Cracked piston

[metriccrescentwrench]

OK, after my last outing I'd noticed that the engine (1990 Johnson 200) was losing power at the top end. (I'd been fighting an idle and off-idle problem since I got this engine and had just finally gotten a handle on that - it had normally run pretty well at the top end). Not too long ago it would turn about 6100 rpms at WOT at around 63 mph. A few outings later it was down to 5800 rpm, then last time it struggled to get to 5500 rpm. Everything checked out ok, spark, timing, compression (somewhat low ranging from 72 to 81 but basically unchanged) so I pulled the heads to take a look. I saw some scuffing in a few cylinders that wasn't there when I started this misadventure so I'm tearing it down.

Here's the piston from #5 hole (the one with the worst scuffing):

And here's a closer view:

Any thoughts on what could do this to a piston? I got the engine from the same guy that Sparke Boat had the problems with so nothing's going to surprise me.

 

[HighTrim]

Re: Cracked piston

I would guess something is off balance? Just a guess.

Havent seen a crack like that myself before. Any irregularities with the rod from that cylinder?

 

[boobie]

Re: Cracked piston

What's the top of the piston look like ??

 

[metriccrescentwrench]

Re: Cracked piston

Any irregularities with the rod from that cylinder?

Not that I can tell. There's a very slight amount of play when you twist the rod, but it's about the same as 3 or 4 of the others.

What's the top of the piston look like ??

It's a little strange how that one side is "clean". At first I thought maybe a slight amount of water was getting past the head gasket, but the gasket and head both looked perfect.