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GM Marine Power 5.7 liter engine pistion hole at edge

cahhtravis4

New member
I have a 2005 Tige boat with a Marine Power GM 5.7 liter 340hp multi-port fuel injection engine with low hours (300-400 hours). I was using it for wakeboarding/skiing last weekend and started noticing that I had some blue smoke out of the exhaust, then later I noticed that oil pressure was very low at idle but ok at RPMs above that, so I shut it down immediately and checked the oil level and found that it was 3 quarts low. After pulling the boat, I checked the (warm engine) compression on all the cylinders and noticed that cylinder 5 had no compression at all and cylinder 3 had only 70 psi (all the other cylinders had around 160-170 psi). I pulled the head and noticed that the head gasket between 3 and 5 was burnt through, cylinder scoring on both cylinder walls where 3&5 come together and cylinder 5 had a chunk out of the inner piston showing the ring underneath, see attached pictures.

My plan is to replace the engine with a new 5.7 long block, $2750. However, I would like to know the root cause of my failure, so that I don't have this same issue with the new engine in a couple months.

Thanks ahead of time for any help.
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I would have the injectors flowed and cleaned. I would also make sure the timing is correct. When the new engine is installed you want to make sure you are running in the recommended rpm range at wide open throttle.
 
Ayuh,.... Looks like Detonation from a Lean condition,....

Have ya had a look at the contents of the fuel filter,..??
Anything but gasoline is Bad,...
 
That does look like detonation damage. This style piston is more prone to Detonation because it cannot possibly offer a "Quench" area or "squish zone".


Yes..... you must be able to achieve the OEM WOT RPM in order to stay within a safe load range for your engnine.
Test/check this as you would normally load and use the boat.


If you typically load this engine to the point of bording on Detonation, consider a different style build.
IOW... DO NOT go back together with the full dished piston style.

If up for some reading.... start at post #7 here.


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1...... My plan is to replace the engine with a new 5.7 long block, $2750.

2..... However, I would like to know the root cause of my failure,
so that I don't have this same issue with the new engine in a couple months.

1.... Most any GM SBC being sold as a Marine engine, and many to most re-builders, will be using the GM style Full Dished piston. A piston that has no business being used in a SBC that is destine for a Marine application. :mad:
IMO, you'll do much better if you have an engine custom built to your specific needs and with a good Quench Effect built into it.
Any good automotive machine shop can provide this service for you.
The Q/E is nothing new! I've shown some good examples in this thread.... start at post #7 here.


2.... The root cause for detonation damage can be reduced to several areas:
(not necessarily in order of importance)

  1. Poor combustion chamber design (no quench area)
  2. Excessively high cylinder temperatures
  3. Excessively lean F/A ratio for the load
  4. Over-loading for the RPM
  5. Excessive ignition spark lead


(1 above) With the SBC cylinder head, there is a quench surface above the wedge chamber. When the correct pistons are used, this quench surface forces the gasses out into the main flame front.... giving them no place to hide.
However, the GM style full dished piston does not mirror this surface in order to create a quench or squish area, and cannot possibly perform this task.




BTW.... Pre-ignition and Detonation are two entirely different phenomena. Either may be very destructive.

If you do go with a GM style build, make dang sure that you avoid numbers 2 through 5 above.



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Seeing that this piston failure was from a well used engine manufacturer, would all the pistons in this type of engine be the deep dish type and not have a proper quench area.
I assume that all the marine engine suppliers eg. PCM Indmar, Mercruiser use this engine from GM and all would have the same type of piston.
 
1..... Seeing that this piston failure was from a well used engine manufacturer, would all the pistons in this type of engine be the deep dish type and not have a proper quench area.

2..... I assume that all the marine engine suppliers eg. PCM Indmar, Mercruiser use this engine from GM and all would have the same type of piston.
Fair questions!

1..... These are not necessarily "deep dished" pistons. The dish volume does nothing more than to control the static compression ratio.
The problem is not the "dish".
The problem is that this piston profile incorporates a complete dish with NO Quench surface!

2..... Unfortunately yes!
It's less expensive to produce..... one piston p/n fits all 8 bores.... and the guy sitting there operating the machinery to install these onto the connecting rods, can be having a Cocktail. Basically... it saves GM money.

But.... when you get into GM's high performance SBC's, you won't see the full dished pistons being used.
Ask yourself why?


BTW.... doing a Quench build for a SBC Marine engine is not necessarily a high performance thing.... it's more so intended to reduce Marine load detonation potential.
As a result..... spark lead can be increased some........ static C/R can also be increased some....... and the LPCP will end up where it needs to be.... and will offer more torque.

And don't kid yourself........ the GM engineers know this, but the GM bean counters get the final say! :mad:


You'll find some great articles written by people like Dennis Moore, Larry Carley, Jeff Smith, Keith Black, etc. on this topic.
The Chrysler Wedge Head boys are no strangers to this either.



Just as with drugs... say NO to these!

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