Over the past five years, the rock crawling phenomenon has pushed both individual builders as well as aftermarket vendors to develop options for achieving lower gear ratios where the rubber meets the rock. Nowadays, rigs that are designed to negotiate ever increasingly difficult trails and obstacles typically have final crawl ratios ranging from about 100:1 to more than 300:1! The quest for low gear ratios has included lower ratios in each of the three components of the traditional drivetrain - transmission, transfer case and differentials. Options for lower gear ratios at the transmission are limited by the available gearboxes themselves (e.g., SM420, SM465, TH350, etc.). Likewise, for some differentials such as those in the Land Cruisers, ratios are limited by the available ring and pinion gear sets (usually up to 4.88:1). Aftermarket gears for axles such as the Dana 60 have a wide range of R&P ratios (up to 6.17:1) , primarily built to meet the needs of drag racers. Off-roaders have generally avoided the low ratio R&P gear sets believing that the smaller head of the pinion gear is weaker than that of the higher ratios. I don't believe this since I have never seen a pinion break at the head, it is generaly in the shaft area which is the same size regardless of gear ratio.
The transfer case provides the greatest range of options for achieving low gear ratios. The general options include:
I decided to use a double transfer case arrangement in the truggy. This decision was made primarily on a roughly equal balance of cost and strength. I wanted a very strong drivetrain, but also one that was within my limited budget. The possible options were greatly expanded by the long wheelbase of the truggy project relative to an FJ40 or other SWB rig. The 4.3:1 Atlas is an excellent choice for lower gears, and if I were building a shorter wheelbase rig, I would select that transfer case. I opted instead for a combination of arguably the two strongest transfer cases that have been used in 4WD vehicles - a combination of the NP203 and the NP205. In addition to their strength, these units can be found in salvage yeards for reasonable prices.
The New Process 203 is the largest transfer case I have seen in domestic trucks. It was used in 1971-'80 GM K-series trucks, Blazers and Suburbans, 1974-'76 Ford F-150 and Broncos, and 1971-'80 Dodge Ram and Ramcharger trucks. It consists of three basic components: 1) a cast-iron gear reduction box containing massive helical input, idler and output gears with a low-range ratio of 2.0:1; 2) a cast-iron case housing containing a 3"-wide chain drive for the front output assembly, and 3) a differential gearset allowing for full-time operation of the transfer case, The differential, along with the output yoke and speedometer gear is housed in an aluminum tail houising. The overall size, chain drive and the differential have not made this a popular transfer case for serious off-road enthusiasts. What has made it popular is the fact that the gear reduction box (GRB) can be separated from the rest of the unit and used as the front half of a double transfer case assembly. The image to the left shows a complete NP203 that was originally mated to an SM465. The image shows the SM465 adapter, the GRB, chain unit and differential tail housing. Here is an exploded view diagram of the NP203.
The image to the right shows the GRB with the original output shaft which connects to both the chain drive and the differential. As a doubler, this output shaft must be shortened and new splines cut to mate with the transfer case mounted to the rear of the GRB. Aftermarket shafts and adapters are available to mate the NP203 GRB to transfer cases such as the Land Cruiser transfer case (Advance Adapters) and the NP205 (Off-Road Design ).
NP203 transfer cases are common and relatively inexpensive in salvage yards ($50-$150). The frustrating aspect of NP203 gear boxes is that they utilize a female input shaft that connects directly (no spud shaft or coupler) to the transmission to which they were originally mated. This means that each original transmission-NP203 combination used a different input gear, varying in both length of the input shaft and the spline count (e.g., 10-spline for the SM465, 27-spline for the TH350, and all other combinations used on Dodge and Ford transmissions). Even more frustrating is the fact that perhaps the most popular combination for today's conversions, namely the SM465-NP203, was the most rarely used OEM combination. The 465-203 combination was only used in 1974 and rare '75 Chevy Blazers. Consequently these 203 cases and their adapters are very difficult to find.The NP203 that I ended up using was from a 1978 Dodge. I purchased separately the adapter and a new 10-spline input gear. In many cases, purchase of a new adapter and input gear may be the most effective means to obtain these parts.
