About 3 or 4 years ago I looked into this...the silicone stuff and buna-n, that is. There are at least 3 challenges with silicone. Not all is ethanol resistant. Almost all of what you will be able to afford is what is termed "conditionally resistant" which is a nice way of saying "it will last for a while before it fails, but it will fail." It cuts best using a steel rule die/press method. The rule die is going to be very expensive and also is going to leave a lot of wasted product. Also when you compress it, the vacuum passages will shrink somewhat, so they must be cut larger than normal.
Buna-N on the other hand is resistant, but finding it in 1/32" thickness is not so easy and it is costly unless you're buying a lot of it. At that time it was $24 per square foot plus shipping. Like the silicone, it would also have to be cut with a steel rule die and a press in order to get a precision product. I bought 1 square foot of the buna-n and cutting it by hand is very difficult...it moves around on you so even with very sharp tools it is sort of like stacking greased marbles. And...beyond 250 F it begins to fail. It came with a big warning about that. Not that your carburetor should be getting that hot but everybody I talked to agreed a larger comfort margin would be better. And there is another problem. The compressibility and the tendency for people to try to overtighten the screws is going to lead to more warping. Sorry, but everyone agreed that was unavoidable. The buna-n has a shore (hardness) of 40 generally speaking and that is soft enough to allow for a good deal of compression. A compression set rating closer to 90 was what folks thought would be best or setting into the gasket "metal stops" that would keep anyone from overtightening while still allowing for compression. All in all it left me thinking the fiber gaskets were much better. But, hey...I do have some buna-n left over.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
