Differences between C/K-10 and C/K-20

[kobayashi]

Besides the uprated springs what are the visible differences between these two frames?

Is the 20 frame taller, heavier, thicker metal, etc? Is there a frame production tag and if so where is it located?

TIA

[krue]

It's my understanding that the frames are pretty much the same. The trailing arms may be beefed up a little.

[Longhorn Man]

the frame has a little added bracing in a couple areas....but other than that, they are the same.

[kobayashi]

So its mostly in the springs then that increases towing and hauling capacity?

Are there any frame build tags? Location?

[Robert1970C20rstbukt]

No tags, but part of the truck's VIN number is stamped on the left front frame rail on the top. You might have to wire brush the frame to find it like I did.

[kobayashi]

Originally Posted by Robert1970C20rstbukt

No tags, but part of the truck's VIN number is stamped on the left front frame rail on the top. You might have to wire brush the frame to find it like I did.

Robert is that on top of the frame rail and behind the radiator? Meaning can I see it if I pop the hood and look down behind the radiator?

[Robert1970C20rstbukt]

Yep, sure is.

[Longhorn Man]

just behinde the steering gear box on top of the frame rail.

[kobayashi]

Thanks guys. I am hoping I dont see something disappointing when I see it in person. Be looking at it/picking it up Saturday.

[Longhorn Man]

The numbers on the frame won't tell you if it is a real 4X4... but if the #'s match the VIN on the truck...you're in luck. (it is only the last 6 or so of the VIN)

[kobayashi]

Originally Posted by Longhorn Man

The numbers on the frame won't tell you if it is a real 4X4... but if the #'s match the VIN on the truck...you're in luck. (it is only the last 6 or so of the VIN)

If the numbers match the VIN, I will know the frame is original to the body and its the springs/axles/transfer case that was added right?

[Longhorn Man]

if that vin is a c******* and not a K******...then yes, it is a conversion.

[kobayashi]

Originally Posted by Longhorn Man

if that vin is a c******* and not a K******...then yes, it is a conversion.

LH, heres the active part of the VIN and my decode:

C = 2wd
E = V8 gas
2 = 3/4 ton
4 = pickup
1 = 1971
J = Janesville assembly plant

Last 6 digits = serial number of that specific truck

So unless I made a mistake in the decode, its definitely originally a C20. Now its a 4wd with 6 lugs on each wheel which seems to mean a K10 underpinnings at least. I hope its no hack job, I'll be seeing it in person tomorrow AM.

Here's a pic of hopefully my first 67-72 and a closeup of the left rear tire, maybe some of the sharp-eyed people can say what type of rear suspension that is. I cant make out any leaf springs, so maybe its a trailing arm?

[krue]

That's trailing arm(coil), you can see it in front of the tire.

[kobayashi]

Originally Posted by krue

That's trailing arm(coil), you can see it in front of the tire.

Gotcha.

[kobayashi]

I had to let this one go. The truck was apparently in a pretty severe accident as the frame rails were bent and further the engine was sitting at an angle to the frame rails. On drivers side of the frame where they attached the leaf springs for the 4wd conversion they cut out and didnt replace about an 8"x8" section of the frame rail. It was bad, I could go on but wont. The number was ground off the frame rail BTW. Back to the search.

Like krue said, it had a the trailing arm coil spring rear setup.

Now on the way back I saw a Chevy C-20 camper(I think a 69) and it had rear leafs and front coils.

Further I saw a 1971 Chevy K-20(all original) with front and rear leafs.

So is it maybe the 20s that have the rear leafs and the 10s with the trailing arms, not GMC vs Chevy? Is there a book/CD I can buy on this?

[Longhorn Man]

there is no definate rule of thumb....some GMC had coils and it wasn't optioned as so.
However, the general rule of thumb that most ppl follow, is Chevy had coils and optional leafs. (c/10 - c/20) GMC had leaf, with optional coils. (again, c/1500 to c/2500)
The camper truck you saw, it was either optioned with the leafs, or it could have been a longhorn, which ONLY came with leafs, just like the one ton trucks and all 4X4 trucks.

[CyberRanger]

From all my research before I converted my K10 to a K20 the only differences I could see were not in the frame. Not sure about a C10 & C20. I only had to swap axle housings & springs, master cylinder & mount, radiator & rear driveshaft. I already had the HD 4spd & transfer case. I have a K20 frame to compare & it is identical to my K10 frame, hard lines & everything.

[Longhorn Man]

on the 2 wheel drive coil spring frames, there is additional bracing...someone on here has pics. He took the pics after he bought a 3/4 ton frame while working on a 1/2 ton and discovered the new frame was not 100% right.

[kobayashi]

Originally Posted by Longhorn Man

there is no definate rule of thumb....some GMC had coils and it wasn't optioned as so.
However, the general rule of thumb that most ppl follow, is Chevy had coils and optional leafs. (c/10 - c/20) GMC had leaf, with optional coils. (again, c/1500 to c/2500)
The camper truck you saw, it was either optioned with the leafs, or it could have been a longhorn, which ONLY came with leafs, just like the one ton trucks and all 4X4 trucks.

