Author Topic: Axle swap  (Read 2638 times)

Offline fm19993

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Axle swap
« on: January 24, 2021, 05:42:31 PM »
Alright guys i have a 1979 GMC K15. Th350 transmission. My rear axle grenaded a few months back so I just did an axle swap. I’m not sure what the gearing is but it’s a GM14 Bolt. I’m going to go out on a limb and say it’s a 4.11. Regardless I searched through these threads but I’m looking for someone to help me understand the mechanics behind the problem. So before I did the swap it drove like a normal truck would. I just finished the swap and now I can’t get the truck to go over 30 mph.

Offline MIKE S

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Re: Axle swap
« Reply #1 on: January 24, 2021, 07:00:16 PM »
If you have  4wd and have different ratios front to rear that is your problem. You will probably destroy something if you continue to drive.

Offline JohnnyPopper

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Re: Axle swap
« Reply #2 on: January 28, 2021, 07:30:55 PM »
What transfer case do you have? If you have a NP203 and your gear ratios don't match, you will grenade something soon, like the transfer case. NOT pretty.

If you do have a NP203, the cheapest fix you can effect is a Miles Marker part time kit. You won't be able to use 4x4 until you match the gear ratios, so don't bother with locking hubs till you make the match.

At least you will be able to drive it.

Why didn't you just get your rear end rebuilt to OEM?

Keep us posted, would like to know how this turns out  ::)
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline JohnnyPopper

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Re: Axle swap
« Reply #3 on: January 28, 2021, 07:35:33 PM »
If you're not sure what you have, you may have a diagram on your driver side visor, describing the function.

If it says 4 HI, 4 Low, 4 Hi Loc, and 4 Low Loc, its a NP203.

If the sticker is gone, look at the  blue sheet in the glove box. It will say what it came with.

If yes, do not attempt to drive it till you make the changes.

1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline Mike81K10

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Re: Axle swap
« Reply #4 on: January 28, 2021, 08:46:17 PM »
Check the tag attached to the differential cover by the cover bolts or pull the cover and find the numbers on the ring gear. Verify and fix prior to driving. Some have spun wheels and counted the wheel and drive shaft rotations. Someone on this forum can probably help you out with that info.
« Last Edit: January 28, 2021, 08:52:06 PM by Mike81K10 »
"Tell me and I forget. Teach me and I remember. Involve me and I learn." -Benjamin Franklin

Offline fitz

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Re: Axle swap
« Reply #5 on: January 29, 2021, 04:15:07 AM »
I'm no expert on 203's but could he pull the front driveshaft untill he gets the part time kit installed?
Hopefully someone can verify that this won't damage the tcase running around without a front shaft.

Offline JohnnyPopper

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Re: Axle swap
« Reply #6 on: January 29, 2021, 03:11:26 PM »
I thought about that Fitz, but there is an internal differential that makes the full time 4x connection between the rear and front output.

If you pull the driveshaft the rear wont engage, would be like lifting one of rear wheels off the ground.

You just go to spinney town.  :-[
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline ehjorten

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Re: Axle swap
« Reply #7 on: January 29, 2021, 03:50:13 PM »
The differential in the back of the NP203 is just an open diff, so if your gearing was a little off, you would probably never notice it, because it will just bias the output towards the rear. It is like a peg-leg 10-bolt...one spins faster the other slower. The difficulty would be if you put the "non-converted" NP203 into Loc!

An old 14-bolt would more likely be 4.10 and not 4.11. Nowadays you are more likely to get 4.11. I would prefer to have 4.10 because the pinion has 10 teeth vs. the 4.11 where the pinion has only 9 teeth. It is a stronger gear set.
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline JohnnyPopper

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Re: Axle swap
« Reply #8 on: January 29, 2021, 05:40:27 PM »
So what accounts for him not getting above 30 mph?
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline blazer74

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Re: Axle swap
« Reply #9 on: January 30, 2021, 03:30:37 AM »
Are you sure the Tcase is not in low gear.
You should verify the gear ratio in the 14 bolt rear and the front axle.

Offline Mike81K10

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Re: Axle swap
« Reply #10 on: January 30, 2021, 04:28:27 PM »
You opened this thread saying you did not know the gear ratio of the rear end you installed. It should have been matched to the front. Bottom line, as blazer just said and others previously, you need to check your gear ratio of the front and back prior to doing anything else, to include driving. No need to wonder if anything else might be the problem until you know the gear ratio of the front and back are a match.
"Tell me and I forget. Teach me and I remember. Involve me and I learn." -Benjamin Franklin

Offline JohnnyPopper

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Re: Axle swap
« Reply #11 on: January 30, 2021, 07:48:22 PM »
Here we are blasting away and it's crickets... ::)
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline fm19993

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Re: Axle swap
« Reply #12 on: August 05, 2021, 11:02:20 PM »
Sorry everyone I went away for some military things for a while. I just checked on here and saw all your replies which I am very grateful for being apart of a community that shows this much support. I did the axle swap, the truck runs and drives fine nothing has grenaded.....yet. I am looking into getting the NP205 instead of rebuilding the current transfer case which I believe is the 203. and yes I had it in low gear thinking I was in 2wd because I thought I had a np205.