Does anyone know much about the 3951511 400sbc 2bolt replacement casting? Mortec says 'some replacements have 2 bolt mains.' Why did they make replacements anyway? The stock one suck that bad? When were these replacements made and how few are there? Are the rarer 2 bolt ones with the 511 casting any more/less strong then the 509 2 bolt casting? Also the 511's don't have accessory holes in the block on the drivers side, but a boss where one looks like it could have been machined. Can you drill/tap them for a serpintine setup or is this a no-no.

 

Can someone at least tell me if some 400's had the lower two drivers side accessory holes drilled while others didn't?

 

All of them have them drilled.

Those holes have always been drilled on all Chevy small blocks since 1955, when they were the original location for the motor mount bolts. They have been used for power steering brackets in most applications since 1969. They didn't suddenly appear for late-model accessories; they've always been there. If you have one that has one of those holes that's not drilled, then it's a factory defect, and that's why it was a "replacement" block instead of going in a new car; and you can just get it drilled by somebody that can locate it in the exact right place (not just "on the boss somewhere") and get it perfectly straight and so forth.

They probably made replacements for the same reason that they always make replaacements; to repair cars in the field that fail or get destroyed somehow.

The 509 casting is the better one, it has more metal around the main webbing area. I wouldn't worry about it though, unless you're planning on turning it more than 6500 RPM continuously and produce more than 500 HP or so like the circle track guys like to do. It's a total non-issue for a street build.

 

what cars came with a 400 chevy? and are they all 2pc rear seal? I can only assume since I belive all the 400's were built before 85.

 

I have a 400 that doesn't have the accesory bolt holes on the driver side. The block casting date (L 16 72) indicates that it was cast December 16th, 1972. I need to locate and run the other casting numbers and see if it was a "replacement" block.

What exactly is a replacement block?

 

Yep still wondering what a 'replacement' block is too.
At least I know now that more then one came without the lower accessory holes. Now what would be a logical reason for this?
Could it be it was made for an industrial truck that wouldn't have accessories requiring those holes? That a 100% guess by me. Any other ideas?

 

Replacement blocks were cast and machined for stocking as a bare block. The factory didn't intend to build those up into shortblocks or engines. They were sold through dealers for instances where only the block needed to be replaced.

My dad had a '70 Impala that was such a case. In 1974, with ~50k on it, the original owners had a radiator hose break and they cracked the block. They traded the car in, the dealer got a replacement 400 block off the shelf, rebuilt it (new rings, bearings, gaskets, but reused crank, rods, pistons, cam, lifters, heads, etc.), sold it to my dad.

The engine had just under 200k when I got the car. A couple of years later, the lower radiator hose burst on me. The car got hot, but after letting it cool down, I cut off the burst end, reattached, filled with water, and drove it home. Replaced the hose that weekend and filled with antifreeze. 6 months later, I gave the car to my older brother, he tore the engine down a year later, found broken rings. New rings & bearings, still stock bore, he put another 50k on it, gave it to his son. It was still running when the body fell off around it and he abandoned it in the streets of Topeka about 5 years ago.

On a "replacement" block.