Would you run this piston?

[ttankmoran]

This is the top piston out of my 1966 Evinrude Fisherman. I tore it down because the base gasket failed and was leaking water under the hood. Since nothing has been touched under the hood for 50+ years I decided to freshen things up. Compression test showed 65 PSI on the top cylinder and 75 PSI on the bottom cylinder. Only the top piston is shown in the pictures. The scuffs are are on the transfer port side, exhaust side was fine.

The pictures actually make the scuffs look a little worse than they actually are, you can feel the texture with your finger nail but nothing grabs it.

So what's every one's opinion? Pistons and rings are getting more expensive by the day for these things. Im hoping to get away with a hone and a set of rings, It would probably be fine to re-use but on the other hand if I replaced them there is no reason this engine wouldn't go another 50+ years. Opinions?

 

[fhhuber]

I would have to actually feel the piston's surface and use a micrometer and "Prussian Blue" (a dye that is used to detect cracks and scratches) to check it out properly.

 

[Scott Danforth]

you also need to mic the piston to see if the skirt has collapsed.

i myself would go with new pistons and a bore/hone

 

[89Aluma]

Look at it like this, you know that there is obviously some issue with the piston and it shows slightly already in the compression, yes new rings and a hone will help the compression for a while but honing will remove a few thousands or so from the cylinder, do you want to take the chance of gauling the cylinderthe skirt is what's keeping it straight in the cylinder not sure what the tolerance for out of round cylinder is on your engine but I would look at that as well and put a new piston in, if you don't plan on keeping it long by all means do as you please but if you want another 20+ years out of it a new piston is the absoluty best decision

 

[fhhuber]

With an overbore, you NEED a new piston.

With a hone job you MIGHT need a new piston.

.010 oversize the original piston won't fit right. Rings won't compress properly and if they meet the walls well, the end gap will be excessive. That's just the first oversize.

You have to measure the cylinder at several locations and measure the piston in at least 6 places if there is any chance the piston will be reused. Any scratches you can feel are a fail.

 

[Chinewalker]

In my experience, these motors are pretty forgiving. A light hone to remove the worst of the scuffing, fresh rings (if you can find them) and it'll likely outlast you. I've cleaned up lightly scuffed pistons like that in old motors with 120 sand-paper, cross-grain to the scuffs. Don't go too crazy with the sandpaper, you don't want to remove much material. Keep in mind, you're building a fishing motor, not a highly tuned racing motor. Pretty good will run pretty well.

I believe 2-ring pistons out of a later 6hp (up to 1979) will fit, too, if you want to replace them outright with something that might be more readily available.

 

[Silvertip]

If you look at the first picture carefully, right at the top of the ring land you will see two pretty significant grooves as if something went past the piston in that area. I would also check the cylinder wall in the same area. I would not put that piston back into the engine. You have it apart, Fix it right or you will likely end up doing it again.

 

[fhhuber]

In my experience, these motors are pretty forgiving. A light hone to remove the worst of the scuffing, fresh rings (if you can find them) and it'll likely outlast you. I've cleaned up lightly scuffed pistons like that in old motors with 120 sand-paper, cross-grain to the scuffs. Don't go too crazy with the sandpaper, you don't want to remove much material. Keep in mind, you're building a fishing motor, not a highly tuned racing motor. Pretty good will run pretty well.

I believe 2-ring pistons out of a later 6hp (up to 1979) will fit, too, if you want to replace them outright with something that might be more readily available.

I have no doubt the engine an be made to run with that piston.
The question then becomes: How long will it run before the engine has major issues again and requires the piston to be replaced?
A piston that has gone out of round and has scratches and gouges, it may not take long at all before it gouges the cylinder and loses compression again.

Sure, "pretty good" will run OK for a while.
You might be rebuilding again in 2 years, or next week.

No measurements and no in person inspection = I have to say replace it if you can feel the scratches.