That opinion seems to vary slightly, The stock track was something like 5/8 or 1/2 inch, and most peeps run a 3/4 to 7/8 inch stud in them with much problems, aslong as the track was propperly adjusted. I have a ~7/8 inch on mine now, with 1 inch studs (I belive it is anyway) and I have had no problems at all. The 1.25 inch track stuff will usally have to be trimmed where the tunnel coolers are so they dont rub, but that will pretty much kill anything for studs that would be longer than the track. Now if you swap a newer proaction or rx1 skid into it, you have to move where the skid is anyway, and can adjust for as big a track as you want to run!