If you go onto AFR's website and look up the AFR 265cc Oval Port Magnum BBC head. About 3/4 the way down the page they tell you which head gasket to use, under the Specifications, Features, and Optional Supporting Components section. They are more than $20.00 bucks for the recommended head gasket.
http://www.airflowresearch.com/index.php?cPath=68_111

 

Your head gasket needs to be somewhat larger than the bore; maybe .030" - .050" might be ideal, up to .100" might be acceptable. The fire ring needs to be far enough back from the edge to get some cooling, without giving up too much compression or leaving a large volume of gas out around the edge of things that won't ignite due to a convoluted chamber shape, as is already an issue in general with the BBC design.

The "regular" felPro gasket is just fine, as far as "quality". Might not have all the super high-perf hot-rod type features, but unless you're building onehelluva hot motor, it would do fine. But, I would not put a gasket with a nominal bore dia or .250" on a 4.250" bore; what if your dowel pins and your bores were mis-indexed by .010"? It happens you know. Especially with either factory blocks known for loose tolerances in such areas, or "rebuilder" motors where they bored on the wear instead of on the measured indexed bore centers as they usually do.

 

As pointed out already, go to the website of AFR's and use their recommended head gasket for the bore of your block.

As far as putting too large of a bore gasket on your block, you have no choice if going with a head designed around a 4.5 bore, like my 305's. So you have to purchase the 4.540 bore 1017 gasket so the ring is not sitting in the chamber of the head.

 

Hello all, I am going to be upgrading heads in the next couple months and was re-reading the "best head gasket" thread so that got me thinking and looking on Summit's website what head gaskets are available. I am confused about what the head gasket bore should be though. My engine is a Gen VI 454 with stock bore of 4.25 but there is only one gasket with a 4.25" bore listed and it is a regular FelPro $20 gasket which doesn't make me feel good quality-wise at only $20!

The engine will more than likely be getting a set of the AFR 265 ovals, and is a street car that I am not afraid to run through its paces. I don't want a race only gasket, but I also don't want a weak non-performance type of gasket either if that exists.

Also, should I stick with a Gen VI gasket, or I have heard of people using Mark IV gaskets as well.

Somebody shed some light for me.

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Hi Scott,
The gasket you want is the Felpro 1037 for the 265 heads
It is 4.370 for a few reasons some of which have been touched on
We "blueprint bore" a lot of blocks & many are far enough off that you sometimes cannot put all or even any of the cylinders where they belong which is what RB touched on above
Also you normally have at least a slight chamfer or break on the top of the bore to get the rings started & as Walter said on some heads the CNC chamber option dictates what gasket you have to use not the block or bore size.

Anyway, don't spend time worrying about it,, all has been figured out so you do not have to