ST dual springs
 

Hello. Am about to install Supertech DuaL Valve Springs in a Bp4W head. Do i need to install the washer below the ST seat?
​​​​​​I also have +1mm ST valves. Shall I use them or are the stock valves sufficient? Gtx2867gen 2 turbo.
TIA

The dual springs should have come with a fitting they sit on, that is what you use, not the original washer.
Unless you had the valve seats ground for over size valves you should not use oversize valves.

are you sure? When I acquired mine, I’m pretty sure they required re-use of the factory spacer to achieve the listed seat pressure.

I mean, what should really be done is measuring the height of the valve and spring, measure the change in spring height by the cam, measure the spring pressures at both heights, determine if the spring seat pressures are acceptable by referencing the service manuals and other people's builds and data, then determining whether or not the stock .015" shim should be used. Throwing parts in willy-nilly can work, but building without a specifically stated goal is kind of pointless.

That "washer" in the ST kits that I've seen is a big thick puppy that ST uses to achieve the spring pressures they quote while using a spring that is not optimal for the installed height.
I believe they found a spring (for another application) that was close and adapted accordingly.
It fits on the bottom and the original shim is NOT used per their instructions.
If you leave it out, I'm guessing you will have extremely low pressures at both check points. It's 3+ mm thick

It's wise to check the ST springs for closed and open pressure anyways as they often require more shimming than just the spacer to hit their advertised numbers.

The oversized valves require having a valve job done and their greatest advantage is gained with careful port throat/pocket blending.
I would not put oversized valves in without 8+ hours of port work done on the head. Just the valves alone will not give you much improvement and if they are done incorrectly, they can be worthless.
Good idea to VERIFY that the increase diameter valves clear the valve reliefs in the pistons as well.

That is the same advice I received from SuperMiata / 949 Racing, from whom I bought the dual springs.