big hole to the left and right of the water outlet is for the temp gauge. Use one for the temp gauge, can either plug the other, or use it for 1 of your heater hoses. On the back, the two larger holes on either side are for a water crossover, can be plugged, probably 3/8" pipe... Then it has two threaded holes for the distributor hold down(s), and a couple others for linkage brackets, etc. Just make sure you use the right threaded fittings, or you will have a bunch of extra work to do!!!!