Piston anomoly

[cowlum]

Hi

The attached jpg is a periscope photo taken of my number 4 piston after the engine has been sitting for a day.

Questions:
1: What is the screw head like thing in the center?
2: Do the stamped markings mean anything to anyone?
3: Where does the fluid come from? Its not present in the other cylinders.

Regarding question 3. With the flathead design oil wont seep into the cylinders through gravity while at rest, so my best guess is its unburnt additives I've added to my fuel. Seems a lot though and the engine runs great. Number 4 has the lowest compression of all 4 but only by about 5%.

Thanks

[Dave Neptune]

#1 That is the "center hole" for machining and grinding the piston, IE just a dimple.

#2 The stamped numbers are part numbers.

#3 The fluid is probably water. Water will break up the carbon deposits during combustion.

Do you shut off the water valve? Could be back-flow from the exhaust or a minor leak in the head gasket which may be the compression loss. Water can also seep in through a crack, has the engine overheated recently?

Dave Neptune

[joe_db]

Most A4s slope aft and if any water back up the exhaust, it ends up in #4 first.

[Mo]

The engine that I removed from the boat 3 weeks ago had one piston with that mark. It was number 1 cylinder, the high end of the engine on my boat. The other 3 cylinders didn't have anything like that. It did have minor pitting on the other 3 cylinders that I think happened the year prior to me buying the boat. I bought it in 2007 and all the valves were seized, the manifold was broke open from frost etc etc....I looked at that hole with a light and it was almost like it was a machining set.

[cowlum]

#1 That is the "center hole" for machining and grinding the piston, IE just a dimple.

#2 The stamped numbers are part numbers.

#3 The fluid is probably water. Water will break up the carbon deposits during combustion.

Do you shut off the water valve? Could be back-flow from the exhaust or a minor leak in the head gasket which may be the compression loss. Water can also seep in through a crack, has the engine overheated recently?

Dave Neptune

I have a raw water strainer that I unscrew breaking suction to the pump, I then wait for the last big gush of water out the exhaust before shutting the engine down. Usually takes about 30 seconds. Then I shut the water valve.

The valve seat area is always dry as a bone.

Thinking it may be a head gasket leak now.

I should take more pictures overtime and see how it goes.

[Dave Neptune]

It most likely is the head-gasket and perhaps another compression check will expose it.

Another possibility for water incursion and the engine still running could be the intake manifold is rusting through which is an easy check. Both are not uncommon.

Dave Neptune

[cowlum]

I ran the engine for 15 minutes, in gear attached to the doc at 1400 rpm.
The fluid remained in the cylinder. I checked the other cylinders and they too had liquid, though less of it.
Is it possible its marvel oil from the gasoline that isn't being burnt off? Or oil from my compression test a month ago, which would be about 2 hours runtime ago? My engine runs quite cool.
She starts easily and runs perfectly. The spark plugs are a little sooty but clean on the electrode tips.

On another note: I dont suppose theres enough numbers showing on the piston to suggest any history for the engine? Factory originals? rebuild etc?

[ndutton]

Cooling system pressure test:

https://www.moyermarineforum.com/forums/showthread.php?t=11111

[jcwright]

Cowlum,

The markings that are visible on your piston match those on mine, which are original parts. Here is a link to a photo:

https://drive.google.com/file/d/1zTAJo_i3ySQMQVYmOd4uZW4Su-z17a2K/view?usp=drivesdk

This does not rule out a previous overhaul of your engine. For example, the original pistons might have been OK, and only rings replaced.

Jack.

[TimBSmith]

JC what do the registration markings on your pistons reference?

Do you have a larger album from an overhaul or rebuild would love to see. Thank you. Stay well. Tim

[Surcouf]

mine

https://www.moyermarineforum.com/forums/showpost.php?p=119149&postcount=46

[jcwright]

Tim,

Here are my notes on my overhaul:

https://drive.google.com/file/d/0B869NyQaShwZSFFvRGtUQ0JUWTQ/view?usp=sharing

My notes merely describe what I did, not how it should be done. Some people on another forum were (oddly) upset that I did more work and spent more money than I should have. Somewhere near the end I comment on things I would do differently 'next time'.

Hope this helps some. Good luck with your A4.

Jack.

[jcwright]

Tim,

I forgot to answer your question about the numbers stamped on the piston:

One of the numbers is "2.56055". My average cylinder bore measurement was 2.563. The standard cylinder bore in the MMI manual is 2.562. So, the stamp on the piston appears to be its diameter.

The large "2" in my photo is the piston number/position, and the "Front" arrow indicates direction of installation (though "Front" is not what I first thought).

I can't explain the remaining number on the piston ("2.60999"). Perhaps others know.

Hope this helps.

Jack.

[TimBSmith]

Jack, thumbing through notes. Awesome documentation.

Registration notes on markup on piston helpful. Thank you for follow-up.

Favorite note so far in your discussion of rebuild or buy economics..

"An unexpected benefit was appreciating all the tools
I’d inherited from my father, each one of which—right down to the smallest wrench and socket—he’d
engraved with his initials decades ago."

Priceless.

Stay well, Tim

[jcwright]

Which reminds me--my dad engraved all of his tools with an engraving tool that I gave him for Christmas, I would guess when I was about 10 or so.