Hole on the block behind circulation pump... what is it?

[itchyfishnv]

So I noticed when I replaced my circulation pump on my 87 3.0 theres 4 holes for the bolts and another small hole that the water pump body fits over. Can anyone tell me the purpose of this? It was a little clogged and I cleaned it out and put some compressed air down there and it did shoot through the block. All normal here? Just trying to prevent any future blockages.

 

[Bondo]

Ayuh,..... Welcome Aboard,.... It's just a vent/ by-pass,....

Ya did right by cleanin' it,.....

 

[hemi rt]

It's actually the feed from the heater hose, not used on a marine engine.

 

[Bondo]

It's actually the feed from the heater hose, not used on a marine engine.

Nope, not really,....

A heater is fed from the other side of the t-stat, the hot side, the head side,....

The heater would return it's water to that undrilled barb, under the big hose barb,...
That's the low pressure side of the circulator pump, the t-stat side/ head- block side is the higher pressure side,...

Btw,... Nice boat hemi,...
I've got 2 of those, in much worse shape in my backyard, a '61, 'n a '67,....

 

[itchyfishnv]

So I guess my next question, if it was clogged for a few years would it have caused any damage to the engine? Less water flow, etc. Pardon my lack of knowledge, I've read the mercruiser manuals inside and out, but there's just some things it doesn't tell ya

 

[hemi rt]

Due to the fact that nothing goes through the port, with water just sitting in it it will clog up. It will not hurt your engine and should not affect water flow unless a large chunk of it comes off in one piece and gets lodged in a water passage but that happening is really rare.