piston size

[brad luna]

I just pulled my piston out of the cylinder and measured it and read 67.85mm. If I get a 68mm piston it should slide into the cylinder after I hone the cylinder??? The cylinder looks good no scratches or wear marks. The piston also looked good besides the black on the top of it.

[broken1]

Black carbon on the piston top is normal... Before buying a new piston Mic the cylinder to make sure it's not out of round or tapered & has the proper piston/cylinder clearance. If all that checks out you could probably get away with a hone & re-ring the piston.

[croat1]

How did you measure the bore?  Was it low on compression (did you check)? 

Joe

[2ndmoto]

That's a lot of material to remove with a ball hone. Typically a hone will take 0.02-0.08mm of material. 67.85mm seems odd, I have only seen increases of piston size for 250r on 0.25mm increases, so I am guessing you have a 67.75mm piston. Going to a 68mm piston I would highly suggest getting the cylinder bored. Have it checked out, you might just need a hone and a new set of rings.

Best regards
-Jason

[brad luna]

I was able to see the numbers on the top of piston and it is a 68mm. The cylinder looks fine I can still see the cross hatch or the criss cross from the last person who done the engine. I had the engine pulled out to fix a crack in the frame that was under the engine. So I figured I would replace the piston and rings while I had easy access. Not realizing that I'm almost positive it has been done recently. I have not owned this 250R to long and I am doing all this myself because I dont have the extra funds to pay someone to do it for me. Wish I did have the funds to have a professional mechanic build it up. Thanks for your info any extra input is welcome thanks again

[Jerry Hall]

Since your piston is for a 68 mm bore and assuming your piston to cylinder wall clearance was set correctly when it was last bored, using your current piston diameter measurements your current piston to cylinder wall clearance is 68.00- 67.85 = .15 mm  or 0.0059".  A new 68mm piston should have a diameter of 67.92 to 67.95mm 

If you can still see the cross-hatch I would not hone or de glaze the cylinder.  It will take about 15 minutes longer for the rings to seat when you do not hone the cylinder.  The cylinder bore will last longer, the piston and rings will last longer and the piston to cylinder wall clearance will remain tighter when you do not hone the cylinder when replacing rings and or piston on a two-stroke.

A ball hone, flapper hone, 3 stone-spring de glazer will make the bore look great but it will only make the cylinder taper and or out of round worse.  If you are bent on honing the cylinder take it to a shop that has a Sunnen hone that uses the long mandrels.  A shop that has invested in a Sunnen hone will also have the measuring equipment to properly measure the bore and piston to cylinder wall clearance.  The piston to cylinder clearance of a Wiseco should be .0030" for a new bore and .0040" would be ok with a new piston on an  old bore that has not been honed.  The piston to cylinder wall clearance on a cast piston should be .0026 on a new bore and .0035 would be ok with a new piston on an old bore that has not been honed.

[brad luna]

No I am not set on doing the cylinder hone I was just curious about it because this is all new to me. Everything I have read or watch has talked about not putting in a new piston unless you hone the cylinder first. That is why I am here asking questions. I appreciate all your advice and taking it all in to the best of my ability. This is my first time taking an engine apart, I  want to make sure I do it the to the best of my ability and funds will allow me.
I have the 68mm wiseco set ordered this morning and already have the top end gaskets plus the spacer plate because i have a long rod crank in the motor, v force reeds and I think the oem carb if they were 34 PWK. I just want my R to run like a champ , it ran very well before I took it apart I just hope it runs as well if not better when I get it back together.       Thanks again