk as much as ¼”. The fiberglass on this boat is at least ¼” thick and I epoxied in two layers of ¾” marine plywood (1-1/2” total). I had plenty of clamps and bracing so I am really shocked to see this. I would have bet that the fiberglass would have straightened out when clamped to the new plywood.


My question for the group is “how much of a problem is this? Do I have to build up layers of fiberglass mat and resin to make it flat or will a gasket seal it. I know this is a critical area as any leak could be catastrophic.
Thanks in advance for any advice
Jeff

 

1/4" is quite a lot. The transom shield and gimbal housing will deform if you force them. Ideally you would fill the gap. Is it not flat on both sides or just the outside? The quick and dirty fix is to put thin washers as shims and fill the gap with an adhesive sealer like Sika or 4200. To make it fair, put tape on the gimbal housing and mask the transom around the edge. Fit is loosely with the bolt so it just touches the surface. Fill the gap with thickened epoxy and then reinstall the gimbal housing. If the inside is also bent, do the same.

 

Gonzo, Thanks for jumping in. I haven't had a question for a while and was hoping you were still around and kind enough to answer.
Well, I took another look at it last night and did some more measuring. As it turns out, the problem is not as bad when using a straight edge that is roughly the same size as the outdrive. The first test was done with a 4’ level and where the outdrive bolts on is ¼” below level. But when using the smaller straight edge, the gap goes down to maybe a heavy 1/16”. Do you think that is ok as is or do I need to fiberglass it to flatness? I have some pictures on my phone but it is acting up and I can't download them just now. I will add them in a while if you want to see the difference. Thanks again

 

Grind and fair it out. How far to go to fair is up to you.

A couple ways to fair. You can either build up small to large with glass or you can just use epoxy and cabosil. Make a single pass and it’ll shrink. Then make a mix with microballoons to finish it.

You probably used too much bonding putty when you bonded the old skin to plywood, then you clamped at the keyhole and blew out the bonding putty. 1/8” is the maximum to use, or if bolting; you can bond with a piece of 1708..

My 10 cents.

 

Thanks for the reply and advice. I am leaning that way as I am fully aware of the disaster that a transom leak would cause. Here are the new pictures I couldn't get off my phone yesterday

 

That is within the limits of what the o-ring will seal. Transoms from factories are often no better.

 

@gonzo has a lot of experience in this field, so pay attention to his comment

I tend to demand some perfection in my own work, so lean toward fixing it.