Building my first set of dual cases and ran into the strangest problem. When I put the crawl box and adapter all the way onto the rear transfer case, the crawl box gears spin just fine, but the gears inside the rear t-case dont spin at all. If I pull the crawl box off the studs about 1/4 inch, it engages and all the gears spin properly. Almost like the input shaft from the rear case goes too far into the adapter and disengages the splines. Also, the input shaft for the crawl box has about 3/8 of an inch of play fore and aft, could this be related? Anyone ever run into anything like this?
Stock 2.28 front, Advance Adapters 21 spline dual case adapter, Marlin 21 spline 4.7 gears in the rear case

 

Have the inside of the rear case and shift forks been clearanced for the 4.7 gears?

 

Bingo!

 

Yes they have. Rear case with 4.7s works perfectly when indepenent from crawl box

 

I may have misread your original post? So the gears are free in the rear case when all the way together, they just dont spin when you turn the front input?

Or is something stopping the rear gears from spinning when all the way together?

 

Correct, gears are free and work great in the rear case. When I put the dual adapter on and spin the front input shaft from the crawl box, the gears inside the crawl box spin but the gears in the rear case dont. Almost like the coupler splines in the back of the adapter arent engaging with the input shaft splines of the rear case. If I pull the crawl box and adapter away from the rear case about 1/4 inch, but still on the studs, I can get the gears in the rear case to spin like they should. Clear as mud?

 

And you're SURE the rear gears arnt spinning (and the rear case isnt just in netural,but it sounds like not because you can get them to spin)? with the cases bolted together, and the rear shifter off, you should be able to look in and see the input of the rear case spinning. These arnt horribly complicates, you should be able to follow the power through the cases and see where it "stops"

If the rear gears really arnt spinning, then I think you identified the problem, and need to look at the coupler and input/output and see exactly why it isnt. Pictures or a video is always helpful

 

I haven't even put shifters on yet and can clearly see that the gears in the rear case dont spin while the gears in the crawl box do. When I get home from work on friday I'll take a closer look and try to see exactly where the power stops. Thanks for the help