In order to convert an Advance Design Series truck (Pre- '55 2nd Series) to 4 wheel drive, it needed to be a 3/4-ton or 1-ton unit. The 1/2-ton, 3/4-ton and 1-ton Task Force Series trucks ('55 2nd Series and later) could all be easily converted. The conversion kits were different for Advance Design and Task Force Series trucks.
Two different axle styles were used, and were either six bolt or eight bolt (referring to the number of lug nuts used.) On the six bolt style axle, used for the Advance Series conversions ( and commonly used on later 1/2 ton trucks,) the differential was offset to driver's side of the truck, while the eight bolt axle (commonly used on 3/4 and 1 ton trucks) had the differential offset towards the passenger side of the truck. On some earlier six bolt axles, adapter plates were used to convert them for use with eight lug wheel rims, for use on vehicles with eight lug rear axles.
Several optional gear ratio's were available, including 4.57:1, and 5.14:1. The 1 ton units came standard with the 5.14:1 gear sets. Owners or installers also needed to ensure that the gear ratio of the NAPCO front axle closely matched the one in their rear axle as well. Each NAPCO front axle has the name "NAPCO" and the part number either stamped into, or as a raised casting that faces front on the longer side of the front of the axle housing. It's located in the general vicinity of the word "Owners" as you see it on our logo at the top of this page.
The NAPCO front axle assemblies were in essence a modified corporate GM axle, and the differential gear assemblies are interchangeable with the GM corporate rear end of the time. Model's HL-52 for the 1/2-tons, and HO-72 for the 3/4-ton and 1-ton units. The 3/4-ton and 1-ton units are the same, with the exception of the gear ratio's. Manually locking hubs were also an optional item.
The modified section of the NAPCO front axle, consisting of the steering and 4 wheel drive components,
use many of the same internal parts as the Dodge Power-Wagon, such as the Rzeppa constant velocity joints. It will vary from
truck to truck (both Dodge and GM) due to the fact that neither could/would leave well enough alone. Dodge has something like
5 major changes between
1945 and 1961. NAPCO made changes through the years before 1957.
Though highly durable and a very rugged vehicle, after some 40 odd years (in my case) the trucks do occasionally need repaired, so back to the search for parts. Earlier, I said that DANA had bought the rights in the early 60's to NAPCO's Powr-Pak 4x4 conversion package and so, the road lead me back to DANA. After much gnashing of teeth and effort, I discovered that DANA had re-sold the "NAPCO segment" of the business in the early 60's to a company called R. Cushman & Associates in Livonia, Michigan. I made contact with Mr. Byron Holly of Cushman and Associates. Byron was employed by DANA for years, and is really a knowledgeable guy!
Cushman and Associates only purchased the rights to a part of the NAPCO conversion package from DANA. The segment that they bought included the SPICER Model 23 & 24 Transfer Case rights, information and NOS parts stock. Once purchased by DANA, our beloved NAPCO had been subdivided...
The NAPCO transfer cases were not manufactured by NAPCO at their plant in Plymouth, Minn. The transfer cases are actually SPICER 23's for Chevrolet, Spicer 24's for Ford products, and were manufactured by DANA at the SPICER plant in Toledo, Ohio. The completed cases were purchased by NAPCO, and were shipped to Minneapolis for inclusion in the NAPCO conversion package. NAPCO re-tagged many of the SPICER cases with the NAPCO Industries tag.
The SPICER Model 23 & 24 Transfer Cases are DANA Part #300176-1 through -8, depending on the configuration of the version. Though Cushman purchased the rights to the Transfer Case segment of NAPCO, DANA did not sell Cushman the rights to the -2, or the -8 versions because there was still a high demand for those configurations. The cases were manufactured in 8 different versions, all having the same case and gear assemblies. The different versions were created based on the application configuration. Flange in, yoke out / Yoke in, flange out / Yoke in, bigger yoke out, etc. Also, some configurations had an 8" by 2" emergency brake attached. All had front and rear PTO accesses. The key here is that the basic cases and gears are interchangeable. The Model 24 cases had the drop on the opposite side from the Model 23 cases, though the internals remained the same.
The Transfer Case is technically a "shift-on-the-fly" case, and you don't have to stop to engage the 4x4. However, I've been advised by several owners "NOT" to do it. I've been told that if your gears are not going all the same speed you will grind, bend or break things. With the manually locking hubs, they need to be "locked in" to engage the 4 wheel drive which defeats the purpose of the "shift-on-the-fly", regardless.
The NAPCO units are reported to be the most rugged ever built, so if you find a NAPCO assembly, your chances of it still being useable are good. NAPCO sales brochures even suggested that the buyer save the beam front axle and reinstall it on the truck at trade-in time; the 4x4 set-up would be removed for installation on the next truck. That advice speaks highly for the unit's durability. As the proud owner of one of these mighty vehicles, I'll endorse those reports and comments as being "Right on the Mark!"
This is about as good as it gets for information to date. There are still a "ton" of unanswered questions out there, and lots of additional information waiting in some dusty drawer to be discovered and shared. You might have some information we can add or correct in our ongoing mission of discovery, so take a minute-or take an hour, and SHARE your story with us at our forum. See you soon for the next in the series on our "Quest for NAPCO".