Though gasoline engines incorporate crankshaft and camshaft seals like diesel engines, gasoline engine seals tend to be different from their diesel engine counterparts. As gasoline engines have gotten both smaller and more powerful, the need for increasingly heat- and additive-resistant rear crankshaft seals has caused designers to turn away from silicone (which for years was the typical engine seal material) to fluoroelastomers (FKM). FKM seals are now the norm for rear and front crankshaft, camshaft, and auxiliary shaft seals.

Studies have shown that a rear crankshaft seal with a half-rubber, half-metal O.D. has advantages over either a full metal O.D. or a full rubber O.D. In particular, a seal with a half-rubber, half-metal O.D. may be more forgiving than a full rubber O.D. seal during installation. The metal portion of the half-and-half O.D. has a chamfer that helps guide the seal into the bore. The metal edge also eliminates the possibility that rubber will be sheared off of the leading edge of the O.D. during installation (as can sometimes happen with some full rubber O.D. designs). Because there is less rubber on the O.D., and a metal-to-metal pressfit, springback (in which a seal unseats itself after installation due to shearing stresses between the O.D. and the bore) is reduced.

Half-and-half seals are less prone to cocking in the bore, but they may require more force to install than a full-rubber O.D. shaft seal. As temperatures and pressures increase, half-and-half seals are also able to maintain higher retention force in aluminum bores. One thing to be aware of: half-and-half shaft seals are more difficult to manufacture than conventional seals, so it’s imperative that extra care be given to procure quality seals. In addition, the initial cost of the tooling is more expensive and also more expensive to maintain. The cost of a half rubber, half metal O.D. seal is often more expensive than a full rubber O.D. seal. An example of a half-rubber, half-metal O.D. seal is shown in Figure 203.

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