A couple months back I installed a fresh top end on a friend's bike, a '99 Cr125. The piston came without lubrication holes at the exhaust side. So, I thought it wouldn't make any harm to add a couple holes, quite the opposite I reckon.
The piston was temporarily placed in the cylinder with the piston ring just above the exhaust port roof and the exhaust bridge layout was then scribbed on the piston skirt.
Two marks were then punched on the piston, from memory it was about 8mm and 18mm below the piston ring. The punch marks were then spot drilled on the Dore Westbury miller and then drilled through with a 1.5mm drill.
For more info on the subject, check out this older blog post
2t-racelab.com - piston preparation
and also this nice article from the Wiseco blog
blog.wiseco.com - relieving the exhaust bridge and drilling lubrication holes
The piston was temporarily placed in the cylinder with the piston ring just above the exhaust port roof and the exhaust bridge layout was then scribbed on the piston skirt.
Two marks were then punched on the piston, from memory it was about 8mm and 18mm below the piston ring. The punch marks were then spot drilled on the Dore Westbury miller and then drilled through with a 1.5mm drill.
The holes seem a little bit too large due to the chamfering. A drill of 1 or 1.5mm is adequate enough for this modification.
For more info on the subject, check out this older blog post
2t-racelab.com - piston preparation
and also this nice article from the Wiseco blog
blog.wiseco.com - relieving the exhaust bridge and drilling lubrication holes
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