Hi I have an mercruiser 500hp efi engine... Ive just did a refurbishment to the engine... Now after testing the boat for a few miles... I found that the rockerarms nuts are cracking from the middle of the nut.. Any one had this problem before .. any ideas of whats causing the nuts to break ?
Thanks Sam.

Roller rockers with polylock nuts I assume? Maybe over-torqued?

Nuts not heat treated right or set screws over tightened, are the typical causes.