Rebuilt the transom with marine play and have dimensions from 48.87mm (one area only) to 51.37mm against the Merc book of 51mm to 57mm - will that be OK cant believe a mm would make all the difference. Also my external is under 2mm variation and my internal is within 0.2mm above the 3mm limit.

I can clean up the paint and add some gelcoat if needed but I kinda would leave it like it is if OK?

I've seen a lot of transoms done but I've never seen one done with such finely defined 'isolation' before .

Is "isolation" good?

If you look at threads here on the same subject where there a loads of pics you'll probably notice right away that there's extensive grinding and glass work well ahead of the transom and into the stringers and hull bottom.
That appears to be some sort of composite material as well . A load of discussion on that too in terms of its suitability for transoms.

Understand - when I took out the rot I was left with a deep trench all the way around and thought what a great idea to build into it. I cut the sheet to be able to slip it into the gap and then kept the stringers as sound as I could to use wedges to push together - I also used angle iron to bolt through as well.

So when the prop pushes the leg the leg puts bending into the transom and as well as a horizontal shove! Th
e solid Lloyds registered marine ply is buried in a trench and firmly fixed to the outer skin with vinylester. I did take out 7inches of stringer and blend that in but your right - around the edge I have focussed on the trench and only gone about 1.5 inches into the floor and sides. Is that a mistake - only time will show?

Question is still there - is a thickness in one area of 2mm under limit but meeting overall tolerance on inside face of 3mm OK?

Did some more measurement and noted that at the bottom of the transom between the lower gimbal housing that the face was 0.07ins dip. Does fiberglass creep?

Prep'ed and put some gell in plus raised the inner face by a mm to get everything tighter and all above 2.01inch (51mm) which now makes it to spec.