5 speed allison wondering how far i can take the stock shafts? I did convertor n clutch packs Im shifting at higher rpm so i think that will help
How much hp will stock shafts hold
- Thread starter Dave L Weaver
- Start date
sure guys have lasted well over 1000hp imho go for it... im on stock shafts and am close to 700hp
All shafts are different. I've seen shafts break at 600 hp and seen them hold 1,000. Where on the list are you is anyones guess.
I believe tuning plays the biggest role in when things break. I was only running a 20/20 tune in my BURB and twisted the frame under my BURB some. I'm now running a good deal bigger tune without the frame twisting problems.
I don't know how much power a stock shaft can hold, but I DO know that they are made out of 4140 (or a very similar alloy) and heat treated to 28HRc, which makes their tensile and yield strength 130ksi and 115ksi respectively. I annealed, rehardened, and tempered my 4wd output shaft at 800 degrees, which increased it's hardness to 40HRc. That corresponds to a tensile and yield strength of 181ksi and 165ksi respectively, which is a strength increase of more than 40%. I did this in order to compensate for the reduction in strength caused when i reduced the diameter of the shaft and resplined to 32 spline, which is .100" smaller in diameter than the stock 29 spline configuration. After the heat treat, I have a bit of a net increase in strength over a stock 29 spline according to the math. We will see how it really works after I beat on it 
I think the only reason Allison doesn't harden them further is cost. They can get the material pre-hardened to 28 rockwell from the steel mill for very little added cost over annealed material. At 28 rockwell, machining operations leave a nice finish without much if any added tool wear, and once machining is done it's ready to be assembled into a transmission, and the strength at this hardness is adequate for it's intended use. If they started with annealed material, they would have to turn and spline the shaft, heat treat it, pickle and/or bead blast it, then finish grind the journals where bearings or bushings ride. Doing it their way saves money and it will never fail in a stock application.
I think the only reason Allison doesn't harden them further is cost. They can get the material pre-hardened to 28 rockwell from the steel mill for very little added cost over annealed material. At 28 rockwell, machining operations leave a nice finish without much if any added tool wear, and once machining is done it's ready to be assembled into a transmission, and the strength at this hardness is adequate for it's intended use. If they started with annealed material, they would have to turn and spline the shaft, heat treat it, pickle and/or bead blast it, then finish grind the journals where bearings or bushings ride. Doing it their way saves money and it will never fail in a stock application.