A head gasket for a 305 Chevy with a gasket bore diameter of 3.855 inches will physically fit a 400 small block, but the gasket will drastically overhang the bore. As shown here, we have a Fel-Pro FEL-1010 gasket with a 4.165 inch gasket bore on a block with a 4.155 inch cylinder bore. It’s tight, but it clears. Image/Jeff Smith

I’ve got a 400 cubic small block Chevy that I had the machine shop bore to 4.155. I’m collecting parts to put it back together and used some of my old parts for the part numbers. We tossed the head gasket on the block and discovered the inner part of the head gasket is inside the bore. In other words, part of the head gasket actually overlaps inside the bore. I didn’t build this engine and while I’ve not done a lot of engine work, I’m pretty sure that’s not right. I’d like to get your opinion. 


Jeff Smith: The short answer is you are correct. The small block Chevy has been produced with a raft of different bore diameters ranging from an absolutely tiny 3.50 inches for the little-known 267 V8 to the 400’s 4.125 inch diameter. As a result, there are a dozens of head gaskets in various thicknesses and bore sizes for these engines. It sounds like the previous builder wasn’t paying attention and probably dropped a set of 350 Chevy heads gaskets that offer roughly a 4.090 to perhaps a 4.100 inch gasket bore diameter.

If you are not paying attention, it would be very easy to slap a 350 head gasket on the engine and not look to see whether the gasket intrudes into your 400’s much larger 4.155 inch bores. A gasket with a 4.100 inch bore would overhang the cylinder bore by 0.0275 inch all the way around. That’s bad.

This situation could quickly create a hot spot in the chamber area that would cause detonation and possibly even pre-ignition. As a quick explanation, detonation is defined as an uncontrolled combustion that occurs after the spark plug has fired. This uncontrolled combustion creates a pressure spike that can damage pistons, flatten connecting rod bearings, and generally cause internal mayhem.

Pre-ignition is even worse. As the name implies, that hot spot in the chamber from the gasket could become hot enough to light off the air and fuel long before the spark plug is scheduled to fire. When pre-ignition happens, the cylinder pressure quickly builds while the piston is moving upward. As you can imagine, catastrophic things happen. We experienced pre-ignition an engine running on E85 once that occurred without warning. The culprit was a spark plug with too high a heat range, but the damage included a broken piston. We were lucky–it could have been far worse.

So the answer is to use a head gasket intended for a 4.125-inch or larger bore size. For a 4.155-inch bore 400, a Fel-Pro performance gasket like the FEL-1045 offers a gasket bore inside diameter of 4.180-inch, which places it close to but still outside the cylinder bore. They also offer a 4.200 inch gasket (FEL-1044), but the 4.180 inch gasket appears to be the better choice.  

So the message should be clear. Pay attention to the small details when building an engine and make sure the head gasket does not overhang into the cylinder bore. You will save yourself a lot of grief!

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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.