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Yesterday at our Southern California Chapter meet I measured the head gasket thickness of a '70 base engine whose owner thought the heads had never been removed. It was about .018" thick. Also, one of the Special 300 HP engine prototypes (John McRae's '67) had .018" gaskets and we believe those heads had never been removed as the car has only about 60K miles and had not run for at least 20 years.
I've got a working hypothesis that the Flint-built small blocks were assembled with .018" compressed thickness shim gaskets, but they were either never offered as a service part, or the 3830711 .026" shim gasket superseded the OE gasket in service. Joe Lucia thinks the 711 gasket part number jibes with a calendar year 1963 release date.
GM may have decided to offer only a thicker gasket for service based on the assumption that heads or blocks would be machined in the field and use of the thin OE gasket could result in excessive compression and detonation.
So if anyone has a small block that they think the heads have never been off, can you measure the gasket thickness?
It's easy to do by sliding a feeler gage between the block and head adjacent to the stamp pad. It can also be done between the block and head at any of the other three ends, so pick whatever end is easiest to access.
Start with about a .010" thick feeler gage and insert it into a corner, then drag it along the entire interface. Note how far it penetrates. Then stack a couple of gages to equal at least .050". They should not penetrate as far assuming the installed head gasket thickness is less than the gage stack.
Next use a .015 and then a .045 or so and keep going until you narrow it down to the exact thickness. It's like measuring valve clearance. A gage of the actual clearance should slide in "snug" and the next .001" higher gage will either not go in or require significant force.
If you have a mid '62-up SHP/FI engine with Flint gaskets, there will likely be two as they began double gasketing SHP/FI engines about midway through the '62 model year, but I'm not sure how long this practice continued. Double gaskets will measure about .036", but this also means the engine might have an aftermarket compostion gasket.
Also, tell me if you can see the corner of the gasket overhanging the head and whether or not it has a small round hole in the corner that you can see.
Either add to this thread or email me with any data you can gather. Please state the model year and engine displacement/power or engine option code so I know which engine you have.
Right now I'm limiting this research to small blocks only.
Duke
I've got a working hypothesis that the Flint-built small blocks were assembled with .018" compressed thickness shim gaskets, but they were either never offered as a service part, or the 3830711 .026" shim gasket superseded the OE gasket in service. Joe Lucia thinks the 711 gasket part number jibes with a calendar year 1963 release date.
GM may have decided to offer only a thicker gasket for service based on the assumption that heads or blocks would be machined in the field and use of the thin OE gasket could result in excessive compression and detonation.
So if anyone has a small block that they think the heads have never been off, can you measure the gasket thickness?
It's easy to do by sliding a feeler gage between the block and head adjacent to the stamp pad. It can also be done between the block and head at any of the other three ends, so pick whatever end is easiest to access.
Start with about a .010" thick feeler gage and insert it into a corner, then drag it along the entire interface. Note how far it penetrates. Then stack a couple of gages to equal at least .050". They should not penetrate as far assuming the installed head gasket thickness is less than the gage stack.
Next use a .015 and then a .045 or so and keep going until you narrow it down to the exact thickness. It's like measuring valve clearance. A gage of the actual clearance should slide in "snug" and the next .001" higher gage will either not go in or require significant force.
If you have a mid '62-up SHP/FI engine with Flint gaskets, there will likely be two as they began double gasketing SHP/FI engines about midway through the '62 model year, but I'm not sure how long this practice continued. Double gaskets will measure about .036", but this also means the engine might have an aftermarket compostion gasket.
Also, tell me if you can see the corner of the gasket overhanging the head and whether or not it has a small round hole in the corner that you can see.
Either add to this thread or email me with any data you can gather. Please state the model year and engine displacement/power or engine option code so I know which engine you have.
Right now I'm limiting this research to small blocks only.
Duke
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