I wanted to share a write up by a guy named Matt (aka 7387 chevynut) from the wegotmud.com forum.
It's a very well written piece and if you scroll down about 3/4 of the write up you'll find a nice 72' K5 that I'm sure we'd all love to have sitting in the driveway!

I've been wanting to do this for quite some time now.........will take some time to get it typed out and get my photos downloaded on here. So it will take me weeks to get this put together. Much of this information, is from GM service manuals from 60's 70's and 80's............and along with the stock 1967 GMC k2500, 1972 K5 blazer, 1973 K20, 1976 k20, and 1979 gmc k10 trucks that I own and the factory drivetrains they came with when new back in the 60's/70's.

You chevy guys will learn somthing from this (study guide) always somthing new to learn. I'm always amazed at all the little details, and changes that GM offered or did to these old 4x4s when they designed them.

I will start with the 1967 model year & body style chevy 4x4. The new for 67 body style chevy truck was revolutionary compared to other trucks at the time, it was also the 1st year GM began offering its 4x4 with Transmission and T-case bolted together, pre 67 chevy 4x4s (1966-back) had divorce mounted transfer cases. GM made 4x4 trucks as early as 1960, and before.............but it wasnt until the mid-late 60's that they really began to produce them on a large scale............because 4wd was becoming more and more popular. All GM (factory built 4x4s) made before 1970..........were only available with a manual transmission. The option of a automatic transmission wasnt available till the 1970 model (so mid 1969). Many of these early 67-68-69 models have since been converted to automatic's.........since they were not offered by GM at that time.

1967 GMC K2500, This truck's drivetrain is original (except motor) for the 67 model year. This truck came with the closed knuckled HD 44 front axle, SM420 4spd with 7.06 1st gear, and Rockwell T-221 gear driven transfer case, and dana 60 rear axle. Both axles have 4.56 gearing, and the rear 60 has a factory posi unit.

The front dana 44 on these early chevys were closed knuckled........meaning that the axleshaft/U joint was sealed, and not exposed like 1971-later open front axle chevy 4x4's. The dana 44 front axle was standard on both 1/2 ton.....and 3/4 ton's for 67 to 72 model chevys. Pre 71 models also had Drum brake front ends. Alot of guys like to swap out the close knuckle/drum brake front axles on these early models, because they are a pain to work on, and certain axle parts needed to repair them are very hard to find, or discontinued.

GM offered 2 axle options for its 3/4 tons during the 67-72 body style. The dana 60 was one of them, this axle has its original metal ID tag held in place by the diff cover bolts (its hard to see in photo) (ALL Dana built axles come with a ID tag telling what its axle ratio is but over time the tags can rot off, get lost ect.). It also list's that it has positraction and that special additive is required when changing the gear oil. This axle came with 4.56 gears, but other ratios were offered. The Dana 60 was & still is a very popular axle. FORD, DODGE, both used it throughout the 60's , 70's and 80's as a front & rear axle. FORD stopped using it as a rear axle in the mid 80's when they went to the Stirling 10 1/2 inch rear axle. DODGE used it as a front and rear axle into the 90's........and possibly early 2000's. GM stopped using the dana 60 as a rear axle in 1972 in pickup trucks, HOWEVER......some late model Chevy 3/4 ton vans..........and even Ford Vans have been offered with the dana 60 rear axle. GM began using the Dana 60 as a front axle in its 1977 model 1 ton K30's......and continued to use it until 1991. NOTE: GM did use the Dana 70 as a rear axle option in 73-up dually 1 tons.

All Dana built axles come with a ratio tag, and if equipted with a posi unit will have a limited slip tag on the diff cover. Here is what my 67's rear dana 60 came with. 41-9 + 4.56 gearing.

Dana 60 factory Power Lok limted slip my 67 came with.

EATON HO52 rear axle: This heavy 3/4 ton 1 ton rear axle was the 2nd axle that GM offered under its 3/4 ton 4wd trucks, and 2wd drive 3/4 & 1 ton trucks during the 67-72 period. The Eaton was used by GM since the late 50's/early 60's.....and continued till it was discontinued after the 1972 model year. At first glance, this axle looks like a jumbo ford 9 inch. It does have a drop out carrier (3rd member) and is full floating. Axle ratios ranged from 4:11, 4:57, 5:14,......other ratios might have been offered, but these seem to be the most common. This axle has large axle shafts, but its splines are the old style thick/corse 17 spline (unlike the 73-up 14 bolt which had 30 spline axle shafts) A factory Posi & Detroit Locker was offered in this axle, and finding one can be hard. Currently there isnt anyone making traction units for this axle anymore. Brake parts can be hard to find for this axle.........since it hasnt been used since 1972. The Eaton is comparable in strength to the 73-up 14 bolt, i would personally say they are dead even in strength, but the 14 bolt does have larger diameter brake drums than the Eaton does. Some late 60's early 70's 3/4 ton 1 ton 4x4 International Harvester pickup trucks were offered with this rear axle.

