One of the problems in working on old cars and dealing with old parts is that cork and paper gaskets can dry out and shrink. Often all that it takes to bring them back to size and flexibiity is five minutes soaking in hot water. (Lay them horizontally on something flat and absorbent to blot the excess moisture.)
Leather and felt seals need to soak in oil overnight before installation. Leather is actually an excellent sealing material, and has the advantage of swelling to seal small cracks, and was the seal material of choice for many years (do we have to wonder where the seal company CR, Chicago Rawhide, got its start?).
But leather has pretty well been replaced by neoprene seals, which can be molded into more complex shapes, and doesn’t come in awkward cow-shaped pieces, but there is no reason not to use an original leather seal if that’s what is available for your project.
Neoprene seals usually have a little spring inside the lip that helps hold the lip against the shaft that it seals against. If you are having trouble with a seal that seems to keep leaking, check to make sure this spring is not bent or sprung. Obviously, if the lip is cracked or worn down, that will interfere with sealing.
Some seals are “directional”; they have little diagonal ribs on the lip. Even if the seal is the same dimensional size, if the rotation is wrong, they don’t seal correctly. One example is the crankshaft seals on the Fiat 600 and the 850. They are the same size, but the engines turn in opposite directions and the seals are designed for each rotation.
Another problem we see occasionally is that the surface that the seal works against has become scratched, grooved, or pitted. This is common in Toyota front crankshaft seals and often in rear trans seals.
Occasionally a seal can be found that is dimensionally correct on the inside and outside, but slightly thicker or thinner than the original or designed differently, so that the sealing lip of the seal rides on a different place (hopefully less worn) than the original seal.
The best answer is to use a sleeve on the shaft. Several seal companies sell “Speedi-sleeves”, which are a very thin, very smooth sleeve that fits tightly over the shaft and provide a new, clean, smooth surface for the seal. They are sold in sizes to fit many shaft sizes. They are not cheap, but cheaper than a new tailshaft, crank pulley, or crankshaft.
O-rings are used in a wide variety of sealing locations. They are usually round in cross-section, but may be square or even D-shaped (Bosch distributor shaft seals, for example).
What is confusing sometimes is that a seal may be born in a common shape, but when they are compressed into their home they deform and compress into a more complex shape. (One example is the early VW oil cooler seal, which looks like a chunk of little hose when it is new, but develops a bulge around its middle when it is compressed.)
Another thing to watch for is that most o-rings in an internal engine application need to be oil and gas resistent. Rings designed for a water-sealing function aren’t, and if used in a spot where they are exposed to gas or oil they deteriorate.
Gaskets: Not all joints in an engine are gasketed. One example is the upper oil pan on the Mercedes SOHC engines, which is metal-to-metal, or the Mercedes timing covers, which have only several o-rings. If the surfaces are really smooth and straight, they will seal adequately without gaskets.
Sometimes the cure for a gasket that keeps blowing out (one common one was the Fiat 124 exhaust flanges) is to machine both surfaces totally flat and put them together without a gasket. Some originally ungasketed joints, like the exhaust manifolds on Toyota 20R and TR7, are not quite as smooth and true after a few zillion miles in the real world and do better with a gasket the next time around.
Many gasket companies make replacement gaskets for some of these applications. Some exhaust headpipes are flared surfaces held together by clamps with no gaskets, such as early MG, some Fiats, some Nissans.
Sealant: It is not uncommon on newer engines to find oil pans that are factory-assembled with sealer instead of gaskets. Sealers are tricky and must be used carefully.
Read the labels and consider whether the part will need to be disassembled again at some point down the line, in which case you do not use permanent materials. My older son was not a happy wrench when he had to take off an old Datsun head that he had JB-Welded on.
Also, think about the heat to which it will be subjected. (Exhaust manifolds are picky — you are better off without sealer than something that will burn off and leave gaps.) The biggest mistake we see is the overuse of silicone, which tends to get squeezed out and break off into crumbs inside the engine that can destroy camshafts and other important parts if they block important oil passages or clog the oil pickup screen.
A small amount of grease or light-bodied sealer can hold a gasket in place during assembly, but too much can do just the opposite. One day a customer had me come look at his Jaguar valve cover, saying, “I don’t think this is right, is it?” The gaskets, which on a Jag are very long and narrow, had so much glop on them that they had slithered out of the cover like a long skinny peeled grape.
Despite what the advertisements for sealants say, they cannot always be used to replace a real gasket. Even now and them we get a car in with a scary bottom-end rattle that turns out to be the oil pickup vibrating against the oil pan, because a pan that originally came with a thick gasket was reassembled with only sealant.
Most modern head gaskets come with whatever their manufacturer wanted them to have on them, and anything you put on will fight with whatever they put on. Old-fashioned steel-shim or copper head gaskets can be coated with copper or aluminum high-temp coatings if the head and block surfaces are iffy.