Basic piston ring questions

[melugin]

Have a 1/2hp mate with typical low compression. New rings are dimensionally same as my old rings. I would expect worn out rings to have a smaller free gap as the new rings. Why or why not? Cleaned everything up and put old rings back in after light sanding cylinder. About the same compression. Also, what was the procedure to break in rings with Bon-Ami?

[RICHARD A. WHITE]

I would be concerned that your cylinder is worn out, there was one of this little motors that was notorious for that, but do not remember which one.

Rings are broken in with running on 2X the normal oil mixture…

http://www.richardsoutboardtools.com
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[billw]

I have an 18 Johnson that had (actually still HAS) worn cylinders. New, standard rings still had way to much end gap. I happened on to some .020″ over-size rings. I figured it was worth a try; so I spent a little time with a file and made them fit the worn cylinder properly, making sure the ends didn’t actual touch at the bottom of the stroke. It now runs like the wind, with very good compression, power and no unusual noises that I can hear……Maybe something similar can be done with a little engine?

Long live American manufacturing!

[Tom]

Keep in mind that pistons rings need to seal in two places. The cylinder wall is the obvious place, but they also need to seal against the bottom of the ring land. Be sure the ring lands are free of carbon and corrosion. Don’t use anything sharp to clean the lands, scratches and gouges are to be avoided.

I’ve taken an old ring and sanded the top and bottom surfaces with 220 grit wet and dry paper on a surface plate to get them smooth. Then loaded up a piston groove with polishing compound. With that done, load the piston and ring into its cylinder. Move the piston up and down in the bore while twisting the piston back and forth. This does a pretty good job of polishing the lands, but you have to thoroughly clean the piston lands and cylinder when you’re done.

T

[chris-p]

I do not think I have heard of this Bon Ami method?! I too believe your bore is worn, not the rings. Easy enough to quickly check. Are oversize piston even an option for that model? Or alternately, I wonder if there is another piston, similar in height, from the next HP up perhaps? that could be used after boring the cylinder up to fit?

Tom….good tip on polishing ring lands!

• This reply was modified 5 years, 4 months ago by chris-p.

[melugin]

Minimal carbon on piston, rings and ring lands. Cleaned off with toothbrush and thinner, no scraping required. I didn’t check the bore taper. May do that next by inserting ring in cylinder. Not real easy to do on this size motor. I am aware that these often wore out from running too cold and the cylinder would probably show that.

[jeff-register]

Cylinder wears like a cone. Use a ring to measure end clearance in both top & bottom of bore keeping the ring square using a bare piston. Also check ring clearance while on the piston for oversize side slop, Use a feeler gauge to measure.

[fleetwin]

Did you mean 1.5hp mate, or is this engine actually .5hp? I am guessing you are dealing with one of the 68-70 versions of this little OMC engine.
If so, these engines are pretty simple and reliable, they run pretty well also for being just a single cylinder. But, when they are mechanically “worn out”, they exhibit some common symptoms. They are hard to start, have a very poor idle, and a lack of WOT power output. Unlike many OMC outboards, these little engines are not very forgiving when it comes to a worn cylinder or piston skirt. I don’t know why, but have experienced enough of them with these symptoms, and wasted too many hours trying to “tune up” or rebuilding carburetors trying to overcome a worn out powerhead.
I don’t understand what you mean by “…a smaller free gap as the new rings”…. I do think that ring design changed over the years of this powerhead, so newer rings might have different end gap specs than the original specs. You mention low compression, what readings are you getting? Did the engine run OK before you tore it down, or is the engine new to you and you just went ahead with rebuilding without running it?
In any event, if the powerhead is apart, I would spend the time measuring the piston skirt and cylinder wall very carefully….New rings might raise compression readings, but they won’t compensate for a loss of primary compression due to worn piston skirt/cylinder wall…..

[melugin]

1/2 hp or 0.5hp No piston wear, NOS rings are identical dimensionally to existing rings. Motor needed several maintenance issues so a teardown was necessary before trying to start-no fire, nasty gearcase and fuel tank, minimal compression.

[The Boat House]

• This reply was modified 5 years, 3 months ago by The Boat House.
• This reply was modified 5 years, 3 months ago by The Boat House.

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