The Canadian GMCs came stock with 261s, the engine in my truck is a numbers matching truck and engine,it is a solid lifter engine. So if you look at the picture the upper thermo housing has a hose that goes to the water pump ,the flow goes from the housing to the pump , the lower thermo housing goes through the heater back to the water pump.Would this be correct? If so would the heater circuit always be acting like a by-pass no matter what position the thermostat was in? I have been a Diesel engine mechanic for 25 years the engines I have worked on all had by-pass circuits on them ,but the thermostats were double acting,as they opened to rad they simultaneously closed off the by-pass until it was full open to rad and the by-pass was completely closed off. If the seal blocking off the by-pass leaked the engines were prone to overheat.
