NEW GUY PLEASE HELP

[brushybarry]

FIRST POST.
I recently bought an 84 k20. truck runs and drives like new but when cruising if you lay your foot on the gas as if to maintain speed it sounds like the rear end is about to come apart. PO said it has done this since he bought the truck in 08. My build sheet shows a G80 locking diff and GQ1 standard diff but says nothing of gear ratio. can yall tell me what I have and any ideas what kind of mess im fixing to be in trying to fix it. I thought i would just buy everything new and go through th rear end to be sure to get it right. thoughts?

 

[TubeTruck]

I would assume 4.11's. I think some came with 3.73's in the semi floaters but I've only seen 4.11's in the full floaters. Not saying that there isn't something different, just my experience.

 

[Bextreme04]

It depends on the power team. Post a pic of the whole SPID sheet and I can tell you exactly what it came with from the factory

 

[Shorty81]

Welcome from Ohio.

 

[brushybarry]

it sucks but best i could get

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[Bextreme04]

it sucks but best i could get

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So, 1984 K20/C6P with 350/TH400/NP208. As a C6P, you should have a 14 bolt FF rear with the 13" drums.

You can find a lot of good info here: https://www.gmheritagecenter.com/do...its/Chevrolet-Trucks/1984-Chevrolet-Truck.pdf

Power teams for your truck are on Page 23.

The power team in your truck could ONLY be ordered with a 4.10 ratio. GQ1 is the code for standard axle ratio for a given power team. GQ1 is the only axle option for a k20903/C6P with LT9 and MX1 options. I will say that is a VERY beefy setup except for the NP208. You even have the factory heavy duty cooling package with external trans and engine cooler options.

You might want to just drain the fluid from transfer case and take the cover off the diff to see what the fluid and components look like. If you are making that much noise, you should see some metal. Alternatively you could just have a bad mount somewhere and have components hitting other components when a load is applied to the drivetrain. There have been a few posts about collapsed cab mounts letting 4wd truck cabs hit drivetrain components and make a racket.

 

[brushybarry]

cab and bed mounts look perfect, there is no rust under the truck. Also no parts hitting each other. I scotched the front wheels, jacked the back up and had someone run the truck while I listened and it def sounds to be in the rear end. I checked fluid level and it was low, took almost a quart before running out the fill hole. Im guessing I should go ahead and drop the cover to inspect for damage? She also looks to have a seal leaking pretty good on the front of differential. The truck sat for a while before PO got it, it only has 59k miles, documented. He said when he got it they took the diff cover off trying to find the noise and only thing he found was some pitting on gears where it had sat. I assume that could cause a roar, which I have, but does not explain the thumping sound. I try to work on stuff myself but have never fooled with a rear diff and am pretty nervous about it. Thank yall for the help

 

[brushybarry]

also there is no noise at all under heavy or light acceleration. only when you barely lay your foot on the throttle as if to maintain speed

 

[Snoots]

If I'm not badly mistaken those Government rears are bad to come apart. When you take the cover off check the spider gears.
Also it may not be a bad idea to replace the U-Joints.

 

[Curt]

Taking the cover off the rear differential will help you know dramatically if there’s a problem there.I would except PO word that everything looked good.

I know the gov-lock is weak and prone to failure.Surely you should be able to have someone rotate the tires of you rearend on jacks and watch and listen.

If it’s not the differential,maybe a wheel bearing,brakes,or driveshaft ujoint

 

[Curt]

And I meant to say I wouldn’t accept the previous owner’s word.Your truck now!

 

[Bextreme04]

Sounds like maybe the bearings got rusted or wore out from sitting for so long. You might have excessive tolerances now. If you have a dial indicator and a little know-how, you can probably replace the bearings and re-check all the rear end tolerances. The Gov-lock should be disengaged above 25mph, so it shouldn't be the issue unless it has come apart. I would definitely start by pulling the rear cover and see if you have visible parts in the bottom of the case.

 

[Grit dog]

Don’t mean to hijack, but beextreem you seem real knowledgeable here.
My 86, C6P, 350/Th400/4.10 truck has a SF rear axle and it appears to be factory. 8600gvw too.
I read somewhere that the 87 K20s even high gvw got SF axles.
Hard to believe my truck got aftermarket swapped to a lesser axle? Plus it fairly original. Still has some original u joints in it even, lol.
Here’s my options sheet for reference.

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[82sbshortbed]

Backlash in the rear end?

 

[Bextreme04]

Don’t mean to hijack, but beextreem you seem real knowledgeable here.
My 86, C6P, 350/Th400/4.10 truck has a SF rear axle and it appears to be factory. 8600gvw too.
I read somewhere that the 87 K20s even high gvw got SF axles.
Hard to believe my truck got aftermarket swapped to a lesser axle? Plus it fairly original. Still has some original u joints in it even, lol.
Here’s my options sheet for reference.

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I have found that even though the year ranges are very similar, the standard equipment for each model changed quite a bit year-to-year. Looking at the 1986 Vehicle information kit https://www.gmheritagecenter.com/do...its/Chevrolet-Trucks/1986-Chevrolet-Truck.pdf Page 10 shows the standard drivetrain specs for the K20/2500/C6P as being a 6000lb rear axle, which is definitely a 14 bolt. The 14 bolt FF will show as 7500lb axle rating for sure. The 6,000lb rating seems to be more subjective. All three of our trucks show the same 6,000lb rear axle rating. Mine is almost identical to yours(1980 K25 Sierra Classic heavy duty rating) but is originally equipped with the 14 Bolt FF. I don't see why anyone would have possibly replaced a factory FF with a SF in your case, so it was probably originally equipped that way. Mine doesn't have an RPO related to the rear end at all, so it was a standard equipment. It seems like the OP's could be either one... I can't see any relationship between why one would be equipped over the other.