CafeRacer - You haven't mentioned cams - most automotive cams won't work in marine - this is because marine uses wet exhaust with water sprayed into the tailpipe of the marine engine to cool it.

Some guys do get away with automotive cams if they use dry stack exhaust of a complete marine system with full length water jacket right out the transom - no internal water spray to silence the exhaust noise.

Marine cams close the exhaust valve early at Top Dead Center TDC of the exhaust stroke. This allows some pressure to remain in the engine cylinder or at least prevents the intake stroke from drawing water back into the cylinder from the exhaust pipe.

Take the Merc 500EFI for example this is a Crane cam with high overlap and durration in the full context of automotive design except for one change - exhaust event closes at TDC.

If you are using an automotive cam you will pull water back into the operating engine. Sometimes it will rust everything, sometimes makes steam in the cylinders which will create more cylinder pressure than burning gasoline. Get enough water and you can hydraulic lock the engine which will bend connecting rods etc.

They will bend just with the starter motor turning against a water filled cylinder.

Sea Horse. The engine was built by Bullet Engines, for marine applications. During the rebuild the cam was nos changed. The engines ran beautifully when they came from Bullet, and im sure they installed a marine cam. Other than that, the cilinders were dry, no water , and the oil is clean.

The machine shop is picking up the engine today to investigate, they are tearing the engine down. I asked them about milling the pistons and said it could be done if need be.

max on 93 oct with steel heads is 9.5 :1 compression.
with aluminum u can get away with 10.5 :1 everything above needs 110+ octane fuel.

timing max on 93 should be not more then 32 degree.

u can get a thicker head gasket but still need to mill the pistons down a bit.
IMO

GOOD LUCK

I never saw an actual cc measurement of the cylinder head, get that volume and piston deck height then calculate the actual compression ratio. I'm suggesting this because once you know where you are exactly, there are options other than changing pistons. Milling the pistons requires a re-balance of the rotating assembly, as does changing the pistons. I would suggest enlarging the combustion chamber of the head. As a guide if you add 10cc of volume to the combustion chamber, you lower the static compression ratio by one point. The combustion chambers should be smoothed out and sharp radius's removed anyway, it is relatively easy to add 5 - 6 cc of volume and you get the benefit of smooth combustion chambers. Post the camshaft specs as requested, I'm sure the engine builder was good but allow us to complete this investigation process. If I were you, I would have a goal of this engine running safely on 93 Octane, then you can add better fuel as a precaution if desired. With iron heads 9.2 -9.3 static compression is acceptable with polished combustion chambers and the correct camshaft. The camshaft is important because it controls the amount of cylinder pressure the combination will see, thus dictating possible detonation issues.

I don't have camshaft specs, can they be meassured once the engine is torn down?

Yes they can be measured with the cam still in the shortblock by degree wheel, or there may be a grind number stamped into the core. If there is a grind number we can probably ID the specs for you. I would highly recommend checking the details of the cam. You've gone this far in your investigation, might as well finish the journey. Good Luck.

If his combo ran well prior to the rebuild for many hours, the issue is most likely something along the line of Jeff31s head gasket thought, other overheating prob or something in the tune related to timing or fuel afr. Most likely unrelated to engine combo unless fuel quality changed.

I agree, but if its a higher compression engine it would run for a while with low octane fuel at mild driving.

I think its a compression related detonation.

I think he said it ran for many hrs on the original build then only breakin till this.

I think these engines could be made to run fine if the owner has access to good pump premium. The compression works out to be about 9.5-9.75. Look at your HP500 pistons. They're small dome. The engine temp problem needs to be fixed first. At most, 140 degree thermostats need to be used. I use 120 degree thermostats from Arizona Speed and Marine for an added margin of safety. -6 size hoses should be added to the intakes to connect the rear water ports to the front water crossover. This helps prevent steam pockets from forming at the back of the cylinder heads and creating hot spots. The carb jetting seems lean. The pistons and sparkplugs look too clean for a marine engine. Go up 2-4 sizes on primary and secondary and see what the plugs look like. Last, set ignition at 33-34 degrees. Good luck.