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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: pcdreams on October 05, 2017, 12:22:21 pm

Title: 78 chevy rearend swap
Post by: pcdreams on October 05, 2017, 12:22:21 pm: OK folks, I'm kind of new to all this but I've got a 78 chevy c20 (3/4 ton). Just got a new 350 and 4bbl dropped in and she's running good. It's a manual 3 speed with the granny gear. What I'm wanting to do is swap out the rear end for duallys. From what I've read this doesn't change the weight class but gives more stability when hauling. I'm planning to put a flatbed on it as well.

My questions are:

which dually rear ends will fit this thing? Will I need to have driveshaft work done? What other things am I looking at changing to do this swap (Do I need to change springs etc?) I'm going to replace the u-bolts because, well they're almost as old as me :)
Title: Re: 78 chevy rearend swap
Post by: Stewart G Griffin on October 05, 2017, 08:53:40 pm: Can you take a picture of the diff cover?
Title: Re: 78 chevy rearend swap
Post by: jetman on October 06, 2017, 09:02:24 am: Will have to wait on a dually axle xpert to weigh in on which axle(s) will fit up.

Regarding the driveshaft issue, the dually axle will almost certainly have a 10.5" ring gear vs your existing 9.5" ring gear, meaning a bigger pumpkin, meaning the old driveshaft will be long. BEFORE you remove tear everything apart, measure and mark your old driveshaft where it exits the transmission tail. When you refit shaft using dually axle you can see how much excess shaft you have relative to the OEM clearance fit.

I have done several axle upgrades. Leaving driveshaft too long will jam the shaft into the transmission when you load 'er up w/ a heavy load, very disconcerting when you hear it start to grind. I have shortened some shafts, on another I added large rubber replacement bump stops from Energy Suspension to keep the axle from approaching frame and compressing driveshaft. Good Luck!
Title: Re: 78 chevy rearend swap
Post by: pcdreams on October 06, 2017, 09:30:23 pm: I'll grab a pic over the weekend. It's dark out there tonight :)
Title: Re: 78 chevy rearend swap
Post by: Stewart G Griffin on October 14, 2017, 10:11:14 pm: i just wanted to post this here so i wouldn't forget:
https://www.youtube.com/watch?v=t3Sq34axZyc

You'll notice that both the chevy and the ford (i'm assuming they are both 1 tons?) are using a single, very wide tire vs. duals.

Could this be an easier alternative to adding duals. Although the duals would probably ultimately be more stable due to the added width?

2) The reason why i wanted the picture of the rear is were all 3/4 tons definitely equipped with 9.5 semi-floater rears?
Title: Re: 78 chevy rearend swap
Post by: pcdreams on October 15, 2017, 12:15:15 am: Sorry, I been crazy busy. I'll try to snap a shot tomorrow. :)
Title: Re: 78 chevy rearend swap
Post by: pcdreams on October 20, 2017, 01:29:44 pm: Sorry it's taken so long :)

https://www.dropbox.com/s/sqgrchmwo9dn85e/78ChevyRearend.jpg?dl=0 (https://www.dropbox.com/s/sqgrchmwo9dn85e/78ChevyRearend.jpg?dl=0)
Title: Re: 78 chevy rearend swap
Post by: Stewart G Griffin on October 20, 2017, 11:00:44 pm: i want to go out on a limb and say that you have the 10.5" FULL floater. If this is true, then there's no need to swap rears---you have the same one as the 1-tons and then all you would need is a dually kit to add the two extra wheels.

But we should get some other opinions here to make sure.

Also, i hate to be a pain, but could you also take a pic of your rear wheels?
Title: Re: 78 chevy rearend swap
Post by: pcdreams on October 21, 2017, 12:05:59 am: I'm not real clear what full floater means. I do know the axels have to slide out to change the brakes/drums and that they have gears on the end.

I'll snap a pick of the wheels tomorrow
Title: 78 chevy rearend swap
Post by: blazer74 on October 21, 2017, 12:40:15 am: Quote from: pcdreams on October 20, 2017, 01:29:44 pm
Sorry it's taken so long

https://www.dropbox.com/s/sqgrchmwo9dn85e/78ChevyRearend.jpg?dl=0 (https://www.dropbox.com/s/sqgrchmwo9dn85e/78ChevyRearend.jpg?dl=0)

Full Floater 10.5 is what's in your pic

Semi Floater 9.5 below.

(https://uploads.tapatalk-cdn.com/20171021/82582b5de907a728ff908a8f5108b4fd.jpg)
Title: Re: 78 chevy rearend swap
Post by: 75gmck25 on October 21, 2017, 06:51:17 am: The heaviest duty 14 bolt axles are full floating (vs semi-floating), which means the axle shaft "floats" and does not carry the weight of the vehicle (weight is on the hub).
Take a look at this link for a more complete explanation. http://www.fourwheeler.com/how-to/0112or-semi-floating-and-full-floating-axles/.

Bruce
Title: Re: 78 chevy rearend swap
Post by: pcdreams on October 24, 2017, 12:51:31 pm: Stewart, here is the requested pic :)
Title: Re: 78 chevy rearend swap
Post by: Stewart G Griffin on October 24, 2017, 09:23:33 pm: Ok, you definitely have the full-floater.
Title: Re: 78 chevy rearend swap
Post by: raf2330 on October 27, 2017, 10:27:58 pm: Every 3/4 ton truck I've had experience with came with factory Chevy 14 bolt full float rear end, from my dad's 73 to a 79 I used to have. I've had a 78 crew cab dually for 20 years now. It has a Dana 70 under it, which I believe is standard from the factory for these trucks. Hope that helps you know what to look for.