At What point should I be replacing the Injector holders on my M117.968?

I've had them out and they are not cracked or anything like that, I imagine just replacing the O-ring and the injector seals should be pretty adequate? am I right?

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RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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