K20 c6p rear axle questions

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

string-a-ling

Junior Member
Joined
Mar 27, 2022
Posts
11
Reaction score
17
Location
Texas
First Name
Aris
Truck Year
1986
Truck Model
K20 Scottsdale
Engine Size
6.0
How y’all doin? I’m new here and I have a question regarding my ‘86 C6P code k20.

Im led to understand ‘81 and up 20 series trucks with the C6P code have some similarities to the 1 ton crew trucks as far as frame thickness, brake size, spring length etc… My question is, are the axles in the 1 tons (including dually axles) direct swaps for my truck? Mine has a SF 9.5” 14 bolt in it and I make frequent use of a gooseneck and I’d feel a lot better loading it down knowing the wheels won’t fall out of it if I break an axle. Also if it comes to a task that potentially outclasses it with the SRW I’d sooner swap a dually axle in it and swap a bed than buy another truck. I’ve put a lot of work into it and am pretty attached to it.

If this question has previously been answered, please feel free to direct me to the appropriate thread! Thanks in advance! Pic for interest!
You must be registered for see images attach
 

Craig 85

Full Access Member
Joined
Jan 25, 2017
Posts
3,892
Reaction score
4,037
Location
Nashville, TN
First Name
Craig
Truck Year
1985
Truck Model
K30 SRW
Engine Size
454/TH-400/NP205
One ton's are not direct bolt in. Shock mounts are opposite and spring pads are more in board. See my K30 SRW 14 bolt below. A 3/4 ton 14 bolt FF is a direct bolt in though.

If you add a FF rear end, you will need to lengthen your drive shaft. The 9.5" has a longer snout.

You must be registered for see images attach
You must be registered for see images attach
 

Grit dog

Full Access Member
Joined
May 18, 2020
Posts
5,950
Reaction score
9,847
Location
Washington
First Name
Todd
Truck Year
1986
Truck Model
K20
Engine Size
454
And another question answered with great knowledge!
I sound like a broken record but this forum and the folks are a great resource.
@string-a-ling, I’ll add, if you’re hauling more payload than a 14bolt FF will handle with appropriate rims and tires, you’re basically overloading the absolute crap out of a very nice truck. Probably to the point that other damage is imminent.
Find ya a srw 14bolt FF and it will be reliable with 2 tons of payload or pin weight and won’t tear the truck up as long as you use some common sense when loaded real heavy.
Nice truck btw!
 

SquareRoot

Full Access Member
Joined
Mar 18, 2017
Posts
3,695
Reaction score
6,683
Location
Arizona
First Name
Mike
Truck Year
85
Truck Model
K20
Engine Size
350
YUP^^
I put a 14FF out of a CUCV in my 85 K20. Had to relocate the spring and shock mounts. Wasn't a big deal if you have the tools. I highly recommend replacing the factory u-bolts/plates with the ORD ones. Not only do they flip the u bolts to the correct orientation and are much more robust they allow you to move the axle fwd/back an inch. This will make up for the driveshaft length Grit dog mentioned. However, I ran my FF in the original position and never had an issue with it being to short.
 

Attachments

  • Ubolt.JPG
    Ubolt.JPG
    99.3 KB · Views: 49

AuroraGirl

Full Access Member
Joined
Sep 8, 2019
Posts
9,209
Reaction score
6,187
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
How y’all doin? I’m new here and I have a question regarding my ‘86 C6P code k20.

Im led to understand ‘81 and up 20 series trucks with the C6P code have some similarities to the 1 ton crew trucks as far as frame thickness, brake size, spring length etc… My question is, are the axles in the 1 tons (including dually axles) direct swaps for my truck? Mine has a SF 9.5” 14 bolt in it and I make frequent use of a gooseneck and I’d feel a lot better loading it down knowing the wheels won’t fall out of it if I break an axle. Also if it comes to a task that potentially outclasses it with the SRW I’d sooner swap a dually axle in it and swap a bed than buy another truck. I’ve put a lot of work into it and am pretty attached to it.

If this question has previously been answered, please feel free to direct me to the appropriate thread! Thanks in advance! Pic for interest!
You must be registered for see images attach
1980* but yes. A c6p axle will ensure its only ratios above 3.73 (4.10 and 4.56 i think) and the brake drum is 13 inches not 11 like non c6p axle. The axles are the same otherwise to my knowledge. Full floaters that is. The shock mounts MAY differ , only if thats required for the "HD" larger shock absorbers. if they bolt in place of eachother then ignore that. also there are other quirks but for your question that should help a bit
 

string-a-ling

Junior Member
Joined
Mar 27, 2022
Posts
11
Reaction score
17
Location
Texas
First Name
Aris
Truck Year
1986
Truck Model
K20 Scottsdale
Engine Size
6.0
Thank y’all for the insights. Admittedly, I was just curious as far as the 1 ton and dually axles go. I’ve already acquired a FF 14 bolt out of an 84 or 85 k20 with the correct mounting surfaces all around and am currently hoarding fresh suspension parts and disc brake conversion parts so I can slam it all together in one sitting However, on the subject driveshaft length, I just finished up a 6.0/nv4500 swap in this truck. I re-used the trans x-member so I’ve already lost 1/8” or so to the shorter transmission. Will probably have to call the driveshaft shop when I get around to making the swap happen.
 

