The 261 was also designed for a bypass thermostat. Some folks will argue that is only necessary because the larger trucks might have engine strain before proper warm up. As much as I agree, I think this is probably secondary. Others (like me) will argue that the primary reason GM engineers added the bypass thermostat had more to do with the siamesed cooling jacket in the 261 block requiring coolant flow at all times. I think the steam holes and the bypass thermostat were an integrated design by the GM engineers for the 261.
Matt
Canadian Pontiac 261's (of which I bet there are more in existence than the truck version) came with the same non-bypass water pump as on 235's leading me to doubt this theory.
