Bigger valves, original seats

[227bhp]

I am thinking about putting 2mm oversized valves in a head, what decides whether I need new valve seat inserts or not? How do you decide the limit?

[227bhp]

PeterBurgess said:

You need to know if the larger valve will have a bigger OD than the valve seat, if it does you will need bigger OD insert. If you are on or near the OD of the insert it would be worth having a widish seat and bottom cut/s so you are generous with how much insert is left. If you go too large on the ID you may well risk the insert cow-lapsing.
Paul Ivey of REC fame and part owner of G&S valves has, historically, produced oversise valves which are on the limit of the standard inserts fitted on many alloy heads.
Peter

Thanks for the reply, I should have posted measurements up before, but the OD of the valve is equal to the OD of the seat and the head is aluminium.
Also the seat 'ring' material is 2.72 thick so when I enlarge the throat (by 2mm overall) it will reduce this to 1.72.
I wondered if it was a bit borderline, the engine is not known for seats coming loose (at all) with normal valve sizes, but am worried if I would be removing a bit too much of the tension which is holding it in.
My decision I guess, but it's maybe prudent to put some slightly bigger seats in.