It's not threaded, so not for a grease fitting.
Should I block it up or anything?
What is it?

It`s a bleed hole to let the water out for when the seal wears out, don`t block it up, just leave it & when you see water comming from there, then you know the seal is buggered

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

But surely half the reason for the seal to wear is because contaminents are getting in through that hole and causing abrasion on the inside?

nah, the wear occurs on the other side of the seal,,, it runs against the back of the impella, so basically no grit can get to that surfasce through that little hole

If the hole wasn`t there, then what happens is the bearing gets more water through it (than it would with the hole) & therfore causes even more rapid destruction of the bearing than it allready would,,, so , if/when the seal gives up, they (the manufactures of water pumps) allow the water to piss out through that hole rather than have all the water pass through the bearing wrecking it more quickly & making it harder to see where the water is comming from,,, the little hole leaves a nice easilly view-able trace of rusty yukky water marks comming from it so is an easy "tell-tale" that the water pump is rooted

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

It took me a month and a radiator swap to figure out the water pump was leaking
At least I hope it's that as I'm 3/4 of the way swapping it , and it's good to know what that hole is.

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Respect mine and I'll respect yours.