Chevy SFA Transfer Case Guide

Mar. 01, 2005 By Shane Wager

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Last time I took you through the most common of GM transmissions. That sure was fun eh? What then, backs up that transmission, what differentiates your tough, rock chunking four wheel drive truck from captain pre runners admirable dune jumper? The transfer case that's what.

The transfer case serves two purposes in an off road vehicle. One of it's functions is splitting the power from the transmission between the front and rear axle. The vast majority of cases do this by means of gear drive or chain drive. The second purpose it serves is to increase the ratio of engine revolutions opposed to tire revolutions. This is also a consequence of the gear or chain drive splitting the power between two axles. Oh, what marvels.

Throughout the general era of the straight axle GM truck, that wonder of wheelers, Gm used mainly New Process cases, and used only three for the most part. There are the few odd Rockwell cases in the very early years, but in general, NP is what comes for cases.

First I will go through the cases' specifications, and then discuss each individually. Following this I will discuss mounting options and modification.

NP20

The NP203 is a chain driven full time transfer case. Four wheel drive is in use at all times. The overall length of the NP203 case is 22"-23" from the face of the case to the rear output yoke's center. The shift positions of the NP203 are as follows from the most forward position (towards the engine) to the rear most position: Low Range Lock, Low Range, Neutral, High Range, High Range Lock. The NP203 has a high range of 1.00:1 and a low range of 2.01:1

When not in lock, whether high or low, a differential at the rear of the NP203's case lets the driveshafts turn at different speeds. This allows street use, and generally puts less stress on the drivetrain. According to my 1979 K5's visor sticker, street use of an NP203 equipped vehicle in low is acceptable to 35 miles per hour. When in lock, whether high or low, the differential in the case is locked, causing the front and rear driveshafts to turn at the same speed.

The front axle uses solid slugs in combination with the NP203 as there is no need to disconnect the "wheels" from the axle shaft. The speedometer cable connects to the NP203 on the drivers side of the case. The NP203 has fill hole for lubrication in the approximate middle of the rear face of the case. There is no drain plug in the NP203. Fluid is drained by removing the bottom bolts on the passenger side cover. The NP203 uses 10w-30 motor oil as for lubrication The shift linkage of the NP203 is adjustable.

Most manuals for appropriate year trucks will have instructions on how to adjust it properly. The NP203 is composed of four sections. The differential housing, the output housing, the chain case, and the range box. The forward two sections of the NP203 are made of cast iron, and the rear two sections are generally aluminum. The model tag is on the front of the chain case, above the output shaft. The NP203 assists in traction on almost every level, but can decrease gas mileage and wear on the front axle The NP203 was available from 1974 to 1979.

NP205

The NP205 is a gear driven part time transfer case. It can be operated in either two or four wheel drive. The overall length of the NP205 case is 12"-13" from the face of the case to the rear output yoke's center on fixed yoke models. The shift positions of the NP203 are as follows from the most forward position (towards the engine) to the rear most position: Four Wheel Drive Low Range, Neutral, Two Wheel Drive High Range, Four Wheel Drive High Range When in Two Wheel Drive, the NP205 does not turn the front driveshaft.

The NP205 has a high range of 1.00:1 and a low range of 1.96:1 The front axle uses either manual or automatic locking hubs in combination with the NP205. Unlocking the hubs in two wheel drive save wear on the front axle shafts. The speedometer cable connects to the NP205 in the general are of the top center of the rear of the case. The NP205 has fill hole for lubrication in the rear face of the case under the rear output shaft. The drain plug for the NP205 is on the rear face of the case on the lowest part of the case.

The NP205 uses 80w-90 gear oil for lubrication The NP205 has a three bolt cover over the idler shaft. This is one of the identifiable traits of the NP205. The shift linkage of the NP205 Is not adjustable. It operates on a simple lever with few parts. The NP205 case is one piece of cast iron. All of the working parts are in this single housing. The identification tag of the NP205 is located above the front driveshaft output The NP205 was available from 1973 to 1991.

NP208

The NP208 is a chain driven part time transfer case. It can be operated in either two or four wheel drive. The overall length of the NP208 case is 12"-13" from the face of the case to the rear output yoke's center on fixed yoke models.

The shift positions of the NP208 are as follows from the most forward position (towards the engine) to the rear most position: Two Wheel Drive High Range, Four Wheel Drive High Range, Neutral, Four Wheel Drive Low Range The NP208 shift pattern is not straight forward, it has to be pulled to the side and back to enter four wheel drive low range mode.

When in Two Wheel Drive, the NP208 does not turn the front driveshaft. The NP208 has a high range of 1.00:1 and a low range of 2.61:1 The front axle uses either manual or automatic locking hubs in combination with the NP208.

Unlocking the hubs in two wheel drive save wear on the front axle shafts. The NP208 uses automatic transmission fluid for lubrication The shift linkage of the NP208 is adjustable. Most manuals for appropriate year trucks will have instructions on how to adjust it properly. The NP205 case uses an aluminum case The identification tag of the NP20 8 is located on the rear face of the case The NP208 was available from 1981 to 1987.

Mounting Options

With the NP205, it is possible to make the case divorced. Divorcing involves separating the transfer case from the transmission, mounting the transfer case at the center of the area between the front and rear axles, switching the transmission to a two wheel drive model, running a small driveshaft from the transmission to the transfer case, and moving the linkage to work properly. This allows the front and rear driveshaft to be equal lengths. This evens out the angle of both shafts allowing the same chance for contact on each. This also allows you to swap front and rear shafts, and carry fewer spare shafts.

Modifications

NP203

The NP203 can be converted to part time operation with a kit that is commonly available. This requires the front slugs to be replaced with manual locking hubs for full effect. The operation of the NP203 is now very similar to the operation of the NP205.

The NP203's range box can also be used in conjunction with an NP205 to create a three speed gear box. Off Road Design invented and sells this product. It is a very strong option to get 4.00:1 low range.

3:1 low range gear sets are available

NP205

NP205 models with slip yoke can be easily converted to fixed yoke

The NP205 can be used in conjunction with a NP203 range box to create the aforementioned doubler

The NP205 shift rails can be modified for twin stick, allowing the independent shifting between high and low range, and two and four wheel drive.

NP208

There's not much to be done with this case. Some have tried to make their own slip yoke eliminators with modest success.

Well, that's about it, have fun