PCM 351 Cracked intake manifold

My friend inherited 79' MasterCraft S&S and asked me to help get it going. When I opened the motor cover the first thing I noticed is 3 of the 6 freeze plugs were sitting in the bilge , "that ain't good". Having zero history for the boat I wasn't sure what else to do but pop them back in the best I could, hook up the hose and wait for the milkshake to be made. To my surprise, after running for 20 mins the oil still looks good and the motor ran extremely well. However, there was a leak at the front of the intake manifold where water enters and I found what look to be some cracks. 

Jonny Quest wrote: If the intake has cracks that go all the way through...then water intrusion into the lifter valley is very likely.  At the very least, the intake should be removed and inspected.  If you want to replace the intake, I would go with an aluminum (not cast iron) intake -- such as the Edelbrock Performer manifold.  Should even pick up a few HP with a better intake. Also...the term "freeze plug" my be incorrectly used.  Those are really core plugs.  Core plugs (aka Welch plugs) are used to fill the sand casting core holes found on water-cooled engine blocks.  The term "freeze plug" was coined many decades ago out of ignorance of the true purpose of the core plugs - they were never designed to be displaced by water freezing inside the block.  Displacement of a core plug is merely coincidental if water freezing inside the block does occur. JQ

Good advice so far Ken and JQ.  As far as the term Freeze Plugs, In the engine aftermarket ALL of the suppliers identify them as freeze plugs.  Our old Buddy Pete, who I hope is still tuning in always corrected us on the proper terminology.  Sealed Power, Pioneer, Melling , Clevite all call them Freeze plugs.  All the engine rebuilders I called on who buillt over 3000 engines per month used the same term so don’t feel like you called them wrong.  Technically we are all wrong but in the industry EVERYONE UNDERSTANDS FREEZE PLUGS WHEN YOU ASK FOR ONE.  Hope that helps.   The new intake will be better looking, weigh much less and perform better.   Running 30 minutes with zero oil in the water is a good test.  Hope your block is good.  If it takes 20 hours use to cause milk in the oil, change the oil and fix it someday when you are bored.  Not mid summer when the boat should be in service.  Just do not park it over the winter with water in the oil.

With first hand experience I second J Q's recommendation for the Edelbrock Performer intake. It adds a lot of much-needed pep to the bottom end.