Fits Filters from 3 1/8" to 3 7/8". The large range enables this wrench to fit most standard size filters. Steel jaws grip filter without slipping. Works great in tight spots since it works from the top of the filter. Use with 3/8" drive. *The wrench has a spring to pull the jaws in. As you turn the wrench with the square drive in it keeps tightening tighter. It will only remove the filter. You can't use it to install a filter. per MW