string-a-ling

Junior Member
Joined
Mar 27, 2022
Posts
11
Reaction score
17
Location
Texas
First Name
Aris
Truck Year
1986
Truck Model
K20 Scottsdale
Engine Size
6.0
84 or 85 k20 didn't get a ff
the k30 did

might take a look at that acquired axle.
good luck!
Maybe it’s from an earlier truck. I’ve seen 70s 3/4t trucks with em. Everything seems to measure out the same. And the shock mounts are in the correct place. Guess I won’t know for sure till it all goes together. Thanks for the heads up!
 

WebMonkey

Full Access Member
Joined
Jan 3, 2020
Posts
685
Reaction score
1,398
Location
Missouri Ozarks
First Name
Chris
Truck Year
1985
Truck Model
K20
Engine Size
350
Maybe it’s from an earlier truck. I’ve seen 70s 3/4t trucks with em. Everything seems to measure out the same. And the shock mounts are in the correct place. Guess I won’t know for sure till it all goes together. Thanks for the heads up!
sounds right!
they definitely put the ff in the 3/4 in the earlier trucks.
it'll be sweet regardless
(the only thing i would get hung up on is the switch in parking/e-brake backing plates that changed from earlier to later trucks)
 

string-a-ling

Junior Member
Joined
Mar 27, 2022
Posts
11
Reaction score
17
Location
Texas
First Name
Aris
Truck Year
1986
Truck Model
K20 Scottsdale
Engine Size
6.0
sounds right!
they definitely put the ff in the 3/4 in the earlier trucks.
it'll be sweet regardless
(the only thing i would get hung up on is the switch in parking/e-brake backing plates that changed from earlier to later trucks)
I’m goin to discs. Currently hunting for a caliper with e-brake functionality that fits. I’ve heard tell of a Caddy caliper that apparently fits, is robust enough for the application and has an e-brake in it but haven’t had time to follow the lead. Worst case my factory axle has the big drum on it so I’ll see if that works.
 

AuroraGirl

Full Access Member
Joined
Sep 8, 2019
Posts
9,209
Reaction score
6,187
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
I’m goin to discs. Currently hunting for a caliper with e-brake functionality that fits. I’ve heard tell of a Caddy caliper that apparently fits, is robust enough for the application and has an e-brake in it but haven’t had time to follow the lead. Worst case my factory axle has the big drum on it so I’ll see if that works.

The wheel end parts for a k30 srw I think should match if you wanted to put a 13 inch drum on your ff

Do you have slide on drums or is the drum retained kinda like a front hub with bearings is
 

string-a-ling

Junior Member
Joined
Mar 27, 2022
Posts
11
Reaction score
17
Location
Texas
First Name
Aris
Truck Year
1986
Truck Model
K20 Scottsdale
Engine Size
6.0
The wheel end parts for a k30 srw I think should match if you wanted to put a 13 inch drum on your ff

Do you have slide on drums or is the drum retained kinda like a front hub with bearings is
Its the early style that’s retained by the axle shaft bolts. I’d be interested to know what disc brake assemblies from more modern 14 bolt axles if any fit. I like going to the junkyard for stuff.
 

AuroraGirl

Full Access Member
Joined
Sep 8, 2019
Posts
9,209
Reaction score
6,187
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
Its the early style that’s retained by the axle shaft bolts. I’d be interested to know what disc brake assemblies from more modern 14 bolt axles if any fit. I like going to the junkyard for stuff.
No it’s not. Unless you mean the semi float. The axle shaft bolts are holding the ff shaft, behind it are spindle nuts I think you meant.

If you wanted a more modern 14 bolt the aam 11.5 or whatever gm used into the 2000s I think is relatively easy to adapt to a square iirc.
 

WFO

Full Access Member
Joined
Jan 6, 2017
Posts
3,397
Reaction score
4,448
Location
Texas Panhandle
First Name
Dan
Truck Year
1986
Truck Model
K20
Engine Size
350
84 or 85 k20 didn't get a ff
the k30 did

might take a look at that acquired axle.
good luck!
My 86 came with a full floater. It's C6P optioned though.
 

string-a-ling

Junior Member
Joined
Mar 27, 2022
Posts
11
Reaction score
17
Location
Texas
First Name
Aris
Truck Year
1986
Truck Model
K20 Scottsdale
Engine Size
6.0
My 86 came with a full floater. It's C6P optioned though.
Mine's a C6P as well. Ive wondered why it didnt have a FF since i figured out what C6P meant. Maybe because its a 4x4?
 

Forum statistics

Threads
42,087
Posts
908,638
Members
33,560
Latest member
bgeise
Top