Detonation melted my piston..... but what caused the detonation?
05-21-2015, 11:21 AM
#11
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Wrong piston for the application too. A hypereutectic piston has no business being in a performance marine engine. Count yourself lucky that the piston stayed together.
Some bad news.... you have a .060 over 454 (468 cid) not a 496. The part number (H661CP) indicates a low compression standard stroke replacement piston. With an open chamber head you're probably at 8:1 or less compression-wise. I hope it wasn't sold to you as being a 496 because if it was, then you were lied to. I wouldn't be surprised if it had a cast crank and 2-bolt block at this point . Since you're looking at a rebuild anyway, I'd look at some offerings in the Swap Shop. There a few engines for sale there that could get you back on the water.
I would say that the engine was just plain built incorrectly for the intended usage. Hard to say without knowing the cam's specs, etc, but this looks like it wasn't built for the marine environment.
Some bad news.... you have a .060 over 454 (468 cid) not a 496. The part number (H661CP) indicates a low compression standard stroke replacement piston. With an open chamber head you're probably at 8:1 or less compression-wise. I hope it wasn't sold to you as being a 496 because if it was, then you were lied to. I wouldn't be surprised if it had a cast crank and 2-bolt block at this point . Since you're looking at a rebuild anyway, I'd look at some offerings in the Swap Shop. There a few engines for sale there that could get you back on the water.
I would say that the engine was just plain built incorrectly for the intended usage. Hard to say without knowing the cam's specs, etc, but this looks like it wasn't built for the marine environment.
I was sold the boat with it being advertised as a 575hp 496. I don't know for sure but I believe it has an aftermarket crank, stroker crank or not I'm not sure. I'll find out for sure Saturday when I get the block pulled out and pull the other head. I was told it was a 9.5:1 CR.
Thanks for all the input. Going to evaluate everything once the engine is out. The engine was supposedly "built" for the boat. Who knows. I'll give an update this weekend on the findings of the parts in the bottom end.
Last edited by BajaKevin; 05-21-2015 at 11:23 AM. Reason: Spelling error
05-21-2015, 11:22 AM
#12
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05-21-2015, 11:42 AM
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Simple to check stroke. Measure piston distance from top of piston to block deck with piston all the way down at bdc. If it is 4 inches it is 468. If 4-1/4 it is 496. It would take a strange short rod to work with 4 1/4 crank with those pistons. So i believe you were...let's say misinformed. If block is toast i have a mk4 std bore needs a 60 cut for free. An hour north in s. Jersey. If block is ok, get a piston, hone it up and go boating.
05-21-2015, 11:57 AM
#15
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Simple to check stroke. Measure piston distance from top of piston to block deck with piston all the way down at bdc. If it is 4 inches it is 468. If 4-1/4 it is 496. It would take a strange short rod to work with 4 1/4 crank with those pistons. So i believe you were...let's say misinformed. If block is toast i have a mk4 std bore needs a 60 cut for free. An hour north in s. Jersey. If block is ok, get a piston, hone it up and go boating.
05-21-2015, 12:20 PM
#16
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I saw your post here when you bought the boat. It is a cool looking old 28. With that being said, I hate to be the bearer of bad news, but after looking at your pics, I'm not at all surprised you are in the situation you are in now.
The general rigging of the engine looks horrible. First, what is up with the transom where the tailpipes go thru?? Second, nice throttle cable adapter modified by drilling a hole in the plastic throttle end. Cables and wires hanging on the exhaust pipes, screw holes all over the inner transom, Trailer too small for the boat, etc
Now, the engine is broken after a couple boat rides, had cheap hypercrackit pistons, is not a 496ci like you were told, car fuel filters, cheapy fuel pump, and who knows what kind of shape the other mechanicals of the boat are in.. my guess is poor.
Some friendly advice here. STOP and really think about what you wanna do at this point. Before sinking a bunch of money into the engine, I would thoroughly check the integrity of the transom, and stringers. The old scarabs were known to rot when treated like cream puff garage queens, and this one looks like it may have been violated a bit over the years.
If you really like the boat, and wanna pour money into it, like transom replacement, stringers, adding external steering, rebuilding a nice engine for it, possibly better trailer or modifying yours, etc, that is cool. But, be prepared to have 3 times as much into the boat, as its worth.
If the hull is rot free, a bilge makeover will go a long way. Filling in old holes, fresh repaint, redo all the wiring, cables, etc, and install a fresh engine in there. I was in your exact position at the age of 21 with my first boat. I bought a boat from some redneck who thought he was a real awesome engine builder/rigger. What did I know. Looking back, this guy had know business installing a license plate on a car, let alone building an engine and rigging it. After a complete summer of sweating my azz off in the bilge, blowing a bunch of money, working late nights on the boat, covered head to toe in mosquito bites, finally got the engine rebuilt and running good. Then, second time out with engine running good, the drive let go, and destroyed the upper housing. In the end, I was out over 10,000 dollars, and boatless. I got 4 boat rides out of it. That was alot of money for me at 21 years old. If I had only had someone to teach me, or a site like offshoreonly, I may have been saved a major headache and wallet draining. Good luck with the project.
The general rigging of the engine looks horrible. First, what is up with the transom where the tailpipes go thru?? Second, nice throttle cable adapter modified by drilling a hole in the plastic throttle end. Cables and wires hanging on the exhaust pipes, screw holes all over the inner transom, Trailer too small for the boat, etc
Now, the engine is broken after a couple boat rides, had cheap hypercrackit pistons, is not a 496ci like you were told, car fuel filters, cheapy fuel pump, and who knows what kind of shape the other mechanicals of the boat are in.. my guess is poor.
Some friendly advice here. STOP and really think about what you wanna do at this point. Before sinking a bunch of money into the engine, I would thoroughly check the integrity of the transom, and stringers. The old scarabs were known to rot when treated like cream puff garage queens, and this one looks like it may have been violated a bit over the years.
If you really like the boat, and wanna pour money into it, like transom replacement, stringers, adding external steering, rebuilding a nice engine for it, possibly better trailer or modifying yours, etc, that is cool. But, be prepared to have 3 times as much into the boat, as its worth.
If the hull is rot free, a bilge makeover will go a long way. Filling in old holes, fresh repaint, redo all the wiring, cables, etc, and install a fresh engine in there. I was in your exact position at the age of 21 with my first boat. I bought a boat from some redneck who thought he was a real awesome engine builder/rigger. What did I know. Looking back, this guy had know business installing a license plate on a car, let alone building an engine and rigging it. After a complete summer of sweating my azz off in the bilge, blowing a bunch of money, working late nights on the boat, covered head to toe in mosquito bites, finally got the engine rebuilt and running good. Then, second time out with engine running good, the drive let go, and destroyed the upper housing. In the end, I was out over 10,000 dollars, and boatless. I got 4 boat rides out of it. That was alot of money for me at 21 years old. If I had only had someone to teach me, or a site like offshoreonly, I may have been saved a major headache and wallet draining. Good luck with the project.
05-21-2015, 12:21 PM
#17
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One more thing. If you have a standard firing order cam and # 5&7 plug wires are crossed this could cause the melt down 2. I don't mean wrong wire to wrong cylinder I mean wire laying over top of one another...
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05-21-2015, 01:33 PM
#20
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