Scored Pistons. Cause? Solution?

[cheburashka]

'69 500, serial #2626372<br /><br />After removing the power head and the exhaust cover (hey, only three sheared bolts!!) I found that all of the pistons show vertical grooves--really more like deep scratches. #3 is the worst. It has very little compression and it looks as though the aluminum has been pressed down around the rings, which may account for the bad seal. The grooves are right down the center of the two outer exhaust ports. The insides of the bores look good, but who can tell, since I can't see the damaged side. There is some yellowish staining on some of the pistons too. <br /><br />I'm wondering what might be some common causes of this kind of damage, if it's an indication of something else being seriously wrong with the motor, and whether I should just part this one out, or try to rebuild it. Anyone have experience with this?<br /><br />Thanks.

 

[bb_ed]

Re: Scored Pistons. Cause? Solution?

Lack of lubrication, stuck rings, overheating, a combination of all three. Overheating could have occurred first causing the others. Carbon buildup/gummed-up rings could have caused the other two. Lack of lube could have caused overheating and thus smearing of aluminum from pistons/cyl walls. Also lugging due to incorrect propping or excessive load in boat (which is related to propping)

 

[cheburashka]

Re: Scored Pistons. Cause? Solution?

OK, so does that mean that these pistons are shot, or can I count on a new set of rings and a cylinder de-glaze to bring things back to running condition? I'll have the bores measured out, but they appear to be in good shape. <br /><br />I have to say, I'm amazed by the casual attitude of most of the folks here who do extensive work on their motors. Phrases like "compression was low, so I threw in a new set of rings" used to sound to me like "I was hungry, so I made a bag of popcorn." Now it sounds like "I didn't have anything else to do, so I brought a child to full term and gave birth to it."

 

[crem1]

Re: Scored Pistons. Cause? Solution?

Hi!<br /><br />Could be just bad timing the guys that really know their stuff(They are busy).<br />Usually Rodbolt, Clams Canino, and AMDRules are there like dirty shirts with proper recommendations.<br />To me it sounds like the pistons probably will have to get replaced with new rings and a minimum deglaze of the bores if they haven't been damaged. Rodbolt would probably ask for bore measurements(9 of them) for each cylinder just to get an idea which course of action to take<br /><br />Alex

 

[rodbolt]

Re: Scored Pistons. Cause? Solution?

dang I think I post to much.<br /> but yes a teardown for further inspection is nessasary.<br /> if you can catch the edge of the scratch with your fingernail its bore time. another test is rub a penny across the scatch, if it leaves copper its bore time.

 

[Clams Canino]

Re: Scored Pistons. Cause? Solution?

You'll need at least to replace the melted piston, and all the rings. From there.... you'll get out of it in terms of longevity what you put in. <br /><br />-W

 

[crem1]

Re: Scored Pistons. Cause? Solution?

See, I told you they were dirty shirts <br /><br />Alex

 

[cheburashka]

Re: Scored Pistons. Cause? Solution?

So the piston is melted? That's what causes the groove down the middle of the exhaust port? I guess that makes sense. Well, looks like I'll be looking for a piston.<br /><br />I know they need to be weight matched. How do I figure out the weight of the damaged piston? Is it marked somewhere on the piston itself?

 

[crem1]

Re: Scored Pistons. Cause? Solution?

Why don't you call the parts dealer state the year and make of the engine, the serial number and ask for the specific part. I'm sure they will tell you what the weight is, if that is what you want.<br /><br />Alex

 

[Laddies]

Re: Scored Pistons. Cause? Solution?

Mercury pistons are all the same weight as long as the are for the same model engine, don't matter if they are standard or oversized

 

[cheburashka]

Re: Scored Pistons. Cause? Solution?

Thanks Laddies. Crem1, the reason I was asking is that with a lot of motors there are weight-matched piston sets and if you mix, bad things happen. I wasn't sure about Mercury. Nice to know that Mercury manufacturing tolerances are tight enough that I don't have to worry about that. <br /><br />Thanks again.

 

[Dhadley]

Re: Scored Pistons. Cause? Solution?

Most aftermarket pistons like Wiseco match too.

 

[Clams Canino]

Re: Scored Pistons. Cause? Solution?

Wiseco = Bad (for a few reasons)<br /><br />Stick with Mercury, Sierra, or Pro-Marine)<br /><br />-W (Favors Pro-Marine)

 

[nelham1]

Re: Scored Pistons. Cause? Solution?

general rule i always followed about piston scuffing. below the wrist pin lack of lube. above the wrist pin , air fuel mix. heat. believe it or not most 2 stroke engine will live with one or two pistons going .020 over . the outboard companies do this because of covering the warranty on a new engine.

 

[Clams Canino]

Re: Scored Pistons. Cause? Solution?

Mercury uses .015 and .030 over as the two oversizes. So dues all the aftermarket. <br /><br />EXCEPT for Wisco that does .010,.020,.030, and wants "special" bore specs, and has heavier forged pistons (OEM and aftermarket is cast), and has too much wrist pin to piston clearance, and uses proprietary rings, etc etc.<br /><br />-W

 

[crem1]

Re: Scored Pistons. Cause? Solution?

Are you still around Clams? If so, tell me this. Recently I was doing some spec work for my 1973 MERC 500 and found that the Seloc specs for the piston are different from the MERC repair manual. i.e 2.550 for MERC and 2.880(approximations) for SELOC
Thanks

 

[Faztbullet]

Re: Scored Pistons. Cause? Solution?

500 Merc 4 cylinder (68-74 years)
O.D above ring land....................... 2.551
O.D at piston skirt......................... 2.558
Cylinder finish hone........................ 2.565
Measure cylinder at : top then 90 degrees. port area then 90 and bottom then 90. If readings are .003 or more ,cylinder needs bored.