it all depends on the tunnel you use..most tunnels with the exception of JM are made for the wave washer and bolt right into the diffuser at the other end. The space at the end of the tunnel is for the rear bearing and wave washer, it is necessary to keep the bearings (front and rear) in good contact with the races as they do not use a bearing cage ( angular contact). with the bearings in place and the wave washer as well the shaft with compressor and turbine; use shims to make sure things turn freely..you may not need them at all...post pics of this assembly...diffuser and tunnel bolted together ..with shaft bearings and turbine/ compressor all together