2004-2009 Honda TRX450R/TRX450ER Service Manual, Page 237Get this manual
CYLINDERPISTON Apply engine oil to the piston pin holesInstall the piston with the "IN" mark toward the intake side and insert the piston pin through the piston and connecting rodDo not align the clip Install new piston pin clipsend gap with the piston cutoutNr.TF NOTE
PISTON PIN
Make sure the piston pin clips are seated securely