91 240sx KA24DE valve questions

[peterm]

Im in the middle of a rebuild and am stuck on the valves now. The valves in number 4 were bent so i ordered new ones and replaced valves and oil seals. The thing is the new valves say T5 on the bottom and all of the old valves say T5S. I ordered 91 valves because thats what i was told the year of the engine was but i know it had been replaced in the past and its possible the head is not actually a 91. It does fit the s13 valve cover and vacuum pipes on the back, so im pretty certain its an s13.

They fit fine, but now that im trying to get the lifter clearances right im running into a problem. There isnt enough clearance, and the backside of the cam is running into the lifter shim, and i dont think theres a shim small enough to get the right clearance.

So my real question is are T5 and T5S valves interchangeable? Could it just be grit or the lifter shim not seating completely?