What caused this? Blown head gasket, cracked block between cylinder 4&6

[blamtro]

I spoke to a technician who reviewed my photos. He indicated that I either had a leaking injector or a knock sensor that failed and allowed the timing to advance too far.

 

[Lou C]

The plugs look like the fuel mix was too rich I would think if the knock sensor failed it would default to retarded timing rather than advanced too far. My thoughts are perhaps a previous overheat before you owned it, and how was your wide open max rpm? Over propping over a long period of time could also do that....depending on use...

 

[blamtro]

was propped perfectly. 4600 with a FULL boat, 5000 when only a captain.

The tech I spoke to indicated that that was one of the failures of mercury in his opinion. That if the knock sensor failed it would allow full advance.

The plugs look like the fuel mix was too rich I would think if the knock sensor failed it would default to retarded timing rather than advanced too far. My thoughts are perhaps a previous overheat
before you owned it, and how was your wide open max rpm? Over propping over a long period of time could also do that....depending on use...

 

[tank1949]

Here are the pics of the manifolds. No leaks, they appear to be in great shape.

Don't just look at them, use acetone to test. You may have to make some sort of tool to block off the flat part of riser in order to fill with acetone. I used aluminum flat stock to create one. Check head too! There is a thin area near water passage that may have rusted through just enough that the input stroke could pulled in water into combustion chamber and not just fuel/air. I had it happen on a 5.7. It destroyed a motor!

 

[Lou C]

was propped perfectly. 4600 with a FULL boat, 5000 when only a captain.

The tech I spoke to indicated that that was one of the failures of mercury in his opinion. That if the knock sensor failed it would allow full advance.

very poor engineering that is! We had a Subaru that had 2 knock sensor failures, in failure mode it retarded the timing to base level so for sure you knew what it was, check engine light and no power = failed knock sensor. They were not fun to replace being on top of the flat four block UNDER the intake manifold!

Also...
You can prop the manifolds up level and fill them with Acetone (careful, very flammable) and watch the level in the cooling ports where the elbow bolts on, and also the exhaust ports. Any wetness in the exhaust ports or drop in the level means leaks.
BUT, that burned up HG sure looks like overheat damage to me. As I said you can have this happen and sometimes the HG does not blow right away. Mine lasted nearly 3 seasons! And it ran perfect, normal compression and no water in the oil....but...I did a oil analysis and a slight uptic in sodium showed up...(raw water cooled in salt water) then one day in August of 2016 it did not want to start. Not hydrolocked, just not wanting to start and this engine was always a reliable starter as old as it was. I went through the ignition system, noticed a curious orange cast on the plug center electrode insulators...then just thought...lets check for water in the cyls...then a bit of water shot out of #2 and just a fine mist of water out of #1. Took it apart, block deck and cyls looked fine, heads were cracked from overheat, so did the top end overhaul...