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View Full Version : 86 merc 115 piston or ring failure what caused this



jon
06-18-2003, 08:40 PM
"This engine was rebuild last season do to a bad crank, it rotted out and the splines stripped. A locating pin on piston #4 was found to be missing at that time, bearings were replaced as well. Engine had 125-130 compression after the rebuild.

In spring compression was good low 120's or so on all; ran it approx. 25 hours. Just recently decarbed it. On sunday, it developed a knock and was shut down immeadiatly. Compression on engine was found to be about low 120's on all except bottom, which was 80.

Bearings all seem tight, spin nice, and look good. Upper and lower crank bearings a bit noisy, but I think they just want lube, both spinning perfectly. Nothing strange I can find with any other part, just this piston, missing a big chunk and pieces of it everywhere.. cylinder even looks good - just needs cleaning, no scoring found yet.

Bad Cylinder:

"

jon
06-18-2003, 08:43 PM
"Lets try that again:

"

jon
06-18-2003, 08:50 PM
" The bottom was the only one that I find anything wrong with too, this is the top, the others all look exactly like it - even the carbon pattern. Does anyone have any insight on this? Thanks in advance,

Jon

top, and all others look like this:

/image{top piston} "

jon
06-18-2003, 08:50 PM
"The bottom was the only one that I find anything wrong with too, this is the top, the others all look exactly like it - even the carbon pattern. Does anyone have any insight on this? Thanks in advance,

Jon

top, and all others look like this:

"

jon
06-18-2003, 08:52 PM
"Again.. one more try:

"

tonyaus
06-19-2003, 10:14 AM
Jon....this to me looks like a classic case of metal fatigue which has cause your piston to crack. From your pics I cant see any signs of detonation just some marks left from the chunk that broke. It doesn t look scored so I doubt that water is getting in. I think you can safely replace that piston and probably have the bore honed and you motor shoulld be right to go.

jon
06-19-2003, 12:37 PM
" tony,

That's good to hear - the .02 I got from a friend who is a mechanic (not a hack like me) was the same. Clean the chunks up, deglaze and replace... something else he said was that he feels decarbing can cause this. Something about carbon expanding and causing rings to catch, breaking the piston - ever heard anything like that?

Jon "

tonyaus
06-19-2003, 08:09 PM
" Jon...no haven t heard of such a thing. Im not a big fan of decarb solutions so i cant comment. Seeing your pistons evrything looks in order, it just seems like you were unlucky. Put it back together and enjoy. "

jon
06-19-2003, 11:19 PM
" Thanks again tony - and yes, it's well on it's way, the bottom deglazed and all flushed out/cleaned. Gasket material removed and case/cover prepped for install. Found piston to be standard size, having a wiseco overnighted tomorrow AM - hopefully will get the crank/pistons in on monday night & will be fishing by sat ;).

Jon "

sparky
06-19-2003, 11:21 PM
Decarb solutions are on the shelf in stores because people buy the stuff.Save your money and buy the good oil.