YZ250 What causes pitting in the cylinder?

[Shooter88]

So I bought a new Apex head and was in the process of installing it. After I removed the old head, I looked down the cylinder and noticed bad pitting above the exhaust port that you could feel with your finger (didnt even need to use a fingernail). So I pulled the cylinder off and will be sending it to Millenium after the New Year. They replated it back in 2014 and they offer a limited lifetime warranty on their plating so I'm hoping they will honor it.  I was just wondering if anyone could tell me what causes pitting like this. Everything else looked good. No significant scratches that I could find in the cylinder or on the piston. Attached is a picture of the worst spot. 

[NPTroopz]

Lots of things. Air filter is typical, crank bearings having play, lean condition which can be due to seals. All sorts of things. 6 years on a cylinder playing on a 125 is pretty good though.

[Shooter88]

38 minutes ago, NPTroopz said:

Lots of things. Air filter is typical, crank bearings having play, lean condition which can be due to seals. All sorts of things. 6 years on a cylinder playing on a 125 is pretty good though.

It's a 250. But I just did a bottom end rebuild a year ago so I dont think it would be bearings. I clean the filter after every ride. I did have an air leak about two years ago, but did not notice this pitting when I rebuilt it afterwards. Maybe the heat weakened the plating and it just started to deteriorate slowly since then. 

[NPTroopz]

2 minutes ago, Shooter88 said:

It's a 250. But I just did a bottom end rebuild a year ago so I dont think it would be bearings. I clean the filter after every ride. I did have an air leak about two years ago, but did not notice this pitting when I rebuilt it afterwards. Maybe the heat weakened the plating and it just started to deteriorate slowly since then. 

Very possible. If you have time maybe put it back together and do a leak down test? You can build a tester for about 30 dollars in common parts and it will save your engine from future failure due to crank seal leakage.

[Shooter88]

50 minutes ago, NPTroopz said:

Very possible. If you have time maybe put it back together and do a leak down test? You can build a tester for about 30 dollars in common parts and it will save your engine from future failure due to crank seal leakage.

Yeah, I'll probably look more into it. It's always been on my list of things to do. 

[FLO_Rida]

10 hours ago, Shooter88 said:

Yeah, I'll probably look more into it. It's always been on my list of things to do. 

Couldn't detonation cause it too? What is fuel your ratio?

Edited January 1 by FLO_Rida

[Shooter88]

1 hour ago, FLO_Rida said:

Couldn't detonation cause it too? What is fuel your ratio?

I never noticed any detonating. The head and top of the piston show no signs of detonation either. I'm running 93 pump at 32:1. 

[MANIAC998]

Could a leaking head gasket be causing this?

[SlowDinoDog]

The pits are a contamination/plating issue.

[hallsy]

20 hours ago, Shooter88 said:

 So I pulled the cylinder off and will be sending it to Millenium after the New Year.

If you don't mind, I would like to hear what they come up with.

[SlowDinoDog]

1 hour ago, hallsy said:

If you don't mind, I would like to hear what they come up with.

i think they have data/pics on plating pitting on their web page, too.

Edited January 1 by SlowDinoDog

[Shooter88]

53 minutes ago, SlowDinoDog said:

i think they have data/pics on plating pitting on their web page, too.

I didnt see it before but I'll go take another look. Thanks. 

EDIT: found it!

"Porosity is gas trapped in the base metal aluminum during casting. These “holes” are exposed when the cylinder is machined or subjected to various chemicals in the plating process. The pits are usually small in size, and in good castings few and far between. When kept under acceptable levels, porosity will not adversely affect the performance or longevity of the engine. In fact, it will improve lubrication. The pits will not cause the coating to peel, nor will a ring hook on them. As far as our specifications on porosity our cylinders carry a lifetime warranty against defects in materials and workmanship. Therefore, through extensive testing we have found that porosity is a cosmetic problem only. In our pursuit to have both cosmetic and functional perfection we do everything possible to remove these pits. In some cases, this is not possible, so we release the cylinder only in cases where it will not affect performance."

Not sure if this is what is happening, so I'll see what Millenium has to say on Monday. 

Edited January 1 by Shooter88

[777654321]

4 hours ago, MANIAC998 said:

Could a leaking head gasket be causing this?

Some how moisture is getting in. You ride allot through water.  Do you put a cover over air box when washing.

[Tepco]

Definitely a playing issue. Never saw that with the company I use.

[marjanboo]

Has it been sitting for some time looks like corrosion maybe water Mark's what do you use for water in radiator?

[Shooter88]

I dont ride in a lot of water or mud. Pretty much only track riding. I was recently deployed and the bike sat for four months in the garage during a very humid summer. I sprayed fogging oil in the engine before I left in order to help keep away any rust or corrosion. When I wash the bike I put a plastic grocery bag around the filter and put the seat on. I then stuff plastic bags in the shroud holes on the side to prevent water from getting in. That's it about it as far as moisture goes. 

As for radiator fluid, I just use whatever generic 50/50 mix I can get from the auto parts store. I dont have a specific brand that I use. 

Edited January 1 by Shooter88

[Skeeter_Fogger]

l used to work in aerospace.  Aluminum castings were impregnated with sodium silicate to fill any porosity.  I've built car engines where someone sold me a basket case and let the bores get rust pitted.  Not having the resources to bore, hone and buy new pistons, I just honed them.  They still had pits but that didn't seem to matter, so I'd have to agree with the cosmetic comments.

[ThumperHead]

Looks like there is smaller pockets or pitting around the cylinder also away from the exhaust port area.

My question, why not build it up again by plating filling in those pits and pockets before final finish allowing for a perfect bore. I don't buy leaving minor pitting as normal and running without issues.  Also long scratch becoming an eroded pit over time by the hot combustion gases, just give it time.

This a Nikasil cylinder possibly having a sulfur erosion issue?

Like chrome plating having the first copper plating process filling in small imperfections and pits to a level surface before the nickel plate process now smooth as glass ready for the chrome process. Plating over pits results in plated pits not smooth surfaces. 

They honor their work not blaming you causing your pitting or erosion issue? Time to test their "lifetime coverage" whch could be good or bad PR.  

Still a fan of older two and four strokes having iron cylinder sleeves, hone or bore next size over......~~=o&o>.......

 

Edited January 1 by ThumperHead

[Kinger317]

  Some porosity is fine and yours shows some but that area above the center exhaust port should be warrantied. It wasn’t like that when you installed the cylinder was it? 

[Shooter88]

11 minutes ago, Kinger317 said:

  Some porosity is fine and yours shows some but that area above the center exhaust port should be warrantied. It wasn’t like that when you installed the cylinder was it? 

No, I didnt notice this when I rebuilt the whole engine a year ago. Is it still fine even if you can feel it with your finger? I always thought that anything you can feel with a finger nail must be repaired. 

Edited January 2 by Shooter88