It depends on the engine you're running. Typically if you run either a flathead or clone engine, your chain should be approximately 40 1/2". If you run an Animal engine, it should be approximately 39".
James, Thank you! we have currently been running flat heads but are looking to switch to clones this season. I have my boys entering their third season & will have to getting new seats & remounting them etc so I want to get everything set correctly for the new class they will running this season. Thank you again.