BTW, I'm going to be in your neck of the woods at Green River Lake this weekend.

sounds like crank was cracked, or just a fluke, and a cast crank is fine for a setup like this, raced cast ones many time at way more rpm than this will ever see..

For about $200 you can buy a 4" stroke cast steel crank from Scat. These are actually a pretty decent crank for a mild to marine build - they are rated to around 650 HP. That would be an upgrade over the stock GM cast crank - the steel used is a higher grade alloy. If you elect to go for more of a performance build, might as well go forged if for no other reason the peace of mind, resale, and the ability to handle more power if you elect to go with a supercharger. If you decide to go the stroker route, prepare to open a can of worms - you will find yourself upgrading everything else too! ;)

There was very little debris in filter, best friend owns a shop and he looked through it... Only debris I've found was in bottom of pan... Oil galleys and filter lines have been flushed since then as well... I used oil cooler and lines on new engine so I had to make extra sure it was clean. It has forged dome pistons in it already about 9.0:1.. H beam rods, I don't know about main and rod bolts tho I didn't assemble engine... But as I mentioned above u can still see the honing "X" on the cylinder walls it's practically bran new... Only thing we can come up with was a bad cast in crank, or a poor line bore? What I was going to do is have rods checked to make sure nothing bent or stretched, have line bore checked, have cylinders checked, buy a good crank, clevite bearings unless you recommend something else, new oil pump, new main and rod bolts, and go to town.... Or should I not?

And have it balanced....

Was at green last weekend... We go to Cumberland more, watch the fish and wildlife, got a new one over there!!!

I'd find a nice takeout gm forged crank for 200 bucks over a new cast scat or eagle crank myself. Teague sells takeout dimple 7/16 rods cheap.

I've been pounding on chevy steel cranks for years ..Also , a diesel engine is nothing like a big block chevy. Yes they both have crankshafts , yes they both have rods and pistons. Things like piston to wall clearance , rod and main clearances , rod side clearances , ring gaps, cylinder wall finish, lifter bore clearances, and so on, are application specific. For many decades guys who built drag race big block chevy engines daily have a hard time making a MARINE big block chevy stay together.

I understand that, just hoping to save some money, by doing lots of research so I'll accept any tips and help I can get! Right now there is just a fresh race engine with different cam and carb in my boat.... It does ok, but I'm scared of it not holding together so I'm very easy on it.... I want what I had, which was a little power and dependability, until crank snapped obviously, hoping I can get it... But u are just suggesting take it all to machine shop and let them go at it? Or is it possible with research and knowledge to DIY.... I have a good friend who builds 454 pulling truck engines also... Which I understand he turns 8500 for 20 seconds .... I'll be turning 5000 for an extended period..,. But different applications, different parts...???

And surely I can find specs and clearances online???

What did the main bearings and main journals look like? If line hone/bore was off, I would expect to see uneven wear on the main bearings. I had to have my line hone done over by a different shop because the first guy had it tapering from .0025 in the #1 main all the way to .0045 in the rear main. Even something like that would not typically break a crank. What kind of balancer was on the engine? Might consider replacing it with a good new SFI approved unit to be safe. Harmonics can break cranks too. Examine the area where the crank broke to see if there is evidence that it was an old crack. However, forged cranks can run for a while with a small crack, while cast cranks usually fail catastrophically when a crack develops.