Marks on intake side of piston

[Sparkroost]

Can anyone tell me wtf these 5 marks are on these pistons? There is excessive carbon on the intake side and a buildup on the outside of these marks. It looks like something was hitting the pistons, but it is not the case. Also, on the bottom side of the piston, in the dome area, it's black from heat. Is this normal?

[Winger Ed.]

I won't pretend to be a engine guru, but I always did pretty well in physical science class.

Bro,

Those pistons don't look like they've been hit, they look burned,, or more nearly- melted.

See the damage at the ring gap?

It looks like (too) hot cumbustion gases tried to escape through the ring gap. And they did,
acting like a cutting torch.

Figuring the bore/piston/ring clearance-gap is correct and within tollerance;

I'd make a wild guess that your engine suffered from high RPM
(which leads to high heat in the combustion chamber) and insufficient cooling.

If it was just too lean, you'd probably have a hole in the piston -- or one growing under the
point where the plug flashes........but you don't.


The next place I'd to look is a old/worn water pickup pump impeller, or clogged up water inlet,

then to figure out if enough water was available to the pick ups.

Was the engine running high & fast, and didn't have a low water pick-up?

The culprit you are looking for is heat.

And why it didn't dissapate fast enough to prevent this damage.
//////////


I ain't a big time guru, but I'd be flattered if I could help you get to thinking and figure the basic properties of why this happened to ya.

In the meantime,,,, I need to get back to backyard bar-b-que and rejoin the neighbors & our game.

*chuckle*

Winger Ed.

[Sparkroost]

Here is #2 piston, cleaned.

[Sparkroost]

Here is the bottom of it

[Sparkroost]

funny burn pattern

[Sparkroost]

#1 piston, looks like 4 point scoring, but only scored in 2 places.

[Winger Ed.]

The pictures are good.

But like putting together a puzzle without 90% of the pieces, its a little
difficult to pinpoint exactly what happened with only that schetchy little bit of info.

About all that is obvious is that the motor burned up.

If it was me, I'd have the block checked real well, and re-build it.

Being as careful as possible about things like timing, cooling, oil injection if applicable as it went back together .

Then, with a tach, water pressure, and temp. guages, you'll have a better chance at keeping it from happening again.

[Hooty]

I don't know the whys but they line right up with the ports.

http://fishingworks.com/Users/UserFolders/hooty/PhotoImages/Evinrude%20Transfer%20Port%20Pix%202A.jpg

c/6

Hooty

[Sparkroost]

I have temp and water psi as well as working thermal sensors.

I did overheat it once at the ramp trying to check timing.

2 psi @idle, 5-7 off idle, 15-17@2k, 22@6100. Heads warm to the touch.

Nothing wrong with the ring-end gaps. Nothing wrong with piston #2 and #4.

Skirt scuffing on #1 and #3 caused the rings to blow by. I was thinking on the other end of what you had said. Maybe running too cold with too much fuel. Carbon build-up.. scuffed piston. Or.. piston was not warmed up well and had not taken shape before engine was ran up in rpm's.

I believe I have a powerpack failure and that is where those marks are coming from on the piston.

I CMM'd the cyls. at work and came up with non-existant cone(.0041mm cone max #1cyl, and min cone/angle of .0023mm. All bores are perfect cylinders(.0470mm cylindricity) with a size of 89.45mm.

The piston tops are that color from the factory.(cept for the underneath, lol)

[Sparkroost]

Hooty, this motor has 8 transfer ports pr. hole. there are 5 of these indentations of carbon on the intake side.

[Hooty]

Sounds like ya got it all worked out.

c/6

Hooty

[84exciter]

pattern from the intakes.
as for the underside i'll bet it's carbon,if you take the one that is scuffed and put it in the oven.
i bet it melts and runs like oil

[Sparkroost]

Does anyone have a flow diagram for a crossflow?

I was thinking that carbon buildup should not be on the intake side of the piston. I was thinking that the flow of the charge would end up on the exhaust side and leave deposits there. Judging from the lack of responses, these marks are not normal.

I had one reply from a fella that said it looked like the intake charge was igniting before the intake port was closed.

It's really hard to guesstimate what went on due to the fact that I had to drive it home 2 miles running on 2 cyls with "fireflys" coming out the exhaust on the way home for 2 miles. Apparently there was raw fuel being dumped out the exhaust and catching on fire in the water or just before it hit the water.

Should be together next weekend. I will go from there.

[84exciter]

1

[racer]

Carbon on the bottom side of the piston is normal on a cross flow. The scuffs on the intake side of your one bank are due to a heat/or lack of and or proper piston to wall clearence. On a 3.5 bore omc with wiesco pistons your bore should be 3.502 and the engine should run at minimum temp of 120 to a max of 145. With any rebuild I always degree the flywheel and check timing on all cylinders. The engine was not lean by the loof of you pistons.

[Sparkroost]

Thanks for the input guys.

Racer, .002 clearance, got it.

In measuring one of the old pistons that came from #2

3.4950 top of piston
3.4965 between top ring and bottom ring
3.4995 just below bottom ring groove
3.5055 just above wrist pin
3.5090 bottom of skirt

Is a new piston a perfect cylinder? This one is too small, right?

[racer]

Piston should be measured 1/2 below wrist pin and it should be round at that point, you need .007 clearance . the 3.502 I mentioned was not for .002 but to show an additional .002 over an OEM piston.

[Sparkroost]

heh, .0145 on these. If I read you correctly on piston measurement(Piston should be measured 1/2 below wrist pin), you refer to 1/2 the distance from the bottom of the piston to the center point of the wrist pin?

[racer]

Basically that is correct.

[Sparkroost]

What do you recommend the ring end gap be set to for 3.530.

This is not a light boat. 4k to stay on plane. What alternatives do I have to get a good break-in.

Just ordered parts from Rickracers!! THANKS RICK

Very friendly people!

Still looking for a set of semi-high comp heads, or 1 73 starboard high comp. head. I know some have you have taken the Dark Side and have some left over. PM me, I won't tell.