piston scrubbed cylinder wall.

Hi folks,
I had gotten some good replies on a ring end gap question so here goes another one. I decided to rebuild my motor myself . for fun I guess. My background is electro mechanical tech. and years as a machinist so I bought the manuals and decided to give it a go.
This motor had just been rebuilt by a marina and with less than a few hours of running time it blew too. Didn't even make it through break in. Any way when tearing down the motor I found that pistons 3 and 5 had scrubbed the cylinder walls but had yet to seize up the motor. All tolerences are within print on the cylinder walls and are not out of rounds or taper. Crankshaft tolerences are good also as well as the bearings and rods. Lots of carbon buildup on the crown of the pistons. I do know that too much oil was added to the gas when the marina test ran the boat when they rebuilt it. They accidently put two gallons of oil into a twenty gallon fuel tank and ran the motor along with the VRO oil. The power pack was going bad and not all cylinders were firing. I am changing out the bad pistons and really checking the other ones as well as installing a new powerpack. I am just trying to figure out the reason for why the pistons scrubbed ? And yes I am honing, cleaning, and rechecking the specs on all cylinders. Please help with the scrubbing reasons if possible, Thanks.

Re: piston scrubbed cylinder wall.

maybe the carbs arnt clean but i wouldnt imagine since the vro is puting oil and u premix y they would scrub like that? carbon build up is only from burning gas so wouldnt it just be oily and dirty if they werent firing my quess? what motor year etc give us info too

Re: piston scrubbed cylinder wall.

Your right I should have given more info. The motor is a 175 faststrike , I was told the powerhead that was installed when I had it rebuilt was a 2004. I dont know that I trust that though. The carbs had been cleaned previously and the pistons and cylinder walls were also oily. The top of those two pistons had what I would call scale on them. This carbon I guess could be almost flaked off. I had theorized that maybe the rings stuck in the grooves and caused the pistons to scrub. I am just learning and I am hoping to find the cause so that I wont duplicate my problem. The mechanic who rebuilt it in the spring put the extra oil in the gas for the break in only. Way too much. I was told by someone that the piston actually should be touching the wall here and there but not to the point of damage to the piston or wall. Did it get too hot ? Was something out of alignment maybe ?

Re: piston scrubbed cylinder wall.

Didn't they guarentee the rebuild?
At least through break in.!

No spark or ignition failure shouldn't cause scrubbing because the engine still gets fuel and lube, but clogged carbs will. I would inspect/rebuild all the carbs but look for the problem in the 2 suspected carbs. Sometimes the sparkplugs can give indication too.

"carbs had been cleaned" , thats not exactly rebuilt though.

What about the whole fuel system itself, its cheap insurance to start from new with fresh fuel lines , clean taNK , filter , seperator etc.

Re: piston scrubbed cylinder wall.

The expensive part of a rebuild is the cost of the piston. If you're going to change the piston anyway you'll be ahead of the game to bore that cylinder. The cost difference between boring and only honing is minor.

Sounds like the original "rebuilder" never figured out why the motor had issues to start with. Make sure you find the cause now - and correct it - or you'll be doing it again. It happens way more than you'd think. A damaged piston is a result, not a cause.

Re: piston scrubbed cylinder wall.

I would check the roller bearing. you can find a piton kit on eBay for less than the market value. if the walls are scored just honing will do this walls are very strong and takes lots off abuse. and last clean the carb like coolguy147said good luck finding the cause

Re: piston scrubbed cylinder wall.

i ya are going to change the pistons or more than half i would buy a piston kit
bore all four and check end gap on assembly ,watch what pistons ya buy,
i buy from the guy that does my machine work ,i can find out what he uses i dont remember ,some of the pistons require correct tolerances .for there brand ,check all your bearings good ,i just did one and rebuilt carbs
got some trash in jet and scored one on fresh motor ,leave no stone unturned and you will come out well,good luck ,got a guy on ebay that does have some good repacement parts cheap .believe its paw paws parts ,if ya
need to get a full set its worth it.

Re: piston scrubbed cylinder wall.

DH is right. Fail to find the cause, and you'll be doing another rebuild.

Re: piston scrubbed cylinder wall.

I?m not so experienced in outboardengines, but ? do some two stroke racing on bikes. Heavy oil load in the gas is there a no no. It can easily make a engine to over heat because the over oiled gas has harder to get thru the jets and makes the fuel/air ratio to lean. Over heating will first make your piston scratch on the cylinder walls, then it will melt. Too much oil in the gas could be the cause.

Re: piston scrubbed cylinder wall.

kandil said:

I would check the roller bearing. you can find a piton kit on eBay for less than the market value. if the walls are scored just honing will do this walls are very strong and takes lots off abuse. and last clean the carb like coolguy147said good luck finding the cause

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hey someone mentioned me lol =D


i basically have u can say new experience. the best thing ive done is replace parts and learn and adjust and learn lol.

well to tell u the truth mainly y i know a lot of stuff is from this forum. when i see and interesting topic i read it and follow it and learn from it my mistakes and to tell others things ive done in the past... mistakes...
if i think of something i put it in the outboard discussions and see what pops up from the experts ( moderators most likely) try it u learn a bunch