Fixed the driveshaft today. So what I bought was 2 x Traxxas 6852x. These are the heavy duty rear driveshafts for the Slash 4x4, and according to the internet they are an upgrade over the stock VXL shafts, which are themselves an upgrade over stock ECX shafts.
First thing I noticed was that on the Traxxas driveshafts, the outer tube connects to the wheel side instead of the motor side. So you can't swap just half of the shaft, it's all or nothing. But when you look at the Traxxas axles, you see that they are different.
The thread end is the same diameter, and the bearing end is the same diameter, but the ECX axle has an intermediate part so it shores up against the inside of the hex, which has different diameter inside and outside holes. Here's the hex from both sides.
The solution is simply to drill out the hex with a 5mm bit.
Next problem is on the motor side. The Traxxas shaft has two little plastic tabs to align it on the diff shaft, which won't work with the round ECX diff shafts. Also, the traxxas motor side cup is not deep enough to slide over the diff shaft far enough to use the lock pin.
Both problems are solved by drilling out the motor side cup all the way through to the ball, again with a 5mm bit. You may also want to shave just a bit off the inside collar, as the plastic has some manufacturing debris on it and it is slightly longer than the ECX collar. I took off maybe a quarter millimeter with a cutter. Then it slides on fine. CAUTION: the Traxxas lock pin is not a metric size. The tiny hex key you get with your ECX will fit, but it is a bit loose and you risk stripping out the lock pin. What you need is a 1/16" hex key.
One final note: the Traxxas axle is slightly shorter. It will only *just* reach the rubber lock ring on the wheel nut. Traxxas put some blue locktite on it, I put some extra on it just to be sure. Here you can see the thread reach.
I put a battery pack through it and it held up fine, but then again the stock shaft held out fine for maybe 30 packs so time will tell if this really is a more solid option.
let me know if you got cheaper options, and I'll get back to you when I've been to the LHS.
