hi guy i have some question about upgrading valve spring kit on gt40p heads. i saw on the kit they have some shim do i have to put the shim and what is for because on stock gt40p spring they dont have shim
Usually after a valve job one or more valves may be recessed further into the head that before. The shims are there to set the installed height correctly so all springs have the same preload.
they aren't needed from the factory because the head is machined to exact specs.
The shims are to set spring height. When the valves are lapped they can change the distance the valve stem sticks out of the head when the valve is closed. If you don't measure and shim the springs you could end up with uneven spring pressure.