The 261 is differant because of the siamesed cylinders and the steam holes in the head. It needs water flow through the head to avoid steam problems and head cracking, especially while the engine is warming up. The bypass thermostat has a basket type thermostat that sends water back to the water pump when closed, allowing circulation. It has a larger housing than the standard 235 type. You need a heater or a bypass hose if you are not using the big truck 261 thermostat housing.
