Are these pistons trashed?

[jrs_diesel]

I got the pistons out of my 72 Johnson 6HP, and have some questions about them.

I had decided to pull them when I noticed some scoring on the lower piston, looking at it through the exhaust ports. After I pulled them both I noticed there was more scoring on them than initially thought.

I want to note that even though both pistons were scored, neither one of them damaged the cylinder walls in any way. On both pistons, the connecting rod and wrist pins were fine.

The upper piston had less scoring on it than the lower, which was much worse. The upper pistons damage only went from the skirt, up to the bottom ring land. The lower piston had three patches of scoring. The worst on that one went up all three rings, and the lowest ring was stuck in that one spot because of it. The stuck ring did pop out with a little effort.

Here are a couple of pictures. The first two are of the upper piston. The last three are of the lower piston.

1 - Are either of these pistons re-useable?

2 - If so, how do I clean them up?

3 - How can the pistons get scored without damaging the cylinder or ports?

 

[Mas]

Re: Are these pistons trashed?

Trashed?...yup

 

[1946Zephyr]

Re: Are these pistons trashed?

If I see any scoring on pistons, I'll replace them. The one in the third picture looks pretty rough.

 

[jbjennings]

Re: Are these pistons trashed?

I'll third this----they look horrible. I wouldn't even consider using them except to beat my wife over the head with one for griping about me buying another old motor.
I'd also get a good used powerhead before I'd buy new pistons and rings. Not cost effective for an old 6 horse.
JMO,
JBJ

 

[Faztbullet]

Re: Are these pistons trashed?

Hard for me to believe cylinders not damaged much less no aluminum transfer to them.. They is goners!!!

 

[a70eliminator]

Re: Are these pistons trashed?

With pistons like that I highly doubt the cylinders survived.

 

[1946Zephyr]

Re: Are these pistons trashed?

You'd be surprised. I'll bet those cylinders will clean up nicely

 

[jrs_diesel]

Re: Are these pistons trashed?

I'm a little suprised myself that the cylinder walls were not scored or scratched or dinged. The cylinders look OK. I can still see the original cross hatching . They are smooth and have what I think is minor staining.

Here is a link to the thread I have on the cylinders:

http://forums.iboats.com/showthread.php?t=378211

I will be replacing the pistons.

I'd also get a good used powerhead before I'd buy new pistons and rings. Not cost effective for an old 6 horse.

I consider myself as a pretty good shadetree mechanic. This motor was essentially free (came with a salvaged sailboat), and in decent condition, considering it was run in salt water, never flushed when put away, and stored for 10 years. I have never torn down a motor to rebuild before, and I find this to be a test of my knowledge and skill. Plus, it's a lot of fun!


I'm still curious though. How could the pistons get damaged like that and not hurt the cylinder walls?

 

[jbjennings]

Re: Are these pistons trashed?

If you got the head off a saltwater motor that had been sitting for 10 years-----you're better than a "pretty good" shadetree mechanic.

I'm thinking that the pistons were probably scored and such by excessive heat and blowby of hot gases due to a bad water pump, eroding the aluminum pistons but not the steel cylinders. Aluminum melts about 900 where steel is at least twice that....

Then again, I'm showing my ignorance here because I'm just a shade tree myself.
I, too, know the fun of tearing one down for the first time!
I bought a used powerhead and wound up using parts off both to build one good powerhead for a weird reason that would take too long to explain.
Anyway, good luck with your project.
JBJ

 

[jrs_diesel]

Re: Are these pistons trashed?

That head wasn't easy to get off. 1 bolt had its head pop off, it was seized to the head, but the threads were fine. I seperated the powerhead, and then had to get the exhaust cover off. 5 out of 10 bolts seized, 2 of those I snapped the heads off. I ground off the other 3. Once I got that off, then I could "unscrew" the head from the block. Getting that seized bolt out of the head was something else too. I used a hydraulic press to push the bolt out and it took about 800 lbs of force to push it out (the head was well supported). This was even after a week's worth of PB Blaster on that bolt

I think you are on to something about it possibly overheating. The water pump impeller was in good shape, but I think the inlet was plugged up some. The head has some paint flaking off. I had thought that was from the corrosion. Perhaps it's a combination of both. Come to think of it, I had to press out the wrist pins too. The hammer and puch method didn't budge them.

I am having a blast with this motor. My ultimate goal with it is to fully fix it up and make it look good. I just hope I can find find a set of pistons for it :redface:

 

[JustJason]

Re: Are these pistons trashed?

Being a 72 and only a 6hp I would just sand and polish the pistons by hand.

 

[jrs_diesel]

Re: Are these pistons trashed?

Being a 72 and only a 6hp I would just sand and polish the pistons by hand.

Wouldn't I risk making them worse by doing that? What I mean is, if I did sand and polish, could I inadvertantly make them out of round and then not fit the bore?

 

[Tig]

Re: Are these pistons trashed?

Pistons are soft aluminum, cylinders are much harder. One would expect the piston to show most of the damage. The answer to the riddle of why the cylinder has no damage at all may be in your question #1. I would suspect that a previous repair recycled the pistons. Pay attention to the size of any parts you order. You may find that your old rings, bearings etc are .010 over. Check everything.

