[68 TT]

Originally Posted by wesmigletz

The heater core on our 70 Blazer has the 3/4" outlet on the bottom and 5/8" on top.

IIRC, the 3/4 goes to water pump and 5/8 goes to the intake, with the hoses crossing over. Will it hurt anything if the 3/4 heater core is on the bottom, and I run it to the water pump? It's my understanding is that if you switched the intake and water pump fittings, the heater wouldn't get hot.

Thanks in advance.

Wes

The assembly manual shows the 3/4" hose at the top on the blower housing and the source for it coming from the water pump just above the lower radiator hose outlet. The 5/8" hose at the bottom is fed from the intake manifold just to the passenger side of the thermostat housing. The 5/8" fitting is coming out of a 45-degree pipe elbow angled at 30-degrees to the front of the engine back toward the heater core. It also shows the hoses crossing over each other in an X-shape with what they are calling a clamp at the X. The picture isn't clear if it is a clamp attached to the engine or if it is floating free with the hoses.

Your assumption about the heater not getting hot if you switched the source fitting locations is correct to a point. You can run the supply through the 5/8" line at the bottom of the blower housing from the water pump and the return from the top out the 3/4" side back into the intake and have the heater work close to correctly in your case. It just won't have the designed in back pressure that slows the flow down to let the water lose its heat through the heater core as efficiently. You will have to decide if this loss of performance is acceptably offset by the time & labor of going through the process of pulling the blower housing to flip the heater core so it is in the correct orientation to make the heater work as intended.

As to where the supply line needs to come from I would stick with the water pump just above the lower radiator hose outlet where the factory put it as that is the hottest water source. Sometimes there isn't a spot for a fitting there on new water pumps like the Edelbrock aluminum one I have and you have to compromise with an alternative location. Square body trucks dump the 5/8" line back into the radiator directly just above the transmission cooler top fitting so you can pick any spot on the engine to pull the 3/4" supply line from if your radiator has that fitting and the heater will still work, just not as well if you pick the wrong place. You want the water before it has gone through the radiator but after it has gone through the engine so it is as hot as it gets and heats up as quickly possible for the cold days when the heater and defroster are needed the most.