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Google AI gave the following:
In certain aircraft piston engines, specifically those from Lycoming and Continental, silk thread is used between the crankcase halves to aid in sealing. It's typically used in conjunction with a sealant, like Permatex No. 4, and is laid into the sealant to prevent it from being squeezed out entirely when the parts are bolted together. This technique helps ensure a leak-proof seal for the life of the engine.
Haven't heard of doing this on 911 engines. Maybe someone here will know differently.
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