Chevrolet Repair: 400 small block chev, australian muscle car, holden monaro

Question
DearVan,I have a 1977 holden monaro which is an Australian muscle car I have done a chevy coversion.The engine is a 400 small block that is mildly worked,flat top pistons,bigger valves,roller rockers,nothing over the top.The problem I have is that I snapped a crank at the snout just behind the timing chain sprockets.the crank is cast.The people I have  spoken to have never experienced a break like this before could you shed some light on the subject please mate.Appart from that the 400 is awsomme.Yours sincerely big danny  

Answer
Hello Danny,
Not necessarily any rhyme or reason to it. Sometimes a cast item will have an undetectable flaw in it.
You might make sure all the main journals were turned to the correct size, and all the main bearings were the correct size, so there wasn't an unsupported section of the crank just hanging out there waiting to break under load.
Maybe have a machine shop check the line bore of the block.

Van