I'm having trouble finding out an answer to this on the 'net. What is the coarse spline- is it the pinion itself or just the pinion flange? It seems so unclear to me and i can't get the images in my head of how it goes together.

The splines on the end of the pinion, opposite the teeth that engage with the ring gear. The splines match with the pinion flange, where your driveshaft bolts to. OEM 40 pinions are coarse. Aftermarket are fine spline. More splines=better. Other than 40 axles, I don't know. Seems like someone told me that 62s and 80s are fine spline from the factory, but I don't know that for sure.

Good luck, Luke
Picture shows a ring, pinion, bolts, bottom left corner is pinion flange. Thanks to Marv Spector for letting me "steal" his picture.

thanks, i just didn't think that one through. i'm a dumbass today. for some reason i thought the 'cruiser 3rd was different. i've set up gears in a 44, a 60, and a 14 bolt, so i should have known the answer to my own question. sorry for the wasted bandwith.

toyota switched cruisers to fine spline at some point...can't remember if it was when they switched to 3.73s. Well some of the cruiser fanatics on here could tell you in a heartbeat and likely will shortly.

PS The toyota pickups all came with fine spline pinions...the flange between a cruiser fine spline pinion and a toyota fine spline pinion will swap. They are the same except for the bolt pattern.

78 is the year, 79 they switched to 3.7
fj62 are also 4.11 fine spline

This may help ;)
http://www.cruiseroutfitters.com/tech_flange.html
&
http://www.cruiseroutfitters.com/lock_pinion_flanges.html
(Not pimping, just showing the dimensions and years)

thanks kurt :) just how prone are the coarse spline pinions to breaking? we're talking 2F, SM420, 35s.

thanks kurt :) just how prone are the coarse spline pinions to breaking? we're talking 2F, SM420, 35s.

Its really hard to quanitatively (sp?) compare the two, but it is fair to assume the fine spline is "much" stronger. Also figure in the fact that the fine spline ones are newer too, much less fatique & stress than an older set may potentially have. Would I be hesitant to run them in a rig like yours... Nope, get a good wrap/traction bar setup and run it. If your axles torn apart and your in a good position to upgrade, do it.

When I swap to fine spline and drop in aussie lockers will I have to re-set up my gears or just set my pinion depth and check the pattern?

Don't mix-n-match ring and pinions... In order to get the fine spline pinion you want, you should get a matching ring gear and set them up properly. The Aussie locker install won't affect your gear pattern. Again, don't mix and match!