Which casting number is better, if any? The 509 or the 511. I heard a long time ago to avoid one of these, but I can't remember why. Any help would be great.

The 509 is the most sought after factory 400. It is a 2 bolt main block and has 2 freeze plugs per side as opposed to 3 on other 400's. The 511 is a 4 bolt block and is actually a weaker.

I don't think the casting is as important as 2 vs 4 bolt mains. A 2 bolt is preferred. Both castings came in 2 and 4. 509 from 74-80 is 2 bolt, from 70-71 are 4, and most 511s are 4 bolt, but some 2s exist from replacement blocks. The 2 bolts have more material in the webs from the lack of the 2nd bolt. If you are building a mild street motor with less than 500hp, either block will work - just have any block sonic checked before machine work to verify thickness. If you go with 2 bolt, the best thing to do for high HP is make it into 4 bolt splayed caps.

-Dan

What Dan said....:thumbsup:
People get way too caught up in a specific casting number IMO, no casting number was immune to core shift or defects, so as long as it's a decent 2 bolt block with minimal core shift you will be good to go.
By the way, I have a 511 two bolt block, some castings may be more than likely 2 bolt but no guarantees on any.

I stand corrected. I've only seen 4 bolt 511 blocks.

I stand corrected. I've only seen 4 bolt 511 blocks.

I believe they most commonly are, I think the majority of the 511 2 bolts are replacements maybe. I have seen so many variations of them, two freeze plugs, 3 freeze plugs, 2 bolt 4 bolt etc. Who knows what reason GM had for it. They are all getting harder to find, my first block was a 509 in fact but had hairline cracks in it when magged, they are pretty used up. Once this block I have is used up I'm going to go aftermarket, for the difference in price you are still money ahead with new I think. :beers:

My cousin races midified class on dirt and his engine builder specifically wanted an 817 cast block. As far as I can tell, all 817 blocks were 2 bolt main blocks.

I ran a 406 in my camaro for 10 years. It was a 4 bolt main and I bought it from Lunati when they sold there 406 super bracket kits. It was a 6" rod motor w/cast crank that they nitride. It had brodix track 1 heads, victor jr intake, holley 850. That was the best motor I had. It ran great and never overheated and would spin 7000 rpm all day long.

I drove this on the street all the time.