As George has stated, it is likely the rear main. If the engine builder will provide warranty service, take it to him first for repair work without opening it up....but read on if you can not get warranty service on it.
FWIW: I had an engine (my own high output touring build) lock up like this when I switched from Babbitt lined inserts to bearing grade aluminum lined inserts for the main bearings. The early aluminum lined insert rear main failure happened while I was running the engines on my dyno under full load and max. rpm.
I consulted with the bearing supplier (he suggests .00175" oil clearance) and my engine rebuilding associates and the consensus was to increase the oil clearance. I built up a test engine and ran test clearances. I found the rear main needs .002" oil clearance at a minimum for a stock output engine. I now set mine at .0022" on the standard builds and .0025" on high output engines. At these clearances, the rear main will not leak if all other clearances are correct and the drainage passage through the rear cap is no less than 3/8" and has the 3/8" drain tube installed. I have a Sunnen Precision hone set up with the proper mandrel and Sunnen stones to hone the aluminum surfaces of the insert. This specialized equipment allows me to adjust the bearing oil clearances in minute increments.
Photos of fixture to hold inserts while honing below:
If your engine builder will not provide service, When you get the engine on a stand and have the rod and main caps removed, lift out the crankshaft and post some pictures for us to evaluate.
If the rear main inserts are aluminum and are frozen to the crankshaft, do not try to mechanically remove them without asking for further input. I will walk you through the procedure of removing them with a solution of muriatic acid. On my original high-output engine and my test engine that locked with aluminum lined inserts, there was no damage to the crankshaft journal when the inserts were removed using a muriatic acid solution. The acid softens the aluminum enough to release it from the crankshaft without damaging the crankshaft. Note: I actually locked up the test engine 3 times while searching for the required minimum clearance....better mine than a customer's.
The block housing bore showed no damage from the insert locking as in all cases, the engine was shut down as abnormal loading was noted and the crankshaft locked as the engine was brought down to an idle.
Please post some photos of the crankshaft when it is removed and the block and cap of the locked main. Hopefully, we can make this a minor issue for you.
Good Day!
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