I'm currently fixing up an older Silvia and the previous owner threw away the old grouphead cover and the screws for it
And I've not been able to find a replacement kit with both the screws and the grouphead cover.

So now I have a grouphead cover but no screws.
What is the right size and type of screws to attach a new black plastic grouphead cover?
The holes on the cover appear to have a diameter of about 4,5mm. So I'm thinking M4 or M5 screws?

They're not bolts, they're pointed screws. They're meant to screw into the little metal fixers that go on the plastic columns. I believe you will find that the same screws that fit the fixers for the body panels fit the fixers for the group cover, if you have the fixers, any wood screw similar in size should hold it.



If you don't have the fixers, you might be able to use some wood screw that is narrow enough not to split the plastic, and see if it works.
Part number for round fixer 37900908 (it would need 2 of them)

Part number for the kit with the group cover and 2 fixers is 10701685 (does not contain the screws)

Part number for the Rancilio screw is 37154212

That's really helpful, thank you.
I see now that I've made a bit of a mistake.

I have several small orders for different Silvia parts coming from different shops. And I've actually got those fixers without remembering/knowing what they were for!
I had mistakenly thought they were alternative fixers for the Rancilio name plate.

Bit of an embarrassing question, how are those fixers supposed to be placed in relation to the cover?

Remember last week's discussion about how much you'll love your Knipex parallel pliers once you buy them? This sort of thing is why.

If you don't have parallel pliers, you can put the plastic on a table and tap the little fixers into place very gently with a small hammer. Don't try to pound them on in one strike, slow and steady wins the race. I have broken more than one, many years ago when I used to hammer them on. I have parallel pliers now.
But now I make sure the retail store side of my company always has some of them available with the fixers already on. Too many customers want free ones after they break theirs (I kid you not).
https://www.espressoplanet.com/Rancilio ... Cover.html

So the fixer sits like a hat on the screw pipes on the cover?
Tap gently all the way down with a rubber mallet, and screw to fasten. Got it.