Those are your piston coolers and if you don't know what those are you honestly need to not be building that yourself.
There's always that guy on every single forum who always says you shouldn't do it yourself, yet has no damn clue who you are, what your experience or skillset is.
You should stop trying to be a forum elitist and just help instead of being a condescending ass.
Let me give you a little background about me so you can really see how stupid that comment makes you look.
I have gone through and completed and entire technical college program on mechanics (Gas engines, not diesels). I am an outboard technical...I work in the marine industry, not the diesel industry. There was a national marine mechanic competition recently where I competed at the state level and took 1st, and moved up to the national level and competed and took 2nd over all.
Possibly I should mention too that I am the owner and lead tech of a marine shop that I opened up here. I have rebuilt MANY MANY engines from cars to outboards to some things inbetween...all gassers though.
So for you to come in here, and rattle off some half-cocked "you're too incompetent for this" remark...it really just makes you look like a twat. It really makes you look like you stuck both your feet in your mouth too when you didn't exactly have the right answer either. According to the guy underneath you, these are actually just block off plugs (which is what I assumed they were in my very first post) and the oil cooling jets are actually underneath the piston.
So please, take your cocky **** elsewhere mate. This forum is here to learn. You show me where in the service manual those blue plugs are talked about. You can't. I know you can't because I have read that service manual probably about 5 times now. It talks about the Jets, but not the block off plugs.
The people that shouldn't be building engines are the ones that don't know something/what they're doing and they dont bother asking questions; they just slam it together and go.
People that ask and learn and improve their knowledge/skills are the people that should continue to be working.
Have a good day.
To the others, thank you for helping. I talked with my machine shop and they said the same thing.