Here is a picture of a 1967 4.57 gear ratio equipted Eaton axle that I have, the ring gear is dated 10/66. This axle came out of a 67 3/4 ton 4x4.

SM420 4spd: This was and still is considered by many 4wheelers as GM's best cast iron granny gear trans, although it is not well known, and not as common as the Sm465 is. It was first offered in 1947 and was used until the end of the 1967 model year. It's short compact size (the case is less than 12 inches long) made it ideal for those wanting to use it in short wheel base rigs. But the 7.06 1st gear made it the lowest geared light truck manual trans offered. Finding one of these transmissions can be hard (especially a 4x4 version) since all 1966-back GM 4x4's used 2wd Sm420's and a divorce mounted transfer case.........and that 1967 was the only year it was used mated to the rockwell T-221 T-case. The shift pattern for this trans is nearly the same as the Sm465...............the only difference in the Sm420.......is that Reverse is to your left & back........instead of right & back on the 465. The SM420 only had 1 inspection cover/pto plate on it, it was located on the driverside as the photo shows.

Here is a picture of Sm420 trans/221 T-case levers inside my 67 gmc k2500, notice the rockwell lever, situated on left side of the trans hump (much like that seen of Ford 4x4's)

ROCKWELL T-221 transfer case: This cast Iron gear driven T-case is the older brother to the NP205, its low range is 1.86 (less than a Np205)........but these were behind GM's Sm420 4spd...which had a 7.06 1st gear....so that alone made up for it. The Rockwell was used from the early 1960's, till it was discontinued after the 1968 model year (it was replaced by the Np205 & dana 20 with the 1969 model). The Shift pattern for the T-221 Rockwell is ( 2hi 4hi N 4Lo)....4low being shifted towards the bench seat.

Here is a picture of the NEW for 1968... SM465 4spd......bolted to the Rockwell T221..........68 was the 1st year for the SM465....and last year for the rockwell T-221.You will notice the 4x4 lever.........it was re-bent to fit a 69-up (right side hole) chevy floor hump......the rockwell T-case is about the same size as the Np205. but has a large inspection cover on the top side of it. Also the 4spd knob came with a shift pattern engraved on it..........this was not seen on 67-back Sm420 knobs.

The lower photo shows the fill/drain plugs located on the driverside of the T-221.

SM465 4spd: This very popular GM cast iron 4spd is the most popular (granny gear) chevy manual trans. It was used from 1968-1991/2. It came with a 6.54 1st gear, and had the option of PTO on the side of its Iron-cased housing. Several varations exist of the 4x4 Sm465, and this really only depended on what transfer case was mounted behind it. The 1968-1980 10 spline rear output. And the 1981-92 32 spline rear output. Shown here are pics of a mid 70's 465 that is 10 spline...and was used with the Np205. Also shown is a top cover showing the shift forks.

1968: 1/2 ton GM 4x4's came with dana 44 front, 12 bolt rear..... ( some sources claim that GM offered a dana 44 rear axle in its 1/2 tons........but I've never seen one.........and I'm not sure this was offered just in 2wd's or also in 4wd's. 3/4 ton trucks used the HD 44 front axle.........and Eaton or dana 60 rear. 4x4 Trannys were 3 speed on the column or SM465 4spd......Rockwell t-221 was standard T-case.

1969: The 69 model GM 4x4 saw the introduction of the new & venerable NP205. This gear to gear all cast Iron transfer case would be used by GM until 1991. Both Ford and Dodge made good use of this T-case as well. Known for being able to take a hell of beating and survive, its main drawback is that its low range is a 1.96. While the Np205 was used in the 1969 model 4x4's......and another transfer case was offered by GM as an option.

NP205: This cast iron gear driven transfer case was used by GM, Ford and Dodge, and even some international 4x4's, it was used for a number of years. Gm used it till 1991........Ford used it until 1979..........and Dodge used it till 1993. While it is strong in terms of strength/ hard use, its low range is only 1.96 and in the mid 70's ( speaking of GM trucks ) many were used behind manual transmissions that had a granny gear, and that sort of made up for the lack of a good low range. There is no linkage adjusment on the Np205, and in the late 1970's.......(78/79) GM began to offer the NP205 with a slip yoke rear and this continued until it was discontinued in 1991. Shown here is a Np205 that was used behind the SM465, and this has the (flat) (Saginaw) front driveshaft flange that GM switched to in 1977. The Np205 also came with a PTO cover on the side of it.

