What is “hard blocked”?

[blown motor]

I saw a 340 engine block for sale and the seller said "engine block is hard blocked" What does that mean?

[BigSig 6 Barrel]

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I've heard of this, but was many years ago ... and someone can correct me if I'm wrong, but seems to me that it's a block that has had the lower half of the water jackets filled with epoxy ... something to do with racing engines. 

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[RUNCHARGER]

Yes: Water jackets filled to bottom of freeze plugs using epoxy or even cement. Kind of takes it out of being street usable.

[cuda hunter]

Is it possible to reverse the hard blocking?   On a hemi? 

[MoparLeo]

A common , old school practice of filling the cylinder block withsome type of cement/similar producys to add rigidity to the cylinder bores. This is similar to the reason of having your cylinders bores honed with a deck plate.
 Better ring seal, cylinder bores could stand higher pressures.

[cuda hunter]

So, this process makes the engine Non Streetable? 
Because of overheating? 

Specifically a keith black 426 hemi. 

[MoparLeo]

Thatis why there are street KB blocks and racing specific blocks. Early 426 aluminum KB blocks with jackets ahd a problem of staying water tight for street use.
Another reason why parts made for racing do not usually work good on the street and vice versa.
Before the Gen 2 Hemi blocks were being reproduced, used racing blocks were the only option and many guys tried unsucessfully to use them on the street. Many reasons why.
Custom, race blocks do not always even have any water jackets. provision for standard mtotr mounts. Clearance or bosses for mounting external street accessories.

[johnshotrods]

Hard blocking is still not uncommon . Ive done it in a few engines that i wanted to save , usually on an engine that I wasn't sure how thick the cylinders were before boring . I would have the block sonic tested first to see how thick the cylinders were and if they were kinda thin i would first clean the inside of the block really good and then fill the lower part of the block ( usually to the bottom of the freeze plugs ) with an epoxy . You want to do this BEFORE you bore the block. As far as removing it from a block that has it in it i,m not sure thats possibly .
 I have actually heard that some engines it helps coolant flow and they ran cooler . Theres several companies that make it , I know Moroso makes it .

[Flatdad]

It would be neat to see a back to back dyno test with the same combo, but block filled vs not to determine if it actually is beneficial or if it is just one of those apocryphal mods.

[6PKRTSE]

Half filled is up to the bottom of the freeze plugs. Full fill is all you can get in it. We half filled my buddies street/Strip 468" Pontiac and it was the coolest running street car that I have ever ridden in. I believe being half filled that it filled alot of hot coolant pockets that the coolant did not flow at all or not very welli some areas.