 

[kend301]

Re: Are these pistons trashed?

When you throw them out , make sure to put them in the recycle bin !

 

[JustJason]

Re: Are these pistons trashed?

Wouldn't I risk making them worse by doing that? What I mean is, if I did sand and polish, could I inadvertantly make them out of round and then not fit the bore?

If you took a sanding block to just 1 side then yes. If you cup sandpaper in your hand then probably not. You can always double check with a mic. It's easier to do it with the con rods attached to the piston.

note~ I wouldn't advocate this for a 300hp optimax.... but for a 40 year old kicker, absolutely. Hot rod guys hand polish thier pistons all the time. You just want to sand it enough to get the major scuffs out. You do not however, want to sand out the horizontal "rings" that you see all over the piston.

 

[oldgradywhite]

Re: Are these pistons trashed?

The cylinders arn't scored????? Toughts pistons are trasted!

 

[kahuner]

Re: Are these pistons trashed?

I'll throw my $.02 in here.

One possible cause for the lower cylinder piston being worse than the upper is possibly a lower crankcase seal that is trashed. That would cause a lean condition in that cylinder. Lower seals fail due to being exposed to more heat from the exhaust. Make a cylinder run lean, w/o warm-up, put it under load and you'll get a piston that expands faster than the liquid cooled cylinder and wahlah, scoured pistons. R & R the lower crankcase seal.

Are the pistons trashed?

The rings seal 2 ways. One is against the cylinder wall and the other is the ring sealing on the bottom of the piston ring groove. Both being aided by compression forcing the ring outward and downward.

If you choose to re-use the pistons, part of your success will depend upon how well you can clean-up the bottom of the ring grooves. I wouldn't worry too much about the scuffs on the piston skirts. They can be roughed-up. (I'll add that the diameter of the piston skirt is somewhat important because it traps the fuel air mixture in the crankcase as the intake port is closed, reducing blow-by or spit-back) But the skirt isn't AS important as the rings ability to seal.

My focus would be on the lower C/C seal and the ring grooves on the pistons. This would be after measuring the cylinders for wear due to it's potential for effecting the ring end gap.

Hope this helps more than hurts.

 

[jrs_diesel]

Re: Are these pistons trashed?

When I pulled the powerhead out, the lower crank seal fell apart, with the spring rusted and broken in two pieces.

This motor was on a sailboat, so I imagine that the previous owner only ran it for a few minutes at a time to get off the pier and out far enough to get the sails up or vice versa. The salt water didn't help things either.

I woud like to find replacement pistons but I am having a tough time finding them. I want to rebuild this engine and get a good working, reliable outboard back in the water.

 

[lowkee]

Re: Are these pistons trashed?

Only tidbit left to add, as everyone else covered the rest. When you sand and polish, try not to make any vertical sanding strokes. You'll want to try and replicate crosshatching as best as possible. You can start off by sanding horizontally and then make the last few passes in a crosshatch pattern, in order to speed things up.

Any pics of the cylinders themselves? I'd be curious what they look like. Maybe a quick honing may be in order to remove any ridges. For practice, you may wish to buy a crappy lawn mower engine off CL or a nearby small engine place and sand and hone that first. Old lawn-boys make good practice, since they suck to begin with.

EDIT: After you finish with the pistons, you may wish to check this guy's page: http://www.sschapterpsa.com/ramblings/OMC_6hp.htm

He goes part by part through your exact motor

 

[kahuner]

Re: Are these pistons trashed?

jrs diesel,

Soda blasting the pistons will probably be the least "invasive" medium to use to clean-up the ring grooves.

As far as the lower seal falling apart, there's your answer as to why the lower cylinder was worse than the top piston.

New pistons (for an engine from that era) will be hard to get or at a minmum cost prohibitive. Soda blast them. Cross-hatch the pistons (staying away from the intake side skirt area) install with new rings. As far as taking off too much material, I wouldn't worry about that. If the pistons are cast, material comes off quicker than if they're forged. Forged pistons have a machined appearance in the interior of the piston. A cast piston should have a P/N cast inside and no evidence of machining. In either case, I think you'll be fine.

Before going any further, measure the cylinder walls. Are the pistons over-sized now, or a standard bore? How much piston skirt clearance? If you have .006"~ .008" piston skirt clearance but new pistons aren't available, there isn't much you can do anyway.

Use a "berry hone" to cross-hatch the cylinder walls. Sand a cross-hatch onto the pistons, assemble with new rings and I think you'll be surprised as to how well the engine will run.

My guess would be that the lower seal failed from heat and or long periods of improper storage. Seals will actually weld themselves to the crankshaft. After sitting for an extended period, the first time you try to start, the seal rips. Then, that clyinder runs lean, and you've got a scoured piston. It has nothing to do with how little the engine was used. Instead, how long or often it was stored.

My next guess would be the crank and con rod bearings are toast.

Hope this helps give you the confidence of using the old pistons.