np203 conversion

[rewardedgamer]

ok so i was wondering if i should get the conversion or not... i already put on the locking hubs in the front. is the conversion worth it or not since i don't plan on going off road with my truck. and where do i purchase a conversion and how much is it also how hard is it to install.

[Irish_Alley]

the locking hubs will be useless without the conversion. if they are unlocked you wont be going anywhere. i think there was a write up on someone putting one in. but without it if you break something in the front such as u-joint, hub, ring/pinion, axle shaft or drive shaft you wont be able to make it home.

[Cabcorner]

                           It depends on how you use your truck . I personally wouldn't bother with a conversion .

[zieg85]

When I was 16 or 17 I converted my neighbors Bronco and I remember it being a piece of cake using a very limited tool selection.  It didn't give her any extra mpg and one of the front u-joint froze up from lack of use.  Nowadays if I ever want a 4x4 I will only consider the full time 4 wheel drive. 

[rich weyand]

One thing to remember is that the NP203 does not have an oil pump.  If you do the conversion and always run it in regular (not LOC), then the output shaft bearing will go dry and trash itself.  The solution to that is to switch it in to LOC (with the hubs in FREE) once in a while.  I run it for the first two miles in LOC when I leave the house for the first time of the day, then switch out of LOC when I hit the state highway.  That keeps the little reservoir above the output shaft bearing from running dry.

That being said, I think I prefer the conversion over unconverted.  Note that with full-time 4wd and three open differentials, once any tire spins, you're done.  I think the best setup is part-time, with posi in the back: two chances to bite, and they both have to spin to stop you.  In deep winter I run around with the hubs locked, and can punch it into LOC any time I need it.

[rewardedgamer]

the locking hubs will be useless without the conversion. if they are unlocked you wont be going anywhere. i think there was a write up on someone putting one in. but without it if you break something in the front such as u-joint, hub, ring/pinion, axle shaft or drive shaft you wont be able to make it home.

i already have the hubs installed and unlocked and the truck drives lol

[rewardedgamer]

One thing to remember is that the NP203 does not have an oil pump.  If you do the conversion and always run it in regular (not LOC), then the output shaft bearing will go dry and trash itself.  The solution to that is to switch it in to LOC (with the hubs in FREE) once in a while.  I run it for the first two miles in LOC when I leave the house for the first time of the day, then switch out of LOC when I hit the state highway.  That keeps the little reservoir above the output shaft bearing from running dry.

That being said, I think I prefer the conversion over unconverted.  Note that with full-time 4wd and three open differentials, once any tire spins, you're done.  I think the best setup is part-time, with posi in the back: two chances to bite, and they both have to spin to stop you.  In deep winter I run around with the hubs locked, and can punch it into LOC any time I need it.

is it bad to run it in 4 hi loc all the time?

[Engineer]

the locking hubs will be useless without the conversion. if they are unlocked you wont be going anywhere. i think there was a write up on someone putting one in. but without it if you break something in the front such as u-joint, hub, ring/pinion, axle shaft or drive shaft you wont be able to make it home.

i already have the hubs installed and unlocked and the truck drives lol

If this is the case then you either have A) a part time transfer case, or B) you have a NP203 that has already been converted.

My '77 Chevy had the conversion done to the transfer case before I bought it. All I had to do was install the lock out hubs. Why somebody would convert the case, and not the hubs is beyond me, but this is how mine came to me.

[rewardedgamer]

the locking hubs will be useless without the conversion. if they are unlocked you wont be going anywhere. i think there was a write up on someone putting one in. but without it if you break something in the front such as u-joint, hub, ring/pinion, axle shaft or drive shaft you wont be able to make it home.

i already have the hubs installed and unlocked and the truck drives lol

If this is the case then you either have A) a part time transfer case, or B) you have a NP203 that has already been converted.

My '77 Chevy had the conversion done to the transfer case before I bought it. All I had to do was install the lock out hubs. Why somebody would convert the case, and not the hubs is beyond me, but this is how mine came to me.

why would it not drive with the lock out hubs? the rear axel is still driving and the mechanic said i had to keep it in 4 hi loc or it would roll after we got the locking hubs

[rich weyand]

4HI LOC all the time is the only way to drive an unconverted NP203 with front locking hubs in FREE.

Some background.

The full-time NP203 has four drive modes (plus neutral).  These are the combinations of HI and LO range, and LOC and un-LOC.  HI and LO range just engages the 2:1 drive, so it takes twice as many RPMs at a given speed.  Also doubles torque at the wheels.

The LOC locks the transfer case's differential.  Normally, the full-time 4WD has the differential unlocked, so the front wheels and the rear wheels can turn at different rates.  This is important when turning, because the rear end shortcuts the corner and turns fewer times going around a corner.  The transfer case differential allows that.  You also have open differentials in the front axle and rear axle.  This means that if any wheel spins, all the other wheels will just sit there.  It also means if you have locking hubs in the front, and they are in FREE, the front axle halves will spin and you aren't going anywhere.

When in LOC, the transfer case's differential is locked up.  The front output shaft and rear output shaft of the transfer case are locked to each other and will spin at the same rate.  If you have front hubs in FREE, then the rear axle will drive and the front will not, though the driveshaft and axle halves will spin.  That's where you are right now.  If you have hubs in LOCK, then the front and back axles are locked to each other and you have classic 4WD, which you can't run on dry pavement or you will scrub tires and put large stresses on the NP203 drive chain to the front driveshaft, causing premature failure, which is a pricey fix.

