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While looking for part numbers it became apparent that m30 & m20 engines (so I would assume m10 as well) have valve seats that are seperate from the head.

I have no experiance of such things and would like an understanding of them.

What is the benifit to them?
How are they fitted?
Are they removeable/ replaceable?

Any info on them would be apriciated as I don't like not knowing about stuff!

Any engine with an alloy head has valve seat inserts. With leaded fuel, Iron headed engines didn't need them, but without lead in the fuel, even they need hardened inserts to to run unleaded.

They have to be machined out, and pressed in, but unless they're badly damaged, or you're fitting significantly larger valves, it's unlikely you'd need to.

Thank you very much!