I assume you are using 3dclust and not 3dcalc. I believe the -1erode and -1dilate options are ignored by 3dclust, but you can use the 3dmerge command with those options to do erosion and dilation of the clusters. Changing the threshold may well change the shape of the clusters, especially voxels on the edge of a cluster. Even if a voxel value is high, its neighbors may have low values, and the voxel will not be included in the cluster.
The interactive clusterize tool in the AFNI GUI is a great tool for seeing the effects of varying the threshold, cluster radius and volume. It can also produce a 3dclust command for you.
