Your engine should be a 9 - 503xxx one. As result the 503xxx cylinders should fit that engine. The early ones might be the ones to go for as they have had upgrades through the years (latest one was made for the 2015 125 engine). I do not know what the upgrades were, so your guess re what year cylinder of them you should go for is as good as mine. I have just done another post reply re this. Here: Exc 125 2008 weird cylinder head
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