Steam Holes, The Cooling Mod For 400-based Chevy Small-blocks

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The 400 ci block is the only Chevrolet small-block that has the steam holes on the deck surface.

The Chevrolet small-block is by-far one of the most versatile V8 known to man. Whether it’s the driving force behind a Chevy platform, like Ed Cole intended, or being part of a Rat Rod Ford Model T rebuild, the influence of this engine is unmistakable. Becoming part of the American condition; just like baseball, apple pie, and coney dogs.

Since its inception in 1954, the Chevy small-block has seen ten different displacement renditions, ranging from a 262 to 400 ci combinations. Gearheads and racers alike have continued to look for cheap and efficient means of making more power. With many trying to fasten the largest displacement engine they can find between the frame rails of their street machines, paying homage to the saying, there is no replacement for displacement. Leading many builders to strap a set of 350 heads atop a 400 based short-block.

This approach has a lot of merits but comes with a slight drawback; all 400 ci small-blocks utilize siamese’s cylinder walls that are known for creating air pockets in the cooling jackets when a set of 350 based heads are used. Over the years one of the most well-known means of addressing this issue is the addition of steam holes on the cylinder heads.

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This is one of the many steam hole templates that can be found on the internet.

Taking these steps to prep a set of cylinder heads can drastically improve the cooling properties of these pieces. Requiring a 400 head gasket, the gasket provides a useable template to scribe out the steam holes to properly drill into the head, without ruining the deck surface and weakening the heads infrastructure.

Utilizing a center punch to create a divot that helps stabilize the drill bit during the drilling process. After this set is when the consensus of how to correctly drill the holes themselves begins to waver. During our searches and discussions, we found multiple ways of performing this modification. While we can’t confirm which one is the most effective method, we aim to lay out all the different variations and let you decide.

One of the technical articles that we stumbled on was performed by Greg’s Engine & Machine  and utilizes a 1/8-inch drill bit and requires the intake side steam holes to be drilled at a 10 degree angle, to ensure that holes reach the water jacket.

Another slew of image diagrams alluded to a set of other, entirely different strategies. With one using a 9/64-inch drill bit and drilling the intake side holes 30 degrees towards the exhaust side of the head, while another employs a 45 degree angle. During our talks with Randy Bolig editor of our sister magazine Chevy Hardcore, we were told that he simply drilled all the steam holes at a 90 degree angle, with no issue.

pr_23_steamholesRegardless which process that you decide to follow, it is evident that this modification has provided a more efficient cooling effect and balanced the temperature range across the head. Tell us what your experiences have yield when it comes to this particular topic, we’d love to iron out the proper way to do this do it yourself modification.

About the author

Justen Spencer

Justen is a Ford modular motor fanatic with seven years of professional drag racing experience, and multiple championship seasons in NMCA West and PSCA. Originally from Las Vegas, he is the proud owner of four Mustangs, one that sees regular track time. When not racing, Justen can be found in the garage maintaining his championship-winning car.
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