A series engine and all Sprite thermostat housing elbow

This housing is often corroded and when you change your thermostat, it's great to have a new one on hand. The flat top version is for the 948 engine. The round version is for the 1275 engine (works on a 1098, too). 

If you are changing from a cross flow radiator to a down flow original type (or aluminum) radiator, you will need a round top.