I'm currently in the progress of building a narrowed 9" for my car. Yesterday I wanted to take out one of my 1/2" screw in wheel studs but to do that I needed to pull an axle from the 9" that's still in my car. I have Strange 31 spline axles. Even with all the hard transbrake launches I've done over the years with these axles, the axles and axle splines are still straight.

Just goes to show that going to 33, 35 or 40 spline axles can be overkill.

Sure it's possible that the next transbrake launch could easily snap an axle but you risk that every time unless you do go overkill. The axle size formula on the Moser web site says 31 spline axles are more than enough for the amount of power I'm putting to the wheels even though upgrading to 33 would be better. 35 or 40 spline is just too expensive because of all the other changes required.

 

Yeah I would imagine 31 spline axles are probably stout enough to handle quite a bit. I'm still running the stock 30 spline axles in my 69 Camaro's 12 bolt doing hard launches with a 4 speed manual trans, and the axles have never even flinched. In fact, I've broken several Muncies over the years, but the rearend has never broken anything, knock on wood!

 

31 spline parts, spool, carrier etc are also easy to find and are cheaper. I could upgrade to 33 spline and still use the same center section but to go larger, I'd need to upgrade to a large bearing center section.

I just see so many street cars with 33 or 35 spline axles and just wonder why? Even race cars that go slower than me with 40 spline stuff. Even with gun drilled axles, it costs a lot of money for that big stuff and adds a lot of extra weight.

30 spline 12 bolt is very strong. Not sure about factory axles but any aftermarket axle will be stronger than a factory one of the same splines.