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Piston clearance on mystery YZ250 cylinder

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Hi All,

 

I have decided to put my CRF450x in the closet this winter and instead an rebuilding a 1981 YZ250 to ride this season. I have had in boxes for years. When I bought it the top end was torn apart, the guy said the piston diameter was written on the cylinder, I said ok and that was the end of it. Today I pulled it out, written on the cylinder was 71.00mm, when I flipped it over 71.50mm was written also, very confusing. So I took out a telescoping gauge and measured it up and down and got a consistent bore diameter of 71.41. So my question is, should I run a 71.25mm piston? This will leave me with a differance of .18mm, or .007 thousandths of an inch. The other option is to run the 3 finger hone through it until I get a hair over 71.50mm bore. Any advice? Thanks

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Get a better measuring tool.  .007 will shatter a piston.  .004 really the upper end of what you should run on a new piston for a 250 cc cylinder with .0025 being more ideal and .0015 bordering on too tight.  All measurements are in inches.

 

Whatever the diameter is, with an iron sleeve you should probably get it bored before putting  a new piston in.  Pick your bore size, select a piston to match, and let the machine shop do the rest.  It seems most people only concern themselves with piston to cylinder clearance but an equally important measurement is taper.  Cylinders wear unevenly over their length.  They wear at the top when the rings dig in during the rapid pressure rise as fuel burns.  They wear in the middle as the piston tries to rock in the bore.  But taper is what makes engines not last long.  It allows the piston to rock and the rings have to move in and out in the groves many times a second as the piston travels through the cylinder.  Piston rings don't live in a happy well lubricated area they don't like to slide in and out of their grooves.

 

Never hone a cylinder to the next oversize.

Edited by 1987CR250R

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Good luck finding a new cylinder.  Just get yours bored but make sure you can get an oversized piston for it.  I have had trouble finding shops willing to bore small cylinders anymore but if you;re willing to ship it consider LA Sleeve.

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Might give Ken OConnor racing a call.  He does nice work and that would be a no-brainer for him.  He will fit the new piston for you.  Thought about cutting the head for correct compression?  Send that to Ken too.

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