Cylinder size

[beck125]

ok so i just went into my good friend and had my cylinder mic'd and honed. Come to find out the piston was wrong from when the guy built it before me it was about .007 over sized to the cylinder. Im furious! right now where it sits its at 3.391 standard (bore), i am looking at what piston i will need to get somewhere between 3.389 and 3.390? I am just needing some sizes thrown out there for me. Also if you have a link to a piston post it please!! Thanks again guys for all of the help!
Respects
Jordan

[bherriman]

I guess I am unsure of what your question really is. If you have a standard bore jug then you will need a standard bore piston which is 85mm. You should file fit the top two piston rings for best performance/ring seating.

[beck125]

ok im currently at 86.06 pr 86.11 is my tolerance gap. i need to find a piston thats inbetween these two, i cant find anything but an 86

[CJM]

you need to either resleeve it, use oversized rings (not common) or bore it for a larger piston. Take your pic..

ETA: and you need a new piston of course.

[togup]

For future refrence buy the piston you want then bore the cylinder to fit the piston

[speedfreek]

Originally posted by beck125
ok im currently at 86.06 pr 86.11 is my tolerance gap. i need to find a piston thats inbetween these two, i cant find anything but an 86 Your only choice is to get an 86.25mm piston and bore to this size because your cylinder is out of spec,

[beck125]

ok well i cant find one at that spec either, FYI i had it honed and mic'd for FREEE so i know what i am at! i have not purchased a piston i am asking what i am needing, as in what would be the easyest way to do this, where can i find a piston of this size before i get it bored

[CJM]

rocky mtn, ebay, motosport, magic racing all carry pistons.

You now need and 87mm piston, this is b/c there is NO such thing as an 86.25 piston. Least none I ever seen.

Like I said, take your pick:
-resleeve it so you can use a stock piston
-use oversized rings (not common on an engine like this at all.I aint ever seen them)
-bore it for a larger piston like 87mm which will give you 416cc. Might as well do a whole build with a cam, and such and you will have to reject the carb.

[speedfreek]

CP, J.E. and Probe make custom pistons.

Whoever miked your cylinder, did they check for bore distortion? Out of round? There is more to it than just honing and slapping in a piston. When a cylinder wears, it wears out differently at the top, middle and bottom.

Pistons for 2 strokes come in .25mm increments. I'm probably wrong with saying get a .25mm oversize.
Ok I just checked, 400ex pistons come in .5mm increments.

If your cylinder is at 86.06mm you need a 87mm piston as mentioned. If you got an 86.5mm you would still be out of tolerance. Your cylinder needs boring, as mentioned. Unless you want CP, JE or Probe to custom make you one.

[togup]

CP will make you a custom piston but youll have to know exact measurements they will barrage you with dimentional questions plus youll have to pay a set-up fee but then youll still have to check it against the bore and possibly re bore it

[beck125]

Yes its out of standards. Its being bored I got it honed so I was a a base line number as to what it was at I'm going to 87