A modified NP203 has had the differential gears removed.  In LOC, operation is as before for hubs in either LOCK or FREE.  When not in LOC, the front drive shaft will not spin at all; all drive is through the rear wheels, whether the hubs are in LOCK or FREE.  This means that you aren't moving all that hardware to the front, and it also means you can't get stuck by spinning one of the front wheels.  It also means that the internal chain in the NP203 does not spin when not in LOC, so no oil gets to the rear output shaft bearing unless you switch it into LOC once in a while, or drive around with the hubs in LOCK, which will spin the front driveshaft and the internal chain.

Most people convert the NP203 when they put locking hubs on in place of the factory slugs, because they find the full-time 4WD of limited use, and want to be able to switch between RWD and 4WD without getting uot of the cab.  Chevy put the full-time 4WD only on the pickups with automatic transmissions, presumably on the theory that by buying an automatic transmission, those buyers already indicated they didn't want to mess with shifting.

Unmodded NP203      
Case      Hubs     Action
4HI         FREE     neutral -- no drive anywhere
4LOC      FREE     RWD
4HI         LOCK    differential 4WD (same as stock)
4LOC      LOCK    locked 4WD (same as stock)
      
Modded NP203      
Case       Hubs     Action
4HI          FREE     RWD
4LOC       FREE     RWD
4HI          LOCK    RWD
4LOC       LOCK    locked 4WD (same as stock)

[rewardedgamer]

4HI LOC all the time is the only way to drive an unconverted NP203 with front locking hubs in FREE.

Some background.

The full-time NP203 has four drive modes (plus neutral).  These are the combinations of HI and LO range, and LOC and un-LOC.  HI and LO range just engages the 2:1 drive, so it takes twice as many RPMs at a given speed.  Also doubles torque at the wheels.

The LOC locks the transfer case's differential.  Normally, the full-time 4WD has the differential unlocked, so the front wheels and the rear wheels can turn at different rates.  This is important when turning, because the rear end shortcuts the corner and turns fewer times going around a corner.  The transfer case differential allows that.  You also have open differentials in the front axle and rear axle.  This means that if any wheel spins, all the other wheels will just sit there.  It also means if you have locking hubs in the front, and they are in FREE, the front axle halves will spin and you aren't going anywhere.

When in LOC, the transfer case's differential is locked up.  The front output shaft and rear output shaft of the transfer case are locked to each other and will spin at the same rate.  If you have front hubs in FREE, then the rear axle will drive and the front will not, though the driveshaft and axle halves will spin.  That's where you are right now.  If you have hubs in LOCK, then the front and back axles are locked to each other and you have classic 4WD, which you can't run on dry pavement or you will scrub tires and put large stresses on the NP203 drive chain to the front driveshaft, causing premature failure, which is a pricey fix.

A modified NP203 has had the differential gears removed.  In LOC, operation is as before for hubs in either LOCK or FREE.  When not in LOC, the front drive shaft will not spin at all; all drive is through the rear wheels, whether the hubs are in LOCK or FREE.  This means that you aren't moving all that hardware to the front, and it also means you can't get stuck by spinning one of the front wheels.  It also means that the internal chain in the NP203 does not spin when not in LOC, so no oil gets to the rear output shaft bearing unless you switch it into LOC once in a while, or drive around with the hubs in LOCK, which will spin the front driveshaft and the internal chain.

Most people convert the NP203 when they put locking hubs on in place of the factory slugs, because they find the full-time 4WD of limited use, and want to be able to switch between RWD and 4WD without getting uot of the cab.  Chevy put the full-time 4WD only on the pickups with automatic transmissions, presumably on the theory that by buying an automatic transmission, those buyers already indicated they didn't want to mess with shifting.

Unmodded NP203      
Case      Hubs     Action
4HI         FREE     neutral -- no drive anywhere
4LOC      FREE     RWD
4HI         LOCK    differential 4WD (same as stock)
4LOC      LOCK    locked 4WD (same as stock)
      
Modded NP203      
Case       Hubs     Action
4HI          FREE     RWD
4LOC       FREE     RWD
4HI          LOCK    RWD
4LOC       LOCK    locked 4WD (same as stock)

so if i drive with the hubs in lock i have to put it in non loc?

[rich weyand]

Yes, on dry pavement.  On poor surfaces -- snow, mud, gravel -- you can have both the hubs and the transfer case locked.

[Jason S]

so if i drive with the hubs in lock i have to put it in non loc?

If your transfer case does not have a part time conversion (i.e., as factory equipped) AND you have front manual locking hubs set to 'free' position, you will use Hi-Loc position. However, your front output shaft will still be turning the front driveshaft/front axle differential/front axle-shafts and front u-joints. Your rear axle will be driving the truck, the hubs simply are not engaging the front axle shafts to power the front wheels.

If your transfer case has a part-time conversion (e.g., Milemarker kit) AND you have front manual locking hubs set to 'free' position, you will use Hi position (between N and Hi-Loc).  Front output shaft will not be turning, nor will the front driveshaft/front axle parts.

With a part time conversion, you essential make the Hi and Lo positions '2WD Hi' and '2WD Lo', the Hi-Loc and Lo-Loc positions become '4WD Hi' and '4WD Lo'.

Chevy put the full-time 4WD only on the pickups with automatic transmissions, presumably on the theory that by buying an automatic transmission, those buyers already indicated they didn't want to mess with shifting

NP203's could be optioned behind SM465 4-speed manual transmissions at least until the 1974 model year.

[rewardedgamer]

Yes, on dry pavement.  On poor surfaces -- snow, mud, gravel -- you can have both the hubs and the transfer case locked.

ok and when i wanna shift do i go in neutral?

[rich weyand]

No.  If the hubs are in LOCK, you can shift in and out of 4HI and HI-LOC while driving.  If the hubs are free, you won't be shifting anyway, because you will put it in HI-LOC and Park before getting out and locking the hubs.