Transfer case info

[kipps]

I was messing around with the 'Vehicle Information Kits' on the GM heritage website, and found some interesting info about transfer cases.

In 1973, the np205 was the only option for the k10 and k20 trucks.

From 1974 and 1975, the new np203 was used on the v8 trucks, and the np205 continued to be used on the i6 trucks. For 1974, the np205 could have been special ordered for the v8 trucks.

From 1976 through 1979, the np203 was used on the automatic-trans trucks, regardless of engine type. The np205 was used on the manual-trans trucks, regardless of engine type. This applied to the k30 as well, when it was introduced in 1977.

I suspect the reasoning here was that GM considered the np203 and np205 to be equally strong. They were favoring the convenient full-time 4wd for the automatic trucks, and figured if you were willing to shift gears, then you would also be willing to lock in your own hubs.

In 1980, the np203 was no longer an option, and the np205 was the only transfer case available for this one year.

Up until this point, the np203 and np205 both had the classic figure-eight bolt pattern, and they had differing spline counts depending what transmission was bolted to the front. The th350 had 27 splines, the th400 had 32 splines, and the sm465 had 10 splines.

I don't know what the spline count was of the heavier-duty and lighter-duty three-speed column-shift transmissions. I suspect they had the same 10-spline count as the sm465, but that's a guess only.

From 1981 through 1984, the k30 continued to use the np205 only. If the np205 was bolted to a sm465, it used the figure-eight pattern and 10-spline shaft that it did before. If the np205 was bolted to a th400, it used the figure-eight pattern and 32-spline shaft that it did before. The np205 was no longer being bolted to any of the lighter 27-spline automatics.

From 1981 through 1984, the k10 and k20 used the new np208 exclusively. This transfer case was only available in round pattern, and 27 & 32 spline input shafts. There was no 10-spline option for the np208. Because of this, the sm465 and th400 switched to the round-pattern 32-spline shaft when bolted to this np208.

The three-speed th350 was phased out and replaced by the four-speed 700r4 between 1982 and 1984. These two transmissions were the only ones that used the 27-spline shaft. These were used in the k10 trucks, and the light-duty k20 trucks.

In 1985, GM finally simplified the lineup. They dropped the 10-spline and figure-eight pattern altogether on the np205. Everything was now round-pattern, and was only 27-spline or 32-spline.

In approximately 1988 or 1989, the np208 was phased out, and replaced by the np241. The two transfer cases are identical in fitment; the np241 is significantly updated over the np208.

The np205 was last used in the 1991 crew-cab trucks.

The military m1008 k30 trucks were model years 1984 through 1987. These were different from the civilian k30 trucks, in that they used the np208/th400 combo exclusively. These did not use the np205. Edit: the m1028 k30 trucks did use the np205/th400 combo.

Any corrections to this list are welcome. Also, if anyone can shed light on how GM chose between the th350 and the th400 during the 1970's, that would be helpful. The GM literature I was reading didn't make any distinction between the two.