The New Process 205 was used in 1971-'80 GM K-series trucks, Blazers and Suburbans,Dodge Ram and Ramchargers and 1976-'79 Ford F-150 and Broncos. In contrast to the 203, the NP 205 is a part-time, all-gear transfer case. It features an all cast-iron case, massive helical gears and large input/output shafts. It is arguably the strongest transfer case ever built for 1 ton and smaler 4WD vehicles. The downside of the NP205 is its low-gear ratio (only 1.98:1) and small sized case, thus preventing development of aftermarket gear sets that might improve the low-gear ratio. Although compact, it is a heavy transfer case, and I have yet to hear of one breaking. Here is an exploded view diagram of an NP205.
Like the NP203, the NP205 came with several input gear configurations and evolved over time. GM used 4 different input gears in the 205:
I had access to a very reasonably priced 1991 NP205 with the long 32-spline input gear, and chose to simply replace the input for the shorter TH400 style input gear. I also selected the Off Road Design adapter. Steven Watson at ORD was very helpful in customizing the adapter for my late-model 205 housing and in clocking the 205 so that its bottom is level with the bottom of the NP203 and the SM465. I am pleased with the quality of the fabrication, and the ORD adpter and output shaft make for an easy conversion.
The complete assembly of gear boxes is shown in these images. The total length of the gearboxes is 37.75 inches and includes the SM465 (12"), 465-203 adapter (4.5"), NP203 GRB (5.75), 203-205 adapter (3") and NP 205 (12.5" from front surface to the center of the rear output yoke). Although too long for a short-wheel-base FJ40, the 105" WB of the truggy is ideal. The combination of big iron cases makes it heavy compared to an Atlas II, but I doubt anyone would question the strength of these gear boxes. After breaking several Land Cruiser transfer cases over the years, I look forward to having a builletproof drivetrain.
In addition to replacing the NP205 input gear with a new unit, two additional upgrades were made to the primary transfer case.
1. Twin Stick Shifting:
In the stock configuration, the NP205 is shifted by a single stick shifter capable of accessing N, 2WD high range, 4WD high range and 4WD low range. The 205 has two shift-fork/rail systems -- one controls the range and the other engages the front wheel output shaft. The two shift rails are connected to each other by two mechanisms: First, at the shift lever relay rod (this allows the box to be shifted with a single shift lever); Secondly, independent movement of the two rails is restricted by two spring-loaded interlock pins, oriented perpendicular to the shift rails and constrained to detents ground into the shift rails.Shiftrails (#36) and interlock pins (#37) are shown in the NP205 Schematic diagram. The shift lever connection (#35), located at the end of the shift rails can easily be removed and the single shift lever replaced by a twin stick assembly, thus allowing control of the two shift rails. However, this modification alone does not allow for any sort of independent movement of the two shift rails. The interlock pins still prevent independent movement. The interlock pins are important because they limit the movement of the shift rails and hence the gear combinations that can be selected. It is possible to remove the interlock pins entirely, but that would allow for completely independent shifting along the two shift rails. This would allow, for example, the transfer case to simultaneously be placed in rear wheel low range and front wheel high range -- obviously not a good thing. Fortunately a compromise exists that allows for independent control of the front and rear output, but not mixing of high and low range. This modification allows one output to be shifted into neutral while the other is in either low or high range. Specifically useful is the possibility of shifting the rear output into neutral and keeping the front wheels in low range. With the modifications described below, one shift lever controls the rear output while the other controls the front. Shift options for each include H - N - L, but a mix of H and L between front and rear is still prevented by the interlock pins. The Atlas also provides this capability and a Dana 300 can be modified to do it as well.
This is a very easy modification. It simply involves increasing the size of the two detents in the range shift rail. In order to do this, the shift rail must be removed from the transfer case. The procedure is easiest if the inspection cover is removed, thus allowing access to the shift fork.
First you need to remove the detent ball and spring (blue) that connects the range shift rail with the front interlock pin. I also recommend removing or at least loosening the spring on the other end of the interlock pin (the one connexting the front wheel drive shift rail). Simply remove the cap and spring. The ball can be retrieved with a small magnet.
