The heater circuit will also act as a bypass, provided there isn’t a closed heater control valve being used, and the core isn’t plugged.
You could always drill a hole in the housing too.
Thanks. Yes, drilling the hole will be on the agenda. I was just surprised to find a housing without a bypass. After trying to cure the symptoms with different water pumps and thermostats for years I was running out of ideas why the engine would idle for hours at 140 F without a t-stat but would jump like crazy between 180 and 230 in seconds when operated with a t-stat.
