Those insulators are called seals.

If you're going to the trouble of changing the injectors themselves, why are you not also changing the holders and seals as well? To not change these along with the injectors is false economy. Those old seals will leak you know, especially when disturbed after being in the car for however many years. Get ready to smell gas and have performance issues.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)