Here is the straight scoop on 273 head gaskets.
There is no use in getting a smaller diameter head gasket since the closed combustion chamber, 64-67, measures about 3.97" at the widest point and 4.04" at the original steel shim head gasket witness marks. Since the gaskets need to be sandwiched between the head and the block, smaller diameter round bores will not work. These heads and witness marks are from an original steel shim gaskets that came off a 1967, 2 barrel 273. Since the smallest round bore gasket I measured was 4.01" on the MOPAR 318 head gasket #2806 984 and the HP Fel-Pro 1008 small block head gasket measures 4.18", both at .039" thickness there is only about .01 difference in compression ratios. Using a +.040" bore (3.665"), as cast combustion chambers (63.5 cc or 3.875 cubic inches), 3.31" stroke, "0" deck, factory 2 barrel pistons with 4 valve reliefs ( .7 cc or .0427 cubic inches each), and head gasket volume the compression ratios are as follows. The MOPAR 2806 984 head gasket yields an 8.54:1 compression ratio. The Fel-Pro 1008 head gasket yields an 8.53:1 compression ratio. The Mr Gasket 1121G at 4.14" bore and .028" thick will jump the compression ratio up to 8.95:1, all things being equal. So in conclusion, usable bore diameter makes little difference while the thickness does make a difference.
Here is a close up of a 4.04" bore .027" Cometic head gasket on the same 273 head. Notice how close the chamber is to the valve reliefs in the combustion chambers.
The second area of concern is a partial cutout on the head and some 273 blocks, see below. Also note the cutout on the heads and 273/318 gaskets. I called Cometic and they don't recommend their MLS head gaskets for the 273 (Not sealing at the cut out?) but offer a 3.97" bore x .043" thick composite gasket, about the same compressed volume as the Fel-Pro 1008 HP head gasket.
Head Gaskets
OEM 318 composite 2806 984
OEM 273 Steel Shim 2843 283 courtesy of @65Val
There is no use in getting a smaller diameter head gasket since the closed combustion chamber, 64-67, measures about 3.97" at the widest point and 4.04" at the original steel shim head gasket witness marks. Since the gaskets need to be sandwiched between the head and the block, smaller diameter round bores will not work. These heads and witness marks are from an original steel shim gaskets that came off a 1967, 2 barrel 273. Since the smallest round bore gasket I measured was 4.01" on the MOPAR 318 head gasket #2806 984 and the HP Fel-Pro 1008 small block head gasket measures 4.18", both at .039" thickness there is only about .01 difference in compression ratios. Using a +.040" bore (3.665"), as cast combustion chambers (63.5 cc or 3.875 cubic inches), 3.31" stroke, "0" deck, factory 2 barrel pistons with 4 valve reliefs ( .7 cc or .0427 cubic inches each), and head gasket volume the compression ratios are as follows. The MOPAR 2806 984 head gasket yields an 8.54:1 compression ratio. The Fel-Pro 1008 head gasket yields an 8.53:1 compression ratio. The Mr Gasket 1121G at 4.14" bore and .028" thick will jump the compression ratio up to 8.95:1, all things being equal. So in conclusion, usable bore diameter makes little difference while the thickness does make a difference.
Here is a close up of a 4.04" bore .027" Cometic head gasket on the same 273 head. Notice how close the chamber is to the valve reliefs in the combustion chambers.
The second area of concern is a partial cutout on the head and some 273 blocks, see below. Also note the cutout on the heads and 273/318 gaskets. I called Cometic and they don't recommend their MLS head gaskets for the 273 (Not sealing at the cut out?) but offer a 3.97" bore x .043" thick composite gasket, about the same compressed volume as the Fel-Pro 1008 HP head gasket.
courtesy of @stroker402
Head Gaskets
OEM 318 composite 2806 984
OEM 273 Steel Shim 2843 283 courtesy of @65Val
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