I believe you, but man is that nuts. A trailing arm/rear coil setup is so different in my mind, I cant see why they optioned that inside of the same load classes. Talk about confusing. Can you imagine what it must have been like for the salesmen? Along those lines, I just found a 69 GMC C-10 with coil fronts and rears(with rear trailing arms.)

LH, FWIW, the C20 was not a longhorn.

[kobayashi]

Originally Posted by CyberRanger

From all my research before I converted my K10 to a K20 the only differences I could see were not in the frame. Not sure about a C10 & C20. I only had to swap axle housings & springs, master cylinder & mount, radiator & rear driveshaft. I already had the HD 4spd & transfer case. I have a K20 frame to compare & it is identical to my K10 frame, hard lines & everything.

Did your K-10 come stock with rear leafs or trailing arm/coils?

Cyber, can you got into a little more detail on each of those and why they needed to be swapped?



You swapped axle housings and axles too correct?

Springs I got.

Why the master cylinder, ms mount? Larger brakes on the 20 need more fluid?

Radiator?

Rear driveshaft? The rear driveshaft I am guessing because of the larger diff housing and/or yoke?

[kobayashi]

Originally Posted by Longhorn Man

on the 2 wheel drive coil spring frames, there is additional bracing...someone on here has pics. He took the pics after he bought a 3/4 ton frame while working on a 1/2 ton and discovered the new frame was not 100% right.

Probably due to the additional twisting load the trailing arm setup applies?

[CyberRanger]

Originally Posted by kobayashi

Did your K-10 come stock with rear leafs or trailing arm/coils?

Cyber, can you got into a little more detail on each of those and why they needed to be swapped?



You swapped axle housings and axles too correct?

Springs I got.

Why the master cylinder, ms mount? Larger brakes on the 20 need more fluid?

Radiator?

Rear driveshaft? The rear driveshaft I am guessing because of the larger diff housing and/or yoke?

The K10 came w/ leafs all the way around, as well as 6.50x16 M&S tires on 16"x5" 6-bolt drop-center wheels. I know it came that way b/c I went w/ my Dad to pick it up at the dealership when we bought it new in March of '71.

I swapped both front & rear axles. The K10 had a Dana 44 front & Corporate 12 bolt rear. The K20 also had a Dana 44 front, but with much larger axle tubes, shafts, u-joints & knuckles as well as larger calipers, 8 bolt hubs & 8 bolt spindles & locking hubs. The K10 was 3.73 gears standard. The K20 was 4.11 gears standard.

The master cylinder was much larger volume-wise as were all the brake components. The K20 master cylinder mount is considerably different than the K10 & the two will not interchange. They also use a different brake booster as well. I also used all stainless steel braided flex brake lines. It vastly reduced the mushiness of the brakes, not that they were that mushy to start with. Spend the money, its worth it.

One thing to note. Virtually every rear axle on a K20 I've run across in Colorado as an owner & as a former mechanic do not match the specs in any parts book or GM specs. All the books & specs say they have 12"x2" brakes. Mine & virtually every one I've seen had a Dana 60 diff, 12"x2½" brakes & 1 1/8" wheel cylinders. None of the rear brake parts match the books either. The drums I've had to simply match w/ existing b/c none of the books listed the correct drums. Same for the small parts kit (springs, levers, retainers, etc., etc.) & wheel cylinders. Most parts stores list these rear axles as only havin' been installed in C20s with leaf springs & not always. Don't know where the disconnect comes from. Depending on the book, the drums & cylinders match those from a G30, stepvan, but not always. Go figure.

The standard K10 w/o A/C used a 2 core radiator unless it had a plow package, then it was 3 core. The standard K20 w/o A/C used a 3 core radiator unless it had the "plow" package or "camper/towing" package & then is was 4 core.

The front yoke on the rear driveshaft was identical to the K10. The K20 used a larger diameter tube, different rear yoke & was shorter. The Dana 60 has a much longer (front-to-back) housing than the 12 bolt, hence the shorter driveshaft. Of course I had to modify mine for the lift as well.

Another funny thing is Skyjacker sells the same springs in the Softride 4" kit for both the K10 & K20. The rear springs work great offroad, but will not carry much of any load--even less than the stock K10 springs. That is why I added from long travel air bags to act as overloads. They don't affect axle travel when offroad, but do allow me to tow & carry the same load as a regular K20.

[Tx Firefighter]

I'm the one with the pictures showing the differences between a c10 frame and a c20 frame. I set one of each right next to each other and took pictures of the differences once. It was all to solve a somewhat heated discussion we were having at the time about frame variances.

[ckhd]

Originally Posted by CyberRanger

One thing to note. Virtually every rear axle on a K20 I've run across in Colorado as an owner & as a former mechanic do not match the specs in any parts book or GM specs. All the books & specs say they have 12"x2" brakes. Mine & virtually every one I've seen had a Dana 60 diff, 12"x2½" brakes & 1 1/8" wheel cylinders. None of the rear brake parts match the books either.

That's funny. Almost all of the K20 rears I've seen are the GM Corporate (Eaton) axle, and usually in 4.57 gears. I've seen one Dana 60 diff, and it was in a '68 K20 with a 6 cyl.

All 67-72 four wheel drives have leaf springs. Four wheel drive frames are straight over the rear axle (when looking at it from above). Two wheel drive frames bow out, I think even on two wheel drives with leafs.