Working on some SBM iron heads and need to shim the valve springs about 0.100". That is too much shim to put under the valve spring to keep the spring accurately located. I'm using 3/8" stem 1 groove valves. Does anyone know of a negative height spring retainer? Comp has -0.050" locks. The Comp lock + shims would probably work fine. I'm just hoping to find negative height retainers and negative height locks to limit the amount of shims needed.