Np205's can sometimes shift hard going into......or more likely shifting out of 4wd.......this can be attributed to the fact this T-case is non-synchronized, so you must stop or slow up to engauge 4wd.......or push in the clutch. I've seen some guys rock the truck back & forth to release the gear bind on 205's to get them back into 2wd.

The Shift pattern for the Np205 was 4Hi ,2wd, N ,4Lo. Shown here is the 69-72 Glove box door sticker found in NP205 equipted Gm trucks.

Dana 20: Very few Chevy guys realize that GM offered this little cast iron gear driven T-case in some of its 69-74 4x4's. The Dana 20 was offered ONLY behind 3speed on the columns.........or with SM465. GM never offered it with the 350 turbo automatic trans option (70-72 models). I've only ever seen the Dana 20 in 69-72 GM 4x4's and they are quite rare to find. And I have never seen one in a 73 or 74 GM 4x4........even though it was offered in those years. The GM Dana 20 has a odd shift pattern similar to a ( J or U ). I was fortunate to purchase a 4x4 Floor hump out of a 72 K5 blazer that came with the Sm465 & Dana 20.......and it even has its rare floor shift retaining plate with shift pattern on it. Some Jeeps in the 70's used this same shift pattern dana 20, while others used the (straight shift) pattern dana 20. The GM dana 20 had a 2.03 low range.............this varied by vehicle manufacturer......as not all Dana 20's have the same low range ratio.

I just picked this up in june 2010 on craigslist, a very hard GM transfer case to find, and only offered for 5 years and only behind manual trans. Show are picks of a common small saginaw 3 speed hooked up to the rare GM version of the Dana 20, the shift linkage and lever assembly are more complex compared to the later NP203's.

While this floor hump came out of 72 K5 with a Sm465/dana 20.........I figured I'd show what the lever assembly looked like with the dana 20 lever.

The GM Dana 20's shift lever and linkage is more complex than any other GM transfer case, The lever itself is interesting in that the base of it is very much like that of an SM465, the lever is held into the control box by roll pins and crush collar. Since the lever has a ( J ) style shift it must be able to Pivot, and the pivot box is filled with grease. Inside the pivot box is a spring loaded bolt which allows the lever to slide and lock into its desired range. The GM dana 20 can be awkward to shift, because of its complex linkage system, which is why it draws complaints from those who've used it.

The pivot box has a grease zerk on the side of it, this is also where the spring loaded bolt is held on at to allow the lever to pivot into each range.

1969: 1/2 tons came with dana 44 front...12 bolt or 44 rear. 3/4 tons came with HD44 front and Eaton or dana 60 rear. 3spd on column or Sm465 offered for 4x4 models.

1970: This was the model year in which GM began to offer its 4x4's with automatic trans, which was the Turbo Hydramatic 350.....aka 350 turbo. Axles were same for previous years for 1/2 tons......3/4 tons. Transmissions were 3spd on column or Sm465.......as well as optional 350 turbo. Transfer cases were NP205..............or Dana 20( dana 20 used behind manual trans only).

1971: 1st year for the open knuckle front axle & Disc brake front end.....both 1/2 ton dana 44 & 3/4 ton HD44 front axles were standard with open knuckle/disc brake front. drivetrain axles T-cases same as 71.

1972: This was the last year for this body style. Shown is my 1972 K5 blazer (front clip is off of a 1969 model). This Blazer came with front dana 44 and 12 bolt rear.......both have 3.73 gears, and rear has eaton posi. Transmission is 350 turbo with Np205.

Shown is a photo of my Blazers Np205 shift lever. 1969-1972 GM model Np205's came with a much longer 4x4 lever than did later 73-up Np205 models did.

1972: HD44 front axle and Ho52 Eaton with posi, the front is 4.09 ratio, and the rear is 4.11. Both these axles came from a early 1972 3/4 ton. You will notice the Posi in the Eaton and the slight change in brake drums (my 1972 Eaton compared to my 1967 Eaton). Also the Front HD44's factory Spicer locking hubs with correct Blue knobs which were used for 1970-72 3/4 ton model's.

1973: Year of FULL TIME 4 Wheel Drive. This was offered in mid (1972) for the new 73 model year. In the 73 model year........ONLY k5 blazers...GMC Jimmys came with full time 4x4. Trucks......and suburbans came with part time 4x4....aka....Np205......regardless of transmission or motor size.

The 1973 model year chevy truck/blazer/suburban were completely redesigned from the earlier 67-72 body style. The 73 body style would last till 1980......and with slight changes in body metal/grilles in 1981.......would last until 1987, & for blazers/burbans till 1991.

The NP203: (....GD it.......its got an Np203 in it!!! sob!!).....thats a common phrase I hear from peaple looking for an older chevy 4x4 from the 70's. The Np203 gets a bad rap for several reasons...........mainly because of its full time 4 wheel drive, and also because the transfer case weights around 175 pounds & shifting the T-case lever could be difficult sometimes. When the Np203 came out it was super popular.........but the gas/energy crisis in the mid 70's didnt help this very much. And by 1979 GM was doing away with its full time 4x4 (the 1979 model was the last year GM offered the Np203).

