2003 Mercury 15 HP 4ST rebuild newbie questions

Hi Guys,

I recently purchased this 2003 Mercury 15ELH 4ST off craigslist non-running and intended on using it to learn how to work on, service, and possibly rebuild basic small engines. I purchased and am following the Seloc repair manual (1705) but still have a few questions.

After repairing the recoil starter I did a compression test that resulted in ~85 PSI on the upper cylinder and ~60 PSI on the lower cylinder, so I decided to pull the powerhead off to take a peak. There is substantial salt in the head and there seemed to be a portion that clogged and lost it's flow and ate away at the head and I'm concerned. I also noticed a strange hole/window in the block, the shape of the hole looks intentional but the hole is rough making me think it's a problem. I've ordered a replacement powerhead gasket set after the low compression diagnosis but before noticing the damage.

What do you think? Is this head & block salvageable?

Questionable thin wall on cylinder head


Cylinder head overall condition


Questionable hole in cylinder block


Thank you for any advice & guidance!

Well, after more digging and finding replacement powerheads on ebay that have the same wierd hole (3rd picture) I think i can safely rule that out as an issue, but i'm still questioning the integrity of the cylinder head.

Any suggestions/questions/critiques/etc are welcome!

Thanks!