So since I have a spare laying around I went ahead and pulled the ADD system apart as far as I could without a lot of work to take a more in-depth look at it.
So here's the overall system. Moving from left to right we have...
The ADD stub shaft (also known as intermediate shaft No. 1 in the Toyota parts catalog)
The ADD locking collar (this is what allows for the part time 4wd function on these trucks, depending on what year they were either electronic or vacuum operated, mine be a 98 is vacuum operated.)
ADD actuator housing, top, and the actuator itself, bottom
Axle side intermediate shaft (intermediate shaft No2 in the Toyota catalog.)
ADD tube that then carries the intermediate shaft out to where the CV itself interfaces.
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So now the measurements. We have seen on other peoples rigs in the past, Dirtco running Lotus shafts and Big Fish All Day running stock shafts, and myself running stock shafts, that the stub shaft is the next glaring weak link. This stub shaft is also the same size as the CV stub that goes into the diff.
1.100 inch diameter measured on the outside of the splines. We know this has been improved on the RCV CVs by being replaced with 300M steel vs whatever alloy the factory used
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On the Opposite side where the ADD locking collar engages. You can also see a large portion of the stub is gun drilled to allow for the locating bearing and shaft that keeps the shaft centered when the ADD is disengaged.
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An interesting thing I noticed about both of Dirtcos failures was that shaft appeared to shear at the same spot where the gun drilling stopped and right at the edge of the splines. To me, this indicates a possible structural flaw due to the contours of the splines combined with the gun drilling there that is causing a stress concentration. Or it could just be at the outer limit of the material itself and it isn't capable of supporting the load. These pictures are from his most recent failure
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So now the ADD locking collar. Pretty basic here and it measures out to have plenty of spline engagement and spline thickness IMO. I would highly doubt this as being the next failure point after the stub shaft. Unless it somehow cracked clean through essentially exploding the entire collar.
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After the ADD collar is the outer intermediate shaft. This shaft is substantially larger in diameter than the stub shaft and it carries this diameter all the way out to where the CV locks into it. I don't see this being much of a concern either since it is not gun drilled which increases the strength some, and is also much larger in diameter. Will it fail? at some point, yes, but most likely not in our use cases
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Just after the splines on intermediate shaft No 2.
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After seeing these measurements I think there is a strong likely hood the stub shaft is the bottom of the rabbit hole. After that, you will be either breaking the second intermediate shaft, ADD collar, or R&P. But again I don't see either of these failing even with dual cases, 35s, and a front locker on, and if it gets to the point where the R&P is blowing I don't think there's anything to do there. Part of it too is just being very selective about the front locker use. I don't know if you guys are buying shitty replacements, just turning the locker on and thrashing, or what but even considering the terrain your guys have been wheeling lately and the vehicles involved the breakage seems excessive.
@theesotericone at this point I can guarantee you will break that stub shaft. Will it happen immediately, no, but its just a matter of time. Shit I started twisting the splines on mine with 265s and an open diff. I am going to reach out to RCV at some point this weak hopefully and see what they say about making a 300M or 4340 stub shaft with.
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