New piston instalation - piston-to-cylinder clearance

5 replies to this topic
  • thefex

Posted May 27, 2014 - 12:48 AM


hi everyone!


the time came to check my top-end, so I took off the cylinder and piston.
the last one was a Wossner B

the cylinder was measured by an automatic Zeiss machine. here are the results:


what should be the propper piston diameter? 

the manual says:
cylinder-to-piston clearance: 0,020 - 0,045 mm; limit = 0,1 mm

In my opinion a Wossner C (94,98mm) would fit and it would give a clearance of 0,035-0,043 according to my calculations... so that`s close to the upper service limit... what is your opinion?

big thanks for any suggestions. I have to order the pistion ASAP because we have an round #2 of EXTREME ENDURO Tournament soon, and so far I am 5 of 32 and plan to hit the podium next time :)

here`s a page from the service manual regarding the cyl-piston topic

  • grayracer513

Posted May 27, 2014 - 06:54 AM


The problem is that according to the measurements you give, your cylinder is unserviceably out of round.  The listed maximum diameter measured is 95.055.  Subtracting that from the listed minimum (94.9) gives 0.155mm.  The tolerance is 0.05mm.

  • thefex

Posted May 27, 2014 - 08:02 AM


thx for the info
I know that and it`s my personal PITA...

I`ve consulted this problem with a mechanic at a local Yamaha dealer and he said that a Wossner C would be ok in this situation, however the motorcycle may burn some oil

I`ll try this option, should be fine IMO.. if not, I`ll just order na new cyl-piston set during the next year

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  • grayracer513

Posted May 27, 2014 - 08:23 AM


According to your information, a "C" will not fit in the bore.  You give the piston diameter as 94.98, and the minimum bore diameter as 94.9.  That yields a .08mm interference fit.


OTOH, it's a bit difficult to believe that the minimum diameter is at a point within the wear zone, rather than at the bottom or the top.  I don't know who measured it, or what instrument they used, but accurate measurements in that small an increment are exceptionally difficult to make.  I'd suggest you have it redone.  Probably help to hone it for cross hatch first.


Whenever I measure cylinders, I always use a measurement made above and below the swept zone so that I have a dimension representing the completely unworn bore.  This should be possible at the top, and is absolutely possible at the bottom, below the ring sweep, and between the clearance notches in the sleeve and the area swept by the skirt.  Then I measure the wear zone in the areas described in the book.


Also, you are mixing up the spec'd fit limit of .045mm with the service limit of .10mm. 

  • thefex

Posted May 27, 2014 - 11:30 PM


thanks for the info !!

I have similar worries about this measurement, besides I may be mixing some things up. I haven`t had a chance to talk with the guy who meausered this cylinder, I just got the results on a sheet of paper.
I`ll take the cylinder once again, this time to my Yamaha dealer, to have it remeasured and let it be their task to put a propper piston there

Edited by thefex, May 27, 2014 - 11:33 PM.

  • thefex

Posted June 04, 2014 - 08:20 AM


I got the cylinder back from measurements and the dealer told me to fit a 94,97mm piston.
I`ve chosen Wossner 8630DB. Wanted the 8631DC first, but it was difficult to order and I would have to wait a few weeks... both pistons are 94,97mm despite different selections

unfortunatelly the yamaha dealer in my city isn`t too good. i asked them to measure D1, D2, D3, D4, D5, D6 as the manual says... and they just told me what size piston I should install.. never more will I give them any of my stuff to tamper with! :thumbsdn: 

again Gray - thx for the help!

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