A friend of mine was telling me he is working on a 426 hemi with an X block..he said it's an old cast iron race block...I thought the race blocks were designated with an R...has anybody heard of an X block. Thanx in advance

Are there any other markings on it.

Pretty much all the Street Hemi blocks 1966-1971 had the large X on them.

I was told by one of the Mopar drag engineers at one of thier old drag race seminars in CA that the X on the blocks indicated heat treating
I've owned and assemble a bunch of stock OEM 426 street Hemi motors and I'm almost positive all of them had that X on them

X on the block is the code for stress relieved blocks.When they are removed from the mold they were placed in a heated room and slowly brought to normal temperature over a period of 70+- hours.This was a standard process with all the early blocks built up to 71.In the mid 90s they did a few runs of blocks that they skipped this process and when the blocks cooled quickly they distorted and had many issues including internal cracking.These were the infamous reject blocks that got sold for $400 to $900 dollars by dealers and swapmeet venders.