Oh NO 1 piston or 2?

[cid]

Upon inspection of my trusty Johnson '70 4hp I discovered one piston pretty scored up cyl walls look reasonable, little AL deposit on the bottm matching one of the piston scores. and a couple small scratches I can detect slightly with my finger nail.
Didnt find what caused it, looked to have been there longer than the last trip.
Do I replace both pistons, the other looks OK. Can I hone the cylinder slightly and just rering them both?
How carefull with honing do I have to be so it dosent get over.

 

[Paul Moir]

Re: Oh NO 1 piston or 2?

Muriatic acid (hardware store) will take the Al off the cylinder wall without hurting the wall.
Oven cleaner might get the job done too.

 

[F_R]

Re: Oh NO 1 piston or 2?

Either overheated or lack of oil (lack of oil causes friction--heat) The aluminum piston has gotten hot enough to melt, at least on the surface.

The piston acts as a valve, sealing the exhaust ports when it covers them. So if it's scored, you will have a leakage path between the exhaust and crankcase. Saying that to say that worn or scored pistons need to be replaced. And if the cylinder is scored very much, it needs to be rebored. Hone it lightly and see how it looks. Move the hone in and out while honing to produce a crosshatch pattern and don't take out any significant metal, except the melted aluminum. If the other piston is not scored and fits well, it is OK to reuse it with new rings.

 

[KYHunter2]

Re: Oh NO 1 piston or 2?

You need to , mic the cylinders , to be sure the heat hasn't caused them to be out of round.

KYHunter

 

[Paul Moir]

Re: Oh NO 1 piston or 2?

cid sent a pic and the damage looks more like coking to my eye than an overheat. Which isn't all that surprising since the top cylinder is apparently OK.

But the piston is well in need of retirement.

 

[cid]

Re: Oh NO 1 piston or 2?

Ad
what is coking?

 

[Paul Moir]

Re: Oh NO 1 piston or 2?

Gooey carbon stuff that sticks rings.
Likely improper oil, too much oil, etc. I bow to the experts though. Mind if I post that pic?

 

[cid]

Re: Oh NO 1 piston or 2?

No problem go ahead and post it. I cant figure out how to or I would.
There was a lot of gooy carbon stuff in the exhaust side, cleaned up easy with carb cleaner though.
Perhaps some one can estimate how long that score has been in there?

 

[Paul Moir]

Re: Oh NO 1 piston or 2?

Judging by the blowby colouring of the rest of the piston skirt, it looks like it's been that way for some time.

 

[cid]

Re: Oh NO 1 piston or 2?

assuming this was caused by carbon gunk build up, question is how often do you need to clean out the carbon, or is there an addtive you can use once in a while.?
Remember taking the ol Pontiac 455 station wagon out and "blowing the carbon" out of her once in a whileo

 

[Paul Moir]

Re: Oh NO 1 piston or 2?

I am still waiting on a piston, think I found one out of Ishopmarine. every one tells me they dont make these any more. :'(
Thanks

Yeah, BRP code "7", No Longer Available.

Good places to try though - sea-way.com, tcoutboard.com (mostly used stuff), outboard-parts.com, laingsoutboards.com, vintageoutboards.com, theoutboardwizard.bizhosting.com.

Most will require an email.


Decarb every couple seasons is a good idea. Running good quality oil and the correct oil (IE, any modern TC-W3), makes it less necessary.

Would like to hear other opinions on the diagnosis though.

 

[cid]

Re: Oh NO 1 piston or 2?

Working on a lead from Ishopmarine who says that there is one in their "vintage" warehouse $43 US (ouch).
I mic'd the cylinders and pistons, they are all round and in spec, still have a slight score on one, but not bad, honed up looks reasonable so im proceeding to redo her.
Great little motor for the back woods on a canoe. Not sure what is available these days that would be equvilent, this guy is so reliable and lightweight, no gears and its a TWIN