buzzback - The 1947 - Present Chevrolet & GMC Truck Message Board Network

buzzback · 05-11-2009, 12:01 PM

I just bought a 1985 GMC Sierra Classic, 4 speed manual, 4x4, 3/4 ton pickup, with a 350 engine; it looks good, runs great and the price was right. I know zip about 1980s GMCs so I need some help.
Does anyone know how I can determine tow capacity and also how to determine gear ratio of the differential. Former owner said it was a "heavy duty 3/4ton" (which means what?). He also mentioned that it was a HD engine with a cast steel crank; would this have been factory installed and how do you tell?. Thanks

**N2TRUX** · 05-11-2009, 11:45 PM

Anyone?

AceX · 05-12-2009, 03:46 AM

The gear ratio can be determined by the cast # on the housing, or by the VIN I believe.

http://www.lmctruck.com/icatalog/cc/full.aspx?Page=9

then...

http://www.lmctruck.com/icatalog/cc/full.aspx?Page=10

followed by...

http://www.lmctruck.com/icatalog/cc/full.aspx?Page=11

which leads us to...

http://www.lmctruck.com/icatalog/cc/full.aspx?Page=12

which should explain everything, I hope...

D13 · 05-12-2009, 05:56 AM

First stop, the parts label. usually in the glove box, or on one of the inner fenders. It will have the option codes that the truck was built with. Be aware that many parts may have been swapped over the years.
For the axles, if all else fails (I have never been able to read a code after MI corrosion was done) - use the LMC catalog to tell you which type of axle you have, get a cover gasket and lube, then upen her up, clean it out (probably needs it anyways, I think I'm the only guy who maintains his rear LOL!), then count the teeth on the ring gear and the pinion gear. The ratio is what gear you have.

For the engine, it's most likely just another 350. But there is a code on a pad on the front of the block, just below the cylinder head, that can be checked at Mortec to ID the engine. Don't have the link handy, but look over in Engines.

There's a towing capacity thread a couple posts down. Makes it pretty clear what's what.

Love them GMC's! Enjoy!

buzzback · 05-17-2009, 11:39 AM

Thanks Gents for the help. You pointed me in the right direction and I learned what I wanted to know.

**N2TRUX** · 05-17-2009, 11:54 AM

That what this place is all about...

**old Rusty C10** · 05-17-2009, 11:54 AM

welcome to the board... so how about some pics of this truck?

05-11-2009, 12:01 PM	#1
buzzback Registered User Join Date: Jun 2008 Location: Gold Beach, OR Posts: 3	buzzback I just bought a 1985 GMC Sierra Classic, 4 speed manual, 4x4, 3/4 ton pickup, with a 350 engine; it looks good, runs great and the price was right. I know zip about 1980s GMCs so I need some help. Does anyone know how I can determine tow capacity and also how to determine gear ratio of the differential. Former owner said it was a "heavy duty 3/4ton" (which means what?). He also mentioned that it was a HD engine with a cast steel crank; would this have been factory installed and how do you tell?. Thanks Last edited by buzzback; 05-11-2009 at 12:05 PM. Reason: clarification

05-11-2009, 11:45 PM	#2
N2TRUX Happy to be here Join Date: Mar 2000 Location: Cypress, TX Posts: 39,021	Re: buzzback Anyone? __________________ Follow me on Facebook and Instagram @N2trux.com Articles- "Jake" the 84 to 74 crewcab "Elwood" the77_Remix 85 GMC Sierra "Scarlett" "Refining Sierra"

05-12-2009, 03:46 AM	#3
AceX >130dB Moderator Join Date: Aug 2006 Location: Sacramento, CA Posts: 2,130	Re: buzzback The gear ratio can be determined by the cast # on the housing, or by the VIN I believe. http://www.lmctruck.com/icatalog/cc/full.aspx?Page=9 then... http://www.lmctruck.com/icatalog/cc/full.aspx?Page=10 followed by... http://www.lmctruck.com/icatalog/cc/full.aspx?Page=11 which leads us to... http://www.lmctruck.com/icatalog/cc/full.aspx?Page=12 which should explain everything, I hope... __________________ Living Life @ 130dB LED Tail Light Conversion; How-to Friend. Father. Brother. RIP El Jay ------------------------------------------------------------------------------------------------------------------------- CLICK HERE and for less than $0.07 a day, you too can help support and build a better forum.

05-12-2009, 05:56 AM	#4
D13 Registered User Join Date: Feb 2007 Location: Memphis MI Posts: 1,851	Re: buzzback First stop, the parts label. usually in the glove box, or on one of the inner fenders. It will have the option codes that the truck was built with. Be aware that many parts may have been swapped over the years. For the axles, if all else fails (I have never been able to read a code after MI corrosion was done) - use the LMC catalog to tell you which type of axle you have, get a cover gasket and lube, then upen her up, clean it out (probably needs it anyways, I think I'm the only guy who maintains his rear LOL!), then count the teeth on the ring gear and the pinion gear. The ratio is what gear you have. For the engine, it's most likely just another 350. But there is a code on a pad on the front of the block, just below the cylinder head, that can be checked at Mortec to ID the engine. Don't have the link handy, but look over in Engines. There's a towing capacity thread a couple posts down. Makes it pretty clear what's what. Love them GMC's! Enjoy! __________________ 1987 2 ton 1982 250/TH350 beater in progress Dad's 1981 3/4 L6 3 on tree posi and no options, awaiting restoration or scrapping Plus a mess o' tractors Last edited by D13; 05-12-2009 at 05:57 AM.

05-17-2009, 11:39 AM	#5
buzzback Registered User Join Date: Jun 2008 Location: Gold Beach, OR Posts: 3	Re: buzzback Thanks Gents for the help. You pointed me in the right direction and I learned what I wanted to know.

05-17-2009, 11:54 AM	#6
N2TRUX Happy to be here Join Date: Mar 2000 Location: Cypress, TX Posts: 39,021	Re: buzzback That what this place is all about... __________________ Follow me on Facebook and Instagram @N2trux.com Articles- "Jake" the 84 to 74 crewcab "Elwood" the77_Remix 85 GMC Sierra "Scarlett" "Refining Sierra"

05-17-2009, 11:54 AM	#7
old Rusty C10 Robert Olson Transport Join Date: Oct 2002 Location: recent transplant to NC USA Posts: 20,310	Re: buzzback welcome to the board... so how about some pics of this truck? __________________ Bob 1951 International running on a squarebody chassis "If a man's worth is judged by the people he associates himself with, then i am the richest man in the world knowing some of the fine people of this board" http://67-72chevytrucks.com/vboard/a...t.php?f=25&a=9 (you can review the site rules here!) PM Me for your vehicle/parts hauling needs in the North East US or see my Facebook page Robert Olson Transport Live each day to the fullest.. you never know when fate is going to pull the rug out from under you... I hate cancer!!

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