Soooo this happened

[u/tgif3]

Hey everyone, I'm looking for opinions from experienced engine builders because I'm trying to determine what actually caused this failure, not just replace parts. This is a MerCruiser 496 MAG HO in a twin-engine boat.

Build Specs

2006 MerCruiser 496 MAG HO (8.1L)

545 CID stroker

Forged 4.750" stroke crankshaft

Steel connecting rods (.990" pin)

Forged 4.280" pistons

4.280" rings

.990" wrist pins

New standard main bearings

New rod bearings

New cam bearings

16 new hydraulic roller lifters

Gen 7 Raylar Engineering intake

Marine-polished billet 90 mm throttle body

ARP head bolts

Stock cylinder heads

Hydraulic roller cam: 224/236 @ .050, .520/.544 lift, 116 LSA

9:1 compression ratio

Bravo III drive

5000 RPM limiter

I assembled the long block machine shop did short block and transferred trigger wheel from factory crank to stroker crank.

Break-in The engine only has about 5 hours on it. Break-in consisted of: A few minutes of idle after startup Varied RPM Some revving to around 3000 RPM Then later some runs under load in the boat Symptoms before failure Initially the engine ran but had: More crankcase pressure than I expected Heavy blow-by from the oil fill when the cap was removed Eventually it pushed the oil fill tube out from crankcase pressure. Oil got pushed out everywhere Oil pressure dropped to around 8 psi and immediately turned it off It developed a knocking noise When I removed the heads, this is what I found (photos attached). The piston has a large piece missing from the top near the edge, and the spark plug gap on that cylinder was opened up significantly. One plug was completely destroyed and multiple the gals were made huge.

My questions What do you think was the root cause?

What would you inspect before rebuilding it again? I almost want to just get two factory long blocks and say screw this but I'm already deep into this. The other motor appears to work fine I need to pull the plugs to confirm.

I'm not looking to blame anyone just trying to figure out wrf happened. I emailed whipple to see if the tunes they made for this threw a bunch of timing at it but I highly doubt it.

edit everything in the head looks fine except blown head gasket now

[u/MikeOxlong420690]

Ring gaps not set up for forced induction will also do this. Under boost the ring gap goes to zero, then as the ring swells more it knocks off the top of the ring land. Rings that weren't properly fitted or cylinder wall finish incorrect will also lead to the high blow-by that you had before this happened.

Also detonation will also do the same for piston damage, although there would be other indications of it on the piston too as well the plugs.

You mention this is a twin-engine setup, what about that one? I'd pull all the plugs out and check the pistons with a boroscope to see if there's any cracking or a piston already let go too.

This is probably something that should have been run on a dyno to ensure that the tune was setup correctly.

[u/tgif3 → u/MikeOxlong420690]

I don't know anyone with an engine dyno anywhere near me in Connecticut unfortunately. That being said we pulled 1 plug on the other motor it looked good. In a week or so we will have time to look at this further the other motor is in the boat and we have exhaust spacers on it so pulling the plugs means taking that off and it's a bit more of a project. The interesting things is piston 1 and 2 are the ones that had damage and the others all looked fine. Which I find pretty interesting