Update: Did some surfing trying to research this hole in the cylinder and found out it is there on all of them. Even the Honda 70cc cylinder head has the same "hole". Not sure of its purpose but its on all of the 50-110cc chinese cylinder head replacements. If you go to Ebay and type in Honda 70 cc cylinder head you will see the pic. Its the 3rd pic and the hole is in the lower right corner.

 

Here is the link to the pics on Ebay:
http://cgi.ebay.com/ebaymotors...0245780420QQddnZParts

Q20Q26Q20AccessoriesQQadnZATVQ20PartsQ20Q26Q20Acce ssoriesQQddiZ2811QQadiZ2869QQcmdZViewItem?hash=ite m160245780420&#ebayphotohosting

 

The 90cc Aeon I bought used and abused has no holes at all in it. And this thing was beat to death.

 

Whatever it is it does not seem to affect anything. On all of the new replacement chinese cylinder heads and the Honda its present. Check out the link to the Honda head on ebay and check it out.