Circulation Pump Old Style?

[Fill Ess]

I just replaced a pump using Merc 8503391, with brass impeller and stainless back cover

The back looks like this, with 3 holes on the side where the big hose attaches. The top 2 holes are for the bolts and the third is a water passage

I used Felpro gaskets, assembeled everthing, and when I started it up, water came shooting of the left side bottom right where that bottom hole is located

I thought maybe I forgot to tighten the bolts on that side, or maybe the freeze plug just to the left of that was the problem

Luckily I saved the old pump because I noticed it's older style GM pump (not LS) that only has 4 holes, not 5

It even has a GM part number on the front

Any idea why I would have this one instead? The PO repowered at some point.

On the block itself, there is a hole to line up with the 3rd hole on the gasket and pump but it looks either plugged or not tapped all the way through

 

[Fill Ess]

 

[Fill Ess]

 

[GirlyGirl1999]

View attachment 130253

So the third hole is a coolant bypass port so coolant can still flow until the thermostat opens, now this is old school gear head Chevy stuff I’m talking about. Many a Chevy person blocked this hole in theory to push all the coolant through the radiator. Effective? Probably not, I always left mine open. Here is the pump on Amazon without the bypass.

For some reason I can’t paste the link, search replacement pump
Mercruiser 46-8M6005225

 

[Fill Ess]

Thanks!

 

[GirlyGirl1999]

Thanks!

No worries, on a side note look at the front pictures to make sure it as the ports/plugs as your old one.

 

[Fill Ess]

I took the new pump off and cleaned up the front. There is a 5th hole but it only goes in maybe a half inch and stops. This hole was exposed before so it clearly doesn't connect to a water jacket I cleaned up the old pump and put it back on since it still works. If everything is good I'm gonna run it like this and look into replacing it in the off season.