I always work on the assumption that if a gasket (or 'o'-ring) was present when you took it apart, then put one back when it goes together!

Speaking from bitter experience, be wary of using silicone based sealants on cooling system joints. The OAT based coolant used in/recommended for the XT660 monitor will quickly dissolve anything containing silicone, even silicone hoses if they're not the "OAT safe" internally coated type (and the vast majority, particularly on ebay, aren't).

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