Read my piston: 2014 YZ125

Bike has 19.5 hrs. Running 25% VP 110 and 75% 93 pump with bel ray H1R mixed at 32-1. I ride 250B MX only and weigh 195 lbs. geared up so I'm pretty hard on my bike. Mods include FMF factory fatty w/shorty, v-force 3 and slightly richer jetting. Riding at sea level and 60 deg. Ok so now that I got that out of the way I think everything looks great except for a slight pitting on the outer most edge of the cylinder and head. Anyone know what is causing this? image.jpgimage.jpgimage.jpgimage.jpgimage.jpg

Doesn't look bad but I wonder if the coloring down the skirt is blow by or just from heat. Was the plug a good color? What was the compression?

if its blowby thats a lot for 19.5 hours, and it does look like the piston was getting hot, any pictures of the plug? oh and the pitting might be from detonation, try using a little more VP and a little less pump

I didn't do a compression test but bike still felt strong and always started on 1st kick. These are some pics of the plug, it's the original and also has 19.5 hrs.image.jpgimage.jpg

Also something I've noticed is that the pitting is only occurring in between the studs. I don't think that this is a coincidence.

I didn't do a compression test but bike still felt strong and always started on 1st kick. These are some pics of the plug, it's the original and also has 19.5 hrs.attachicon.gifimage.jpgattachicon.gifimage.jpg

Also something I've noticed is that the pitting is only occurring in between the studs. I don't think that this is a coincidence.

 

i have no explanation for the pitting, i figured detonation, if its a specific area, i dont know, maybe its occurring right where the transfer ports lead into the combustion chamber? and the plug doesnt look bad, not like it was lean, but like it did get hot

Looks like all is good except for the premix. It almost looks like it burns real dirty.

 

I can highly recommend Super-M from my days of running Honda cr125's in B-class.

The pitting in the head and cylinder almost sounds like "cavitation erosion" from a leaking head gasket.

 

Or it is running too hot/lean. The cylinder wall should be relatively cool and no combustion occurring in the "squish " area. Yet their is pitting at the outermost periphery of the bore and their is so much blow-by down the piston.

 

I would run a clean burning premix like super-m or Yamalube 2r and bump the main 2 sizes. Run for 5 hours and re-inspect. See if you can get a piston with a nice clean wash without all the brown staining.

Maybe have the squish set properly?

The pitting in the head and cylinder almost sounds like "cavitation erosion" from a leaking head gasket.

Or it is running too hot/lean. The cylinder wall should be relatively cool and no combustion occurring in the "squish " area. Yet their is pitting at the outermost periphery of the bore and their is so much blow-by down the piston.

I would run a clean burning premix like super-m or Yamalube 2r and bump the main 2 sizes. Run for 5 hours and re-inspect. See if you can get a piston with a nice clean wash without all the brown staining.

Thanks for all the info guys I really appreciate it! I'm sending my cylinder and head to James Saylor at JMS Performance for port and head work, I'm going to send him the piston also to make suggestions. Edited by Rocket416

Thanks for all the info guys I really appreciate it! I'm sending my cylinder and head to Derek at HP Bikes for port and head work, I'm going to send him the piston also to make suggestions.

Good call...The Pro's should know best...good luck!

Edited by souliog

try using a little more VP and a little less pump

I disagree. I think it's either strait race fuel or pump that should be used. Be consistent with strait race fuel is the dome and/or compression needs it. If race fuel is too expensive, then redo engine to run strait pump gas.

I disagree. I think it's either strait race fuel or pump that should be used. Be consistent with strait race fuel is the dome and/or compression needs it. If race fuel is too expensive, then redo engine to run strait pump gas.

 

race fuel is definetly more consistent, but 93 is always 93, octane ratings dont change, and by mixing it you dont get the consistancy, but you do get the raise in octane levels, and thus, less detonation. I personally wouldnt, and dont mix mine, I think the only reason one would mix is to reduce detonation, not make the bike run as good as it would on straight race because it wont

You will make the most HP with the lowest octane your bike will safely run.

 

You can make more HP with race fuel but it needs to be higher octane and oxygenated. Also the bike needs to be modified to materialize the benefits of higher octane. ie advanced timing, higher compression.

 

There is nothing wrong with mixing fuels. Mitch payton mixes two race gas blends to make the "Pro Circuit" mix.

Ok guys thanks for all the comments and suggestions. Just got off the phone with the tuner and he will take care of the detonation pitting by milling and/or lapping cylinder and head. Also said that the discoloration on the piston skirt wasn't an issue to be concerned about and he didn't think it was blow by. I'm gonna start running 100% VP 110 and I actually found it for a good price at $8.05 gal. I will post a review in another thread once I get it back and put some track days on it.

Ok guys thanks for all the comments and suggestions. Just got off the phone with the tuner and he will take care of the detonation pitting by milling and/or lapping cylinder and head. Also said that the discoloration on the piston skirt wasn't an issue to be concerned about and he didn't think it was blow by. I'm gonna start running 100% VP 110 and I actually found it for a good price at $8.05 gal. I will post a review in another thread once I get it back and put some track days on it.

I agree, see if he can give you some jetting info too. You should/need to richen your jetting because octane replaces fuel on a volume basis. Plus it looked a little lean to begin with. Have him set your squish to 0.035" while he is at it.

I agree, see if he can give you some jetting info too. You should/need to richen your jetting because octane replaces fuel on a volume basis. Plus it looked a little lean to begin with. Have him set your squish to 0.035" while he is at it.

I'm going to bump the main up from a 440 to a 460 and go from there.

Edited by Rocket416

Update: 2nd piston has 13.7 hrs. Everything looks pretty good but I did get a discoloration hot spot on the underside of the piston. I'm guessing this was from the first 2 hrs. of run time because I was lean on the mainjet. I'm making 2 adjustments on this rebuild.... Switching premix from Bel-Ray H1R to Yamalube 2-R and using a Wiseco GP piston with TIN coated ring instead of OEM.image.jpgimage.jpgimage.jpgimage.jpgimage.jpg

That piston looks like it has a lot of life left and H1R is top notch oil. Are you doing this as an experiment or something?

 

What are you trying to achieve?

That piston looks like it has a lot of life left and H1R is top notch oil. Are you doing this as an experiment or something?

What are you trying to achieve?

Main reason I'm switching oil is due to the price. I can get 2-R for less than 1/2 price of H1R.

Switching pistons because I got the wiseco top end kit for only $20 more and it was highly recommended by HP Bikes and JMS Performance.

You are right the pistons have very minimal scuffing and could no doubt go longer but I like to stay on the safe side. Tuners say don't go much more than 12 hrs the way I'm riding and the way the motor is set up.

I plan on running the Wiseco for 18hrs. to offset the cost difference to OEM.

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