new cylinder heads for a 6v92ti?

You haven't owned the boat that long and even if the leaking gear box seal was sanitized for the sea trials by wiping down the bilges and affected areas your surveyor should have caught this with a decent inspection with his fingers / hands. Did you have engine surveys performed by a good engine surveyor ?
Just reading your first paragraph, if it's verbatim from the mechanic's mouth than your being taken for a ride. Stop. reassess what's going on and get second and third engine evaluations . The Hatt. / G.M. people on YF that know the ins and outs of gear box disconnecting for the front seal & can speak to the cost question better than I can but just the verbiage of "cylinder head being thrown" is enough for me to scream shyster. Did he REALLY say that?? Irritated at your questions and sourcing of parts? Another sign that he may have thought you an easy mark. Stick to your guns and engage any diesel mechanic or contractor with questions as they should be more than happy to either physically show you the damage and path taken to affect a repair or negotiate the $$ repair if you can find parts online cheaper.

I've been following your posts the in the past and was anticipating the outcome. I enjoyed all the Neo's of the world falling all over themselves to wish you well and this and that but somehow don't remember any of them telling you to NEVER purchase a boat without a hull and machinery survey...I'm sure a few YF members did but I haven't re- read all the previous posts
Your the customer that's paying the bills, You can stop and reassess anytime that you want to !! If the gearbox is in his shop you can still say no more billable hours until I get a second opinion .If your head is off than you should be shown what the damage is by your mechanic and have him give you estimates as he tears the motor down for what it's going to take $$ wise to make it right.If you gave him a deposit of $4,000than ask for an accounting update on where the invoice parts and labor stands. Did this mechanic come with several verifiable references? (always important) Either you trust this guy or you don't, that's for you to ascertain. That being said your or the mechanics earlier statement makes a little more sense now. Since you only have exhaust valves and not intake valves than it could have been a valve , seat or spring that would have dropped down, balled up and beat a hole in the top of a piston and then maybe exited from what was left of your exhaust valves on that cylinder and traveled to your turbo but not traveled through the engine as first quoted. Sounds like he's elbows deep now with your one side head off , pan dropped and the gearbox out of the vessel so either you go with it or become engaged as one with your mechanic. Did you damage a piston crown?
Was it a bad valve seat or spring that let loose. what are the steps he's taking with your head? Should be just a situation of machining the existing heads deck and pressing in new seats etc.

I agree that you really need to get a handle on this. Are you using a legit Detroit Diesel shop or an independent mechanic? Stop the work until you can control it better. That means getting clear answers from your mechanic or change mechanics or going to Florida or get someone you trust to go.
If the main seal is leaking oil from the crankcase into the transmission you could possibly get by with changing the transmission oil more often. Aren't you going to use this boat as a liveaboard anyway once you get it home?
If as you say you could get into $20K or more keep in mind you can do a complete overhaul on a 6V92 for about $30K.
The part about buying the boat without a mechanical inspection or survey I'm not even going to comment on.

Before you dig a deep hole even deeper, you need someone managing this for you in Florida. I'd personally suggest either a good captain or engineer or a reputable yacht manager. You don't have the experience to wade through what you're told and to know who is being honest with you and who isn't. Nor do you have the experience to know which solution is best in the long run. You need someone representing you. If you had that someone he could go look at what has been taken off. He could give you better insight.

The other option is to go to a top level reputable shipyard, like a Rolly or Roscioli or Rybovich and get them to do the work, but that will be top end prices and their recommendation is very likely to be to do complete rebuilds. Others I'm sure can recommend diesel mechanics I'm not familiar with. But you need real mechanics with real shops and reputations. There are a lot of people in South Florida (and elsewhere) who call themselves professionals when I wouldn't agree with that label for them.

But you can't manage this based either on experience or location.

I think you know by now you made a great mistake but that's not the issue at this point. It is because you have expenses you didn't anticipate doesn't mean now you should take the cheap way out. Now the cheapest way may be right, but it often isn't. You need to get this done right or what you've already spent will feel wasted and you'll have a problem ever enjoying the boat. You can still end up with a great boat. It's just going to cost much more to get there than you planned.

Would you care to enlighten the rest of the forum just how a valve spring is going to find its way into the combustion space of a two stroke Detroit Diesel engine?

The last time I was working on a two stroke Detroit the air entered from the side via ports in the liner, the exhaust valve area has one way flow ( out) and there is always a positive pressure in a two strokes cylinder when the engine is running.

Can we also be enlightened by the OP as to what use this two years of education that will result in the creation of a professional diesel mechanic/technician at the end of the course has gone towards helping understand what they are being told versus what they know from their 2 yrs of study.

I did a full apprenticeship which was 10,000 hr contract with time off for passing exams and was qualified as a Diesel Mechanic before I went to sea.

I have never stopped learning myself over the ensuing 34 yrs and find it incredulous that a course is promoted that claims to teach everything about this complex and continually evolving subject in 2 years.

After re reading the post last night after my 30 min. grace period for editing I knew I was going to take my lumps on this one. Sorry, I stated that no intake vales are present but didn't mention the scavenge ports as its through liner ports, Valve seat could fail & drop but certainly not the spring. Rolled over in bed last night thinking this could be a blood bath! My bad.... Thanks for going easy on me! coulda been a lot worse!!

No thanks needed, I was just curious as to how a valve spring would end up in small enough pieces to get down the guide into the cylinder in the first place. I wondered if there was some history of them breaking into small pieces.

