Noticed coolant level dropping late in the season. Pressure tested and confirmed one tube in heat exchanger leaking into raw water side. Merc FSM stated that you can braze shut up to 3 tubes but will normally result in a nearby tube beginning to leak.

Sen Dure has a drop in Hi Per 5" HE to replace the 3.5" 496HO stock unit. Don't really need the additional cooling capacity but it wouldn't hurt. They also recommended to fill leaking tube with a wooden dowel rod and MarineTex epoxy closed the ends if I wanted to try and repair the stocker. Advised not to braze repair.

Mr. Cool also has a drop in replacement of the stock heat exchanger. Seems they have a good reputation for quality.

What's your thoughts?

From what I've read Mr. Cool units are thinner than OE equaling even faster degradation.

That's simply not true. We use the same material and material thickness as the OEM units on this particular product.
They are dimension for dimension.

Hey there: Russ from Mr. Cool.

I've found through selling lots of these is that the 3.5 needs to be replaced by the 3.5 because it hits the front edge of what we used to call the throttle body if you use a 4 inch where a 3.5 inch used to be.
I know a 4 can replace a 3.5 on some, but not all.

I had mine go bad on a closed cooling system. Took it to the repair shop and was deemed un-repairable ordered a new one from Mr. cool and it and performed and fit perfectly and was cheaper than one from merc

Russ, appreciate the info. I was able to repair the stock heat exchanger. Only one tube was leaking. I prepped the tube and filled both ends with a generous amount of Allmetal compound. Pressure tested ok. No drop in coolant level this season or loss of cooling performance.

^^^^^^ then the water circ pump was going out this year. Common with 500 to 600 hour range. Just heads up 496s do not go that much over 500 hours on the WCP. If you do alot high rpm running and have not changed your coolant in years you might want to change the circ pump 4 sure and coolant - around the 350 to 450 hour mark. .

Excellent. When a tube freezes the tube repair is legit. When the tube repair is due to corrosion, erosion or electrolysis, that's when it's a little iffy. Sometimes it's just the first in a long line of repairs because all of the tubes have seen the same environmental conditions.
Also tube repairs if they leak just a lil - it goes out the exhaust. The glycol circuit is a higher pressure than the SW circuit. It usually doesn't leak a lot at rest.
One tube is not going to affect pressures or cooling in any real sense. So everything you say is completely logical. :-).

Check the oil cooler first for blockage.