The big blue filter housing cap is threaded to match plumbing size. These caps are available in three-quarter, one-inch, inch and a quarter, an inch and a half thread sizes. When you're installing the filter and housing and you have an inch and a half plumbing, make sure the cap measure an inch and a half because you never want to reduce the size from your plumbing size.
If you have a three-quarter-inch cap and an inch and a half housing, you can reduce the fitting down to three quarter with a bushing. If you're using your Big Blue housing commercially, just connect the water into it. The cap has an in and an out. The end is the feed side. The out is the filtered side. Make sure the in and out are lined up properly since the filter housing doesn't come on a mounted bracket. Then install your mounting bracket on to hand the filter housing on the wall