Sherrie...Boat #1, the Roamer, you didn't purchase because of the survey. Boat #2, the Hatteras, then you chose not to have a survey done. Survey saved you from a mistake once. That confuses me more as to why none this time. I hope the price was such a bargain as to make the repairs affordable. Here is your quote from the first one, "It just ends up not being the correct home for me as I am not confident that it would not cost me more than I can afford to keep her in good shape and to do what needs to be done."

Now you also talk about a quick delivery type trip to Knoxville. And you have a mechanic working on your boat that you don't seem to trust. I see a pattern that scares me a bit for you. Getting in a hurry, doing this on a schedule that encourages mistakes. I knew someone last year making the trip to the Tennessee who got held up in Mobile for a month. Too much brush and too many trees coming down the Tombigbee.

I know you're anxious and know you intend to learn, but I have to advise two things. First, slow down, patience, caution. Right now haste has likely cost you a lot of money. One day it could be your life. Second, don't overestimate your knowledge or ability. You will take Captains courses and diesel classes but that doesn't make you a Captain or an Engineer. It makes you perhaps apprentice level. I know you intend to use a captain to take it to Tennessee. Well, right now you need someone to use in Florida to get it ready to go to Tennessee. Someone you've checked out who can be your knowledgeable eyes, ears and advisor there.

This is slow down, take a deep breath time. This is not make another single move or financial commitment until you have someone on site in Florida representing your interests.

Did you get a written estimate of work needed from the mechanic you're using? Let's forget no survey before purchase, but did he not then survey the situation to give you an estimate of all that needed to be done? The very fact that you were out searching for parts disturbs me. A good mechanic would know far more about getting parts than you. You need to back up and regroup. Figure out now the best way to get from where you are to where you need to be. Realize it's going to cost far more than you anticipated but that's already a sunk cost, that's based on a prior decision. You can't get peace of mind doing it the way you're going so you'll have to buy that in the right local advisor.

I am a 200 Ton Master, have covered over 50,000 nm in the last three years, have 25 years experience running companies and yet I could not possibly talk to your mechanic and know whether he was honest or good. Instinct would help me, but lack of technical knowledge would kill me. I have just a grain of knowledge compared to hundreds of members here. We bought a center console recently. I knew nothing about them and the first time in my life I've ever owned an outboard. I found some knowledgeable CC fanatics to advise me.

There are many industry veterans on here who are located in South Florida. Time for you to start your search for not your personal knowledge but someone with the knowledge already, someone you can trust. It's going to cost you. Your dream might be a little delayed and not as attractive at the moment, but it can still be realized. I repeat myself but...

the choices

Someone local you trust to oversee, manage the boat for you while it's there. That will cost you $300-400 a day but let me assure you that's not wasted cost anymore than a surveyor would have been. You're dealing with potentially $30,000 to $40,000 so if you pay 10% on top to get it done right and make sure nothing is wasted, good investment. Paying a mechanic you don't trust (right or wrong), bad investment.

or

Pay a premium to a top yard, one with DD experts and certifications, one that will stand behind their work, one with a reputation they value, one that will give you the facts whether you like them or not. That might cost you 20% over just finding a mechanic. Might save you thousands as well.

Right now you might spend $10,000 and get nothing that will last and that's more expensive than $20,000 or $30,000 and results. You've spent $8,000 with someone already that you don't sound at all confident in. Whether you should be or not, I have no idea. And I will not judge them based on what you've posted because you don't have the knowledge to represent what they've said to you properly. It would be like me listening to a speech in French and then telling a group what was said.

Your next move is the one toward salvaging your dream. Make it without the proper ally with knowledge and odds are against you. Turn in the right direction and you'll one day be boating on the Tennessee River with this an ugly and distant memory.

Now, what you need from this site, isn't advice on your repairs. They can't diagnose your engine issues online. You need advice of expertise in the area of your boat that can be trusted to do the job you need done. And there is no greater knowledge of the area or people there than you can find here. Why don't you start a thread asking recommendations of yacht managers, captains to oversee projects, boatyards in the area? Perhaps even an engine surveyor at this point.

Then just accept this as a new start. You'll then know how when you get the boat to Knoxville to find advisors, captains, mechanics, yards there you can trust. In the long run learning this now might be the best thing that could happen. Some people never learn it and then want out of boating but because they didn't maintain their boat it's not worth half what they owe.

The OP could do worse than speak to CaptJ on here. He and I have crossed swords at times but I know someone else via this site who used him and speaks volumes for the assistance he provided.

I agree as this is just the type owner he deals with frequently and having lived forever in South Florida he knows all the yards well. As he's a friend I hesitated to mention his name specifically, but this is well within his area of knowledge and experience. There may also be others with similar experience. He's just the one I know best. He definitely knows the local landscape and has contacts.

Another member who could give excellent engineering advice would be Marmot if he were still contributing, he might be reached by PM I think. I have an e mail addy for anyone that does want to pick his brains.

Marmot would be excellent once she got beyond the initial hurdle of him telling her what he thought of what she's done. And I've been told he is quite pleasant to deal with in person.

She just really needs to find someone who will represent her interests and in whom she can trust.

I was trying to think of a diplomatic way to write that, well done.

In my experiences with the aforementioned I would say I can agree with that statement 100%

Based on your posts I would strongly suggest you find someone through a referral who you can trust to guide you through this. Otherwise it seems to me you are going to get taken to the cleaners.