Crew C30 vs C20 differences

[kevinr1970]

I am under the understanding that all crew cabs share a "1-ton" frame.
I also understand that a C30 came with a full-float rear while a C20 came with a semi-float rear, but the semi-float is unique to the crew due to the spring perch and shock mount locations?
I am I on the right path here?

So what is different on the front?
Are the Crew C20 and C30 the same upper and lower control arms, or are the lowers different?

I was told that the only difference on the front of a SRW and a DRW was the rotor, but have never confirmed that.

I am going to convert my C30 Dually to single (and short bed eventually) and there is a guy parting a C20 crew.
I have Belltec 3" spindles on my crew now, so I can't just swap all the front end parts over.
Just wondering what all I could grab off this to help me convert to SRW.

[slotard]

My '73 C20 Camper Special has a 14FF, which I believe was also used on the 1 ton trucks and crew cabs. However, the Heritage Center doesn't claim the C20 came with that.

https://www.gmheritagecenter.com/doc...olet-Truck.pdf is the info for the '73, change the url to year of your choice for a different year. For '73, they claim that the C20 and C30 frames have the same section modulus (and therefore shape) and material, and that the Crew Cab frame is significantly beefier than the regular cab (3.84 regular, 7.19 crew). They don't show many differences between 20 and 30, and even fewer for the crew cab.

I wouldn't be surprised if the springs were different on 20 vs 30, but you don't need to swap those if you don't want to.

I believe the 1 ton mounts the springs closer together than the 3/4 ton, so you may need to replace perches on the 3/4 ton axle or mounts on your frame.

I wouldn't entirely trust the specs, as my truck doesn't conform. I'd also advise looking for a SRW 14FF rather than a 14SF, so you don't lose too much capacity.

[kevinr1970]

Originally Posted by slotard

I wouldn't be surprised if the springs were different on 20 vs 30, but you don't need to swap those if you don't want to.
I'd also advise looking for a SRW 14FF rather than a 14SF, so you don't lose too much capacity.

My current spring pack is like 9-leaf with the overload(?) over the top. I have a set of springs off a '91 3/4 (r20!) Suburban that are I think 7-leaf. They are both the same length (56"?) and the eyelets are the same width so I think they should work.
I still need to be able to tow my car trailer, but that is all the capacity I need! Other than that, this will mainly be a driver.

[meatwagon83]

paging kieth sizemore!!

[jetmech85]

I have an '85 C-20 crew with 14 bolt full floater. I can provide specific measurements for you if that will help. I also have an '82 C-30 crew cab dually that is picked clean but still has the rear end and springs to compare. As far as the front I can't confirm but I believe you are correct, just install single wheel hub/rotor assy. Some C-30's used air bags inside the coils and the lower controll arms are different (heavy duty) on those. Noticable mainly in the pivot shaft (eliminates the "U" bolts) and bushing area.

[kevinr1970]

Originally Posted by slotard

I believe the 1 ton mounts the springs closer together than the 3/4 ton, so you may need to replace perches on the 3/4 ton axle or mounts on your frame.

I was 90% sure the frames on the crews were the same for all crewcabs and ElGracho confirms that. I consider him to be one of the forums resident squarebody truck experts!
But... I never considered that they might have used different hangers and shackle mounts between the c20 and c30...

Originally Posted by jetmech85

I have an '85 C-20 crew with 14 bolt full floater. I can provide specific measurements for you if that will help. I also have an '82 C-30 crew cab dually...

I don't need any measurements, but if you could look at the hangers and shackle mounts and confirm they are the same I would really appreciate that!

[andrewmp6]

The crew cab was 1 ton only they never made it in a 3/4 ton.As far as a dually to single wheel swap the front you change the rotor and take off the dually spacer thing.The rear you need a whole new rear end and you need a new bed also.The question is do you want to keep it 8lug or are you swapping down to 6 or 5 ?

[ElGracho]

Originally Posted by kevinr1970

I am under the understanding that all crew cabs share a "1-ton" frame.
I also understand that a C30 came with a full-float rear while a C20 came with a semi-float rear, but the semi-float is unique to the crew due to the spring perch and shock mount locations?
I am I on the right path here?

So what is different on the front?
Are the Crew C20 and C30 the same upper and lower control arms, or are the lowers different?

I was told that the only difference on the front of a SRW and a DRW was the rotor, but have never confirmed that.

I am going to convert my C30 Dually to single (and short bed eventually) and there is a guy parting a C20 crew.
I have Belltec 3" spindles on my crew now, so I can't just swap all the front end parts over.
Just wondering what all I could grab off this to help me convert to SRW.

True on the same frame.

C20 could be full float or semi float 14 bolt rear. Not sure the GVW that made the change. Not sure if any semi floats made it onto crew cabs.

C20 and C30 without F42 heavy duty suspension shared the same upper and lower control arms. With F42 the C30 is different. See here:

http://www.gmpartswiki.com/query

I would think the dually would be heavy duty (F42) no matter what.

Look for part 14026579 and 14026580 which are the lower control arms for the C20 and the lighter C30 and 14026591 and 14026592 for the heavy lower control arms for the C30 with F42.

The C20 and C30 steering knuckles show different part numbers. All C30s show the same part number so that backs up the rotors being the difference between single and dual wheel front ends. 475083 475084 for C20, 363735 and 363736 for C30.

I pulled these part numbers from my 1985-1991 GM parts catalog. You can look at the same one on the parts wiki.

Originally Posted by andrewmp6

The crew cab was 1 ton only they never made it in a 3/4 ton.As far as a dually to single wheel swap the front you change the rotor and take off the dually spacer thing.The rear you need a whole new rear end and you need a new bed also.The question is do you want to keep it 8lug or are you swapping down to 6 or 5 ?

They did make a 3/4 ton crew cab. Not sure where you are getting this. The 2 wheel drive dually does not use a spacer on the front brake. The brake rotors are one big casting.

http://brochures.slosh.com/1987/gmc5.jpg

[kevinr1970]

Originally Posted by ElGracho

True on the same frame.

They did make a 3/4 ton crew cab. Not sure where you are getting this. The 2 wheel drive dually does not use a spacer on the front brake. The brake rotors are one big casting.

I can confirm that the rotor is one casting, with the "extension" cast into it. I know the calipers and other brake/front parts were the same so I assumed that the rotor was the only difference. Thanks for confirming that!

Originally Posted by andrewmp6

The crew cab was 1 ton only they never made it in a 3/4 ton.As far as a dually to single wheel swap the front you change the rotor and take off the dually spacer thing.The rear you need a whole new rear end and you need a new bed also.The question is do you want to keep it 8lug or are you swapping down to 6 or 5 ?

I can confirm that they made a 2-wheel drive 3/4 crew until at least '87, as the '87 sbcc in my sig below was a 2500. But I think the 3/4 tons stopped in '88 or '89 and it was 1-ton only until they stopped them in '91?
Also, I thought they stopped the 3/4 ton 4x4 crews in like '80?
There was a late 70's GMC 3/4 ton crew for sale locally about 6 months or so ago and I would have bought that one in a heart beat, but they guy finally answered his phone when he was on a test drive with someone else!?!

I was told that the difference on the FF between the SRW and the DRW was "from the backing plates out". But I am not sure what exactly that all entails? I am thinking it might be cheaper to find a SRW rear than replacing all the parts to make mine SRW?

As for lug count, it will stay 8-lug. I did 1/2 ton/5-lug on my last one and it rode great as a driver, but this time I still want to be able to tow with it.