Wow, no réponse for 4 years. Crazy. There was someone who came to the suzukisavage.com forum a few months ago asking the same question, and the answer is that you cannot swap the 4spd. & 5 spd. gear sets in the cases. If you want a 5 spd. then you need to find a 5 sod. case wight he gear set in it. BUT why would you want to change ? 5th gear is virtually identical to 4th gear in the 80´s model 4spd case so your final gear ratio is no different. The torque of the engine works just fine with 4 gears; 5 gears adds nothing to its performance at all. I had a ´56 BSA Goldstar DBD 500 flat track bike that was a brute of an animal and those 4 gears work just fine to blow the wheels off of anything except the Kawasaki 500 Mach III´s. It pulled so hard it was literally physically hard to hold on to the grips and keep your eyes focused enough to actually see the road. Anyway, the Savage is not of that type of animal, it is a more sedate street bike but still fun to play with, especially when you tweak the engine here and there. DOUBLING the HP is possible with the right touches. Turn it from a Suzuki Savage to a SAVAGE BEAST...or something to that effect. :)))