Mercruiser 470 (170hp) - Piston #3 failure

[blkzoe]

OK folks. Has anyone seen a piston failure quite like this one? The engine ran flawlessly all last summer. Noticed a wisp of coolant vapor in Nov., right before pulling boat. Decided to do a total upper-end gasket replacement including HG as long as I was going to remove the manifolds. Lo and behold, look what I found in piston #3. Not only is a piece missing, there appears to be extraneous matter welded to the head of the piston. See pics. Cylinder bore is fine (yes, I lucked-out). Any ideas on this mechanism of failure?

 

[Bondo]

Re: Mercruiser 470 (170hp) - Piston #3 failure

Ayuh,.... Looks like Detonation damage to me,...

Probably from a Lean condition, or bad timin',...

 

[JustJason]

Re: Mercruiser 470 (170hp) - Piston #3 failure

Did you clean the top of that piston, or was that the way it came out? How many hours on the engine? What do the other 3 look like?

 

[blkzoe]

Re: Mercruiser 470 (170hp) - Piston #3 failure

Did you clean the top of that piston, or was that the way it came out? How many hours on the engine? What do the other 3 look like?

Odd you should ask:

1. I cleaned the head of piston #3 w/ a wire brush. It did have a carbon accumulation on it prior to cleaning.

2. According to hour meter 700 hrs.

3. Piston #4 was pitted on top. See pic. Pistons #1 and #2 look normal. All rings were free and moving.

4. IMPORTANT additional info. I don't believe piston #3 is original. Compare the pics and look at the "STD" marking. Pistons #1, #2, and #4 match. #3 is the oddball. Previous failure?

5. ALSO, had the presence of lime green buggers (wax like substance) in the exhaust manifold for pistons #3 and #4. Anti-freeze intrusion during combustion?

Piston #4 pic:

 

[JustJason]

Re: Mercruiser 470 (170hp) - Piston #3 failure

Were all 4 equally clean or dirty when you took the head off, or was 3 and 4 somewhat cleaner/noticeably different than the others?

Did you clean #4 in that new picture, or did it come out that way?

The blown crown on #3, and the pitting on #4 is definitely detonation. It's now just a question as to why.

 

[stonyloam]

Re: Mercruiser 470 (170hp) - Piston #3 failure

Looks like #4 was getting some coolant leakage into it. Was the combustion chamber pitted too?

 

[blkzoe]

Re: Mercruiser 470 (170hp) - Piston #3 failure

Were all 4 equally clean or dirty when you took the head off, or was 3 and 4 somewhat cleaner/noticeably different than the others?

Did you clean #4 in that new picture, or did it come out that way?

The blown crown on #3, and the pitting on #4 is definitely detonation. It's now just a question as to why.

#1 and #2 had less carbon deposits.

Yes, I cleaned #4 for the pic to see the extent of pitting.

Note that timing was per specs. I installed a Flame Thrower II ignition system this past summer and repeatedly checked the timing. Spot on the revised spec (4? BTDC). No det noises. Ran like a top. Old issues from previous owner who may have run it at the old spec of 8? BTDC?

 

[blkzoe]

Re: Mercruiser 470 (170hp) - Piston #3 failure

Looks like #4 was getting some coolant leakage into it. Was the combustion chamber pitted too?

combustion chamber #4 clear and clean; no damage.

 

[blkzoe]

Re: Mercruiser 470 (170hp) - Piston #3 failure

Interesting note from Jasper engines concerning timing in the 470's. Looks like it was dated Jan '06.

http://www.jasperengines.com/pdf/37LMerCruiserIgnitionTB.pdf

 

[Scott Danforth]

Re: Mercruiser 470 (170hp) - Piston #3 failure

detonation damage. the "extraneous matter welded to the head of the piston" is the broken part of the piston that then melted and was re-deposited back onto the piston

 

[Eric 3.7LX Alpha 1]

Re: Mercruiser 470 (170hp) - Piston #3 failure

Here's the actual Mercruiser TSB on it:

 

[JustJason]

Re: Mercruiser 470 (170hp) - Piston #3 failure

If the pistons were cleaner then it was detonation due to water intrusion. What more than likely happened is the head gasket blew between 3 and 4, and the engine was sucking in some coolant, giving you the detonation.
The pistons being clean is the key. When an engine ingests water or coolant like that what happens is it flashes to steam, steam cleaning the pistons of all of their carbon. If you flip the head, those 2 combustion chambers/valves should look a bit cleaner as well.

 

[blkzoe]

Re: Mercruiser 470 (170hp) - Piston #3 failure

All:

Thanks for the input. As the head and cylinder bore(s) survived; here's my strategy. Pistons #3 & #4 trashed. 4 new pistons and rings to be ordered. Pistons #1 and #2 to be held as spares. Block & head & crank at the shop as of this post. Gotta' check for exhaust manifold issues. AS LONG AS I'VE GOT IT TOTALLY TORN DOWN; all bearings, seals, gaskets to be replaced. Oil pump was good, but will check clearances anyway. Hey, its winter and there's still ice on the river................

 

[stonyloam]

Re: Mercruiser 470 (170hp) - Piston #3 failure

You might consider hardened exhaust valve seats while you are at it. Use a OEM Mercruiser head gasket when you put it back together, they are designed especially for the 470.

 

[blkzoe]

Re: Mercruiser 470 (170hp) - Piston #3 failure

You might consider hardened exhaust valve seats while you are at it. Use a OEM Mercruiser head gasket when you put it back together, they are designed especially for the 470.

Agree on the OEM gasket kit; its sitting right next to me. What's the advantage of the hardened exhaust valve seats? Extended wear capabilities?

 

[Fastatv]

Re: Mercruiser 470 (170hp) - Piston #3 failure

Looks like the top ring lan ( top compression ring groove ) was broken off and due to heat....a lean condition and this was possibly caused by a coolant....or rather water leak from the head gasket. I would recommend that when you order your new pistons, spend a few extra dollars and purchased "Anodized" pistons. The anodizing process further hardens the top lan, and is less likely to break due to heat.

 

[stonyloam]

Re: Mercruiser 470 (170hp) - Piston #3 failure

Agree on the OEM gasket kit; its sitting right next to me. What's the advantage of the hardened exhaust valve seats? Extended wear capabilities?

My machinist suggested it. Should hold up better with modern lead free gas. Was not too expensive since we were doing a valve job anyway. You might see what your machinist thinks.

 

[blkzoe]

Re: Mercruiser 470 (170hp) - Piston #3 failure

My machinist suggested it. Should hold up better with modern lead free gas. Was not too expensive since we were doing a valve job anyway. You might see what your machinist thinks.

Interesting note on Ford 460 heads (these are the heads on the Mercruiser 470). After 1983, they came with hardened seats from the factory due to the change to unleaded gas. Who would'a thunk that?