It depends on WHICH "positive" seal is being used. The "Teflon" (white with silver "rings") seals are VERY persnikity, as Cliff alludes to. The "condom" (plastic sheeth) that comes with them must be employed, along with a bit of lube (we use assembly lube in a bottle). There are ready-made installers, usually white plastic. Getting them "started" over the valve tip is the more critical part, as they can be distorted or "scraped" at that time. Once started down the stem, keeping them "in line' is the key.
If the Viton (black, rubberish, rings), they're a bit more "forgiving" as they can stretch without distorting. Using the sheeth is still recommended. Usually, they can be pushed over the top of the guide by "hand". The Teflon-lined version (OEM) must be installed carefully, as the Teflon insert can easily come out, making it no longer "round".
SC's method of installing the heater hose nipple is the simplest way we've found, too.
FWIW
Jim
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