like any other mechanical assembly, there are all kinds of "tricks" involved in busting into a d20! I'm no expert and since I do maybe two a year, I haven't developed any proficiency!
But...I do have a major collection of d20 cases busted down and cleaned on the shelf to serve as potential "builders" or parts donors. And that serves me well to refresh my memory when I do bust into one for a build, most folks are not so fortunate!
I certainly agree with ya, even with all the info on these units on the net (the novak stuff is the most complete), the IH service manual is meant for a mechanic with training and experience, it's not the best for someone just learning this stuff!
Scoutboy74 (trever) and I are working up a "redneck" how-to-build video for the tf 727 now to post here exclusively for the use of our members. As soon as that is in the can, we're gonna do the same regarding a d20! But doing a video even redneck-fashion is very time consuming so it doesn't happen overnight!
Since the case you have is not a pressing issue, then go right ahead and use it in learning mode! At least I'd get the mung out of it and see how corroded it appears inside, I don't wanna discourage ya, but I'm just speaking from experience that this stuff that looks nice on the outside, can be a total disaster internally, to the point that nearly no component is useable!
As for using the bull gear that is on your t90 tranny right now and installing on a Scout II 727, no, that will not work. The spline design is completely different. If ya go to the novak website, you will find a fairly complete spreadsheet with an analysis of this issue that includes the IH variations! So what you will need is the bullgear for the 727, then it must match the intermediate gear "pitch" angle. But ya simply do not know which intermediate gear is installed in the tc right now if ya don't look inside and get the part number off the gear (need a flashlight and mirror to do that). Once ya know what ya have, then we can help decide what ya need to make it work. If you are going to overhaul the tc anyway, that is the time you can swap around any internal parts/gears as needed, we May have good used stuff that will help ya in that regard!
There are hundreds (if not thousands) of jeeps, broncos, and scouts running around out there with the wrong bull gear/intermediate gear pitch mis-match! Making much noise, jerkin' around, and failing left and right. But folks that boneyard that stuff just blame repeated failures on "crappy parts", "Dana 20 cases are weak", etc. And this issue is compounded by the fact that new parts for these cases haven't been produced by spicer/Dana in many years, though new parts are on the shelf through many vendors (as well as good/correct used parts).
Everything is in your favor right now for learning how to do this! Take many pics and ask many questions!! I have an early d20 twin-sticker (Scout 800 with t-18) now that needs to be freshened, that's the one that trev and I will use for the video shoot. And guess what...that one had water in it also but it's not damaged in any way!
For teardown, common handtools are used, you will need a big hammer, some soft "drifts" of various sizes, a nice set of allen wrenches (tee handle). But nothing really special that ya can't make on your own if need be...ya gotta be creative and be patient!
Warning though...a good bull gear for a 727 can be a major bitch (and expensive) to find! The last two I sourced came from a transmission wholesale supply back east and were used. But a "marginal" gear can be repaired using a hand-grinding process similar to what a dentist would do!
Here's the novak link regarding the parts interchangeability:
the novak guide to Dana 18 & 20 gears interchange
