We get asked this question a lot in our mailbag: What should I do to my 250 four stroke? Modify the head or drop a big bore in it? Which is better? Well, when we asked Jay Clark to build us two Yamaha YZ250F’s to do this comparison, he already had purchased two beat up 2015 Yamaha YZ250F’s off of eBay to fix up for him and his son to ride. We asked Jay to modify the engine (with reliability in mind) on the standard bore machine and drop a big bore cylinder in the other to see if we could decipher on which engine character would be the most fun to ride/race. Jay trusted Jamie Ellis from Twisted Development to facilitate all the engine work on both machines, but used a Cylinder Works 270cc big bore kit inside the second Yamaha to compare the oversize cylinder against the standard bore. We wanted to do things as equal as possible to both machines (minus the big bore) so Jay and Twisted Development used the following parts on both Yamaha’s to ensure a uniform set up: Vortex ignition, a dual fuel injector system with a YZ450F airboot, FMF 4.1 muffler system, Cycra plastic, Dunlop MX3S tires, Race Tech suspension, DeVol pull rod, stage one Hot Cams, Crankworks rod kit, Hinson clutch kit, Tusk oversize rotor, Renthal bars and sprockets, Works Connection perch, MotoSeat seat cover, ICW radiator strengthening, Scar titanium footpegs, Uni Air Filters, Twin Air Powerflow backfire eliminator, and to make them look race ready Magik Custom Graphics finished off both machines with their customizable graphic kits.