01 YZ426 Piston Size

3 replies to this topic
  • YZ Yankee

Posted September 17, 2015 - 04:55 PM


Can someone tell me why the stock piston size for this bike is listed as 94.93, but the specs in the manual states 94.945 to 94.96

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  • grayracer513

Posted September 18, 2015 - 06:28 AM


Where did you find the listing for the "stock piston size" at 94.93?  


Also, understand that the difference between 94.93 and 94.96 is all of .0011" 

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  • YZ Yankee

Posted September 18, 2015 - 07:42 AM


Where did you find the listing for the "stock piston size" at 94.93?

Also, understand that the difference between 94.93 and 94.96 is all of .0011"

Great! Good question.
I guess I was looking at aftermarket pistons.. Vertex and came across that as being stock oem size.
They also have slightly oversize listed as B,C,D. 93.94, 93.95
The difference seems small, when converted to inches, but when considering cylinder to bore clearance and wear specifications, it appears to make a difference.

My specific concern, my cylinder is worn, still round within specs, plating still good.
I took it apart due to a leaking headgasket.
The compression was pretty good, but I want to replace the piston and rings. They are the same one that came in the bike 1 1/2 years ago.

I found the following marking on the underside of the piston, Y 3.

Edited by YZ Yankee, September 18, 2015 - 07:45 AM.

  • grayracer513

Posted September 18, 2015 - 08:33 AM


Yamaha offers pistons in one size only.


Have your cylinder measured by a machinist using a dial bore gauge.  This will accurately tell you whether it's really serviceable or not.  Maximum variance from straight or round is .002" 


When you use a used bore, the piston, if available in alternative sizes, must be fit so its minimum clearance in the smallest part of the bore (usually just under the ring swept zone at BDC) must be at least the minimum specified by the piston manufacturer, while the clearance at the most worn area of the bore must not exceed the max specified.  If this cannot be accomplished, the bore needs to be plated and restored to a round/straight condition. 

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