Remove the small rubber freeze plug located between the two shift rails and oriented perpendicular to the range shift rail. Removing this plug allows access to the shift fork. The shift rail is connected to the Hi-Lo range shift fork with a roll pin pressed through the fork and rail. With an appropriate punch or allen wrench drive the roll pin out through the fork and rail. It will fall down into the case, but it can be retrieved through the inspection cover. You should now be able to pull the range rail out of the case. If the shift rail will not come out, you might have to shift the front wheel drive shift rail into neutral. In order to prevent the shift fork from moving once the rail is removed, place an allen wrench or punch through the tunnel in the case and the hole in the shift fork.
The key to this modification is to increase the size of the indents in the range shift rail only. The image to the left shows how I modified the rail. Each detent was enlarged by 7/16" (0.4375"). The exact value does not appear critical and I do not know how much slop in this value is acceptable. Note that the detent at the rear of the shaft remains abrupt and the detent on the front remains a slope. Enlarging these detents allows the range shift fork, controlling the rear output shaft, to be shifted to the neutral position while the front-wheel-drive shift rail remains in either Hi or Low, and allows the front-wheel-drive rail to be shifted into neutral while the rear remains in either Hi or Low.
The three shifters I built for the transfer cases are shown in the picture to the right. The stick to the rear shifts the NP203 (H-N-L) and the two sise-by-side sticks are for the NP205. The 205 stick on the left controls the rear wheel drive (H-N-L) while the stick on the right controls the front wheel drive (H-N-L).
Electronic Vehicle Speed Sensor:
The gear boxes in the truggy are mated to a Vortec engine identical to the one in my FJ40. A critical component of the Vortec sensor/computer system is the Vehicle Speed Sensor (VSS). The Vortec computer uses the signal from the VSS to monitor and control a number of important engine parameters, including the fuel:air mixture. I wanted to add capabilities of adding a GM VSS to the NP205. This conversion includes replacing the standard speedometer worm gear with a steel reluctor ring and adding a GM electronic vehicle speed sensor (VSS). The VSS consists of a magnetic tip and coil that produces an electronic pulse each time one of the "teeth" of the reluctor ring passes by the tip. The number of pulses produced per minute is proportional to the rpm of the output shaft of the transfer case.
The most challenging part of this little project was making a new boss in the NP205 tail housing to hold the GM speed sensor. A traditional speedometer gear on the output shaft of the T-case drives the speedometer cable by a gear that sits below the output shaft. In contrast, the VSS must be oriented exactly perpendicular to the output shaft and reluctor ring. This required filling in the old speedometer boss and machining a new one for the VSS. I was worried about welding so much on the cast iron tail housing and the potential for warping the housing. I called on my friend Senkovitch who has expereince welding cast iron. The procedure we used included slow heating to bring the entire housing to a temperature of near 700 degrees. We then welded in the old boss using a TIG welder and a large cast iron welding rod. Following completion of the welding, we insulated the housing and allowed it to cool overnight. The next day we put the housng on the lathe and checked for any warpage. The runout on the bearing journal was less than 0.002" so we were satisfied we had not induced any deformation or warpage.The image on the left shows the housing with the original boss -- note how the speedometer fitting angles down toward the bottom of the output shaft. The image on the right shows the new boss machined into the tail housing. I build a jig to hold the housing on the table of the milling machine and machined the weleded area flat. I then drilled and tapped the hole to fit the GM sensor.
The image to the left shows the output shaft of the NP205. The inner set of splines held the original teflon speedometer gear. Some of the later model NP205s used a reluctor ring instead of the traditional speedometer worm gear. I purchased a 40-tooth reluctor ring for a 1991 NP205 and that fit perfectly on the output shaft. It turns out that GM did use an NP205 with an electronic VSS in 1991 model year one-ton Crew Cab trucks. To my knowledge, that was the ONLY application of an NP205 with a VSS. The transfer case and tail housing are essentially impossible to find. Reluctor rings are avaialble as the same size and spline count were used in the later Borg-Warner transfer cases. The image on the right shows the output shaft with the reluctor ring installed.
The image to the left showns the completed tail housing with the VSS installed in the newly machined boss. The image to the right shows the tip of the VSS (arrow) located approximately 0.040" from the surface of the reluctor ring.
The only other modification I am considering for the NP205 is to add a disc parking brake and new driveshaft yoke on the rear output shaft.
Here is a view of the complete drivetrain, showing the Vortec engine and the gearboxes.