SHown here is the shift linkage and shift box for the Np203, the 203's linkage can be adjusted, and this can loosen up over time. There have been problems with shifting a Np203 (hard shifting) and loose linkage can be one of them, also remember too that since the 203 is full time, drive line bind up is possible while shifting from a Lock position to a Hi/low AWD position.

The NP203 was found in many 1973-79 GM vehicles.........however for 1974/1975, the engine size ordered in the vehicle also had alot to do with getting an NP203, Most V-8 equipted GM 4x4s could be ordered with the Np203......with either automatic or manual trans. But if the owner decided they wanted a 250 or 292 straight 6 engine, they had to settle for part time Np205 T-case option.

NP203/SM465: In 1974 GM began offering its full time Np203 behind manual trannys ( I speak only of the SM465 because I've never seen a 203 bolted to a 3spd on the tree). For some reason unknown to me (because I havent been able to find credable info) The full time Np203 did not want to work behind a manual trans, I dont know if this had to do with letting in & out on the clutch or what............but I do know that GM only offered the Sm465/NP203 combo for 1974......and early 75 after finding out that it just wasnt a good combination to pair up. Finding a 465/203 can be very hard, since it was offered for a very short time.

What is really ironic about GM's discontinue of the Sm465/203 in early 1975 is the fact that Dodge in 1975-1979 offered manual tranmission's with their full time Np203 equipted 4x4's. In 1974/75 Dodge began using the Np203.........and it became the only T-case they used with either a auto trans, or stick until they did away with the Np203 in 1979. And I havent read anything about dodge having problems with 203's bolted to manual trannys. Ford used the NP203 in the 70's also.............but I am not sure if it was offered behind a manual trans........but I have seen them behind auto trannys.

NP203 Operation: The NP203 has a low range of 2.01 (slightly lower than a NP205) has a heavy cast iron housing with an aluminum tail housing. It's full time 4x4 operation is set up kind of like a (open differential axle) with spider gears inside the case allowing both driveshafts to spin at different speeds and allow tire slippage at front axle or rear while in (A.W.D)....all wheel drive. The Np203 has 4 shift ranges. HI LOCK....HI....N...LOW...LOW LOCK. In the HI or LOW position the T-case is in all wheel drive.......and if a rear tire starts to spin.............1 tire in the front will spin, In HI LOC/LOW LOC.........the AWD is locked out.......meaning you have true 50/50 split 4 wheel drive.........1 tire spins in the rear & 1 tire spins in front.

HI LOCK:......This is 4 wheel Hi range for off road use.
HI :........this is All wheel drive range for normal road driving conditions.
Neutral:.....self explanatory.
LOW:.....this a AWD normal driving condition range........but made for pulling or towing, there is a speed restriction for this range.
LOW LOCK:..........this 4 low.

Shown here is the Operation sticker for Full time 4x4 use. This is in my 1979 GMC 1/2 ton and located on driverside sun visor folded down.

1973: This is my 73 Custom 20 3/4 ton 4x4, it has its original 73 drivetrain (except motor). This truck came with a V-8 motor.....350 turbo trans, Np205 T-case.........standard HD dana 44 front axle..........and new for 73 model year, the 14 bolt full floater. Truck has 4.10 gears, and open axles.

Shown here is a Fixed Yoke Np205 from a 350 turbo trans out of a 73 k20 that I purchased from "73chevytruck 8/2/10. The fixed yoke NP205s used behind automatics were not used very long. 1970-1973.......and were optional for 1974-75, but very few GM 74-75 4x4s were produced with Th350/Np205 combos.......as GM was pushing its Np203 behind most of its 4x4 drivetrain combos. Note the T-case shift knob is the later 75-78 version. 1973 and 74 GM 4x4 equipted with the NP205 retained the older 69-72 style Np205 shift knob.

NP205 ID tag: All 69-79 GM np205s and 73-79 NP203s came with a small blue aluminum ID tag stating what model they were, low range ratio, and a build date and asembly #. 1.96 is the low range.........and 3-19-73......is when this 205 was built and most likely would been put in a late 1973 model 4x4.

Here is the Np205 lever for my 73 K20.........GM redesigned it for 73 model, the lever was shortened.......did not have the bend in it towards the top like earlier 69-72 models had. But it did still use the earlier pattern shift knob. GM used this knob as a (carry over) for 73 & 74 Np205 equipted trucks..........they also reused the 68-72 pattern sm465 4spd knob for 73 & 74 models as well. GM also used a range plate at the base of the lever to align it for the correct drive range.