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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Rear Drivelines, Drive Axles and Gearing => Topic started by: jbell1 on June 11, 2011, 05:47:12 PM
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I've checked the radiator bracket, all it says is a bunch of emissions stuff and speed screws. The door jam part of the door only has tires and tire pressure. It's on a 1985 Chevy c20, 305 (well was a 305), big brake drums if that makes a difference.
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Look at the sticker in the glove box.
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Or pull the cover. Count the teeth on the pinion gear, and divide them into the teeth on the ring gear.
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Okay thank you guys
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Or pull the cover. Count the teeth on the pinion gear, and divide them into the teeth on the ring gear.
As our trucks get older you never know whats been swapped in and out of them.
The older they get the less you can can trust the option code sticker.
In the past month I've run across 2 4x4's that had miss matched gears front/rear.
Like Blazin said, pull the cover.
GM gears will have the teeth count stamped on them. Just divide them out to get your ratio (10 40= 4.10's).
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Okay well it's been raining here for the past few days so I haven't been able to check the gears but any opinions on what the best ratio would be for a built 350 2 wheel drive truck? I plan on upgrading to a limited slip anyway.
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It depends on what your driving habits are. In-town truck with no highway use and/or pulling a load 3.73 or 4.10 should be fine. Highway driving mostly with no loads, 3.42 or a 3.21. Automatic, Automatic Overdrive or manual transmission? Lots to choose from. Me personally I like my 3.21 in my 1985 C20.
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It's highly unlikely that the rear gear has been swapped. It's also highly unlikely that the entire rear axle was swapped (in order to change rear gear).
You can take your vin# to the dealer parts counter and they can look up exactly what gear (among other things) you got. Probably best to be a chevy or gmc dealer because the others, b and c, may not really want to deal with you as much.
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Stew, I think Fitz is correct with his statement. The older these vehicles get, the more likelihood of having parts interchanged. I have knonw many of our trucks get the entire axle swapped. When going to salvage yards, the first things stripped from these trucks are the axles/gears. Every 73-87 p/u out in the graveyard no longer has its rear gears.
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It's highly unlikely that the rear gear has been swapped.
why is that? Someone already pulled the 305 and installed a built 350. When your dealing with an old modified vehicle anything goes. Keep in mind he is looking to add a limited slip carrier to his axle. In order to buy the correct limited slip unit you must know your gear ratio.
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Okay to take care of any confusion, the truck was bought by my great grandfather in 85 new. Then my father bought the truck from my grandfather with 10k on it bone stock 305 and all, drove it to 30k as a secondary truck until 03, then he parked. He gave me the truck about a year and a half ago, I replaced the gas tank (hole), entire rear brakes (drums to lines), new rocker panels, then a month ago we pulled the 305. Built a 350 from a 350 block 40 over to a 355, flat top pistons, AFR performance heads, hi flow oil pump, edelbrock performer intake edelbrock performer electric choke carb, compression ratio is 9.5:1. nothing was changed on that truck unless it was changed by me the rear gears are the stock ones. oh and replacement distributer cap and 8mm wires haha if that helps
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Built a 350 from a 350 block 40 over to a 355
.030 over is a 355. So you have like a 356.6 at .040 over. Not prudent, just sayin.
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haha sorry :P
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No need to apologize, haha. And it sounds like a pretty nice build. Is yout truck a manual or auto?
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Yupp its a manual. I checked the glove box sticker and the only things i found that had anything to do with the rear end were that it said it had the standard rear axle and it said this "7.50r-16d TBLS HWY s/I" option number "YWY" have pictures but i need to compress them i will have them tomorrow.
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Thats the stock tires. My options sheet doesnt say my gear ratio either...
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Stew, I think Fitz is correct with his statement. The older these vehicles get, the more likelihood of having parts interchanged. I have knonw many of our trucks get the entire axle swapped. When going to salvage yards, the first things stripped from these trucks are the axles/gears. Every 73-87 p/u out in the graveyard no longer has its rear gears.
It's highly unlikely that the rear gear has been swapped.
why is that? Someone already pulled the 305 and installed a built 350. When your dealing with an old modified vehicle anything goes. Keep in mind he is looking to add a limited slip carrier to his axle. In order to buy the correct limited slip unit you must know your gear ratio.
Because it's too complicated to change the rear gears and it's too much hassle to change out the entire rear axle assembly--- and for what? It's not like you're going to racing with one of these. At least not with a non-posi 3.73.
i would be willing to bet money it has not been changed from stock.
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So pulling off the cover is the only way to know?
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Pulling the cover is the guaranteed way to find out what you have.
Swapping out the entire differential is not that much of a hassle, and most of the trucks I have seen, including mine, have been swapped at some point. Example; the original rear pinion gear on my k10 exploded, and it was cheaper to buy a parts truck with a good axle than to have a new r&p set up for it, so I swapped it out. Because it's a 4wd I had to swap the front too so they would match. In the next month or so I will be swapping both axles to get the lower ratio I want from my suburban parts truck now that I am running taller tires. That will be the third rear end running under just this one particular truck. In other cases, people will swap in a 12 bolt, or upgrade to 3/4 ton running gear for strength. When the 3.08's come out of my truck they will be going in a friend's 1980 k10 to replace the 2.73's. From what I have seen not only is it common but practically guaranteed that the axles have been swapped.
If yours has the original equipment I would say that's the exception, not the rule.
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i would be willing to be money it has not been changed from stock.[/b]
Growing up my parents told me I could be anything. I never thought of being money. Sorry Stew, couldn't resist.
All I'm saying is that if it was my truck and I was going to spend $500 or more on a limited slip for it I would pull the cover to make sure I order the correct parts.
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Okay thanks, but isnt there a catalog somewher that should tell me what the rear end is online? I've tried looking but cant seem to find one anywhere, it is a 3/4 ton axle so i'm thinking its a high ratio, I just wanna know for sure what it is before I go swapping stuff into it.
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3/4 tons usually had lower ratio for towing. Especially with a 305, it would need lower gears to get 'er goin. Im a little shocked it had a 305 from factory. Usually most 3/4's ive seen have 350's. If you look.at my rpo sheet i posted earlier, it says "5.7 litre (350cid)". Does your say 5.0?
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yeah it says 5.0 305, it also says 5.0 on the radiator bracket sticker.
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I almost bought a 1986 C-20 with a 350 4 speed that had a 3.42 but only wound up with the rf fender and some misc. pieces.
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I have an 84 C20 with a 305, TH 400, and a semi floating rear end with 3.42 gears. All original. I got it from the original owner.
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okay I think rather than pulling off the cover I'll just bring the vin number to a dealer and see what i got for a rear end I just really dont wanna screw shoot up when i open it.
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Another way to tell what gear is in the axle without pulling the cover is to mark the driveshaft at say 3 oclock and then the axle housing on the pinion snout so the 2 marks align, then mark one of the wheels at say the 6 oclock position. Have someone roll the truck forward while you watch the mark on the drive shaft(needless to say...DON'T BE UNDER THE TRUCK look from the side out of the path of the wheels) and count how many times the marks pass each other and stop when the wheel makes 1 complete revolution. (wheel mark is back at 6 oclock) Guesstimate the last fractional turn. 1/4 turn would be .25, half turn would be .5, 3/4 turn is .75 then add the full driveshaft revs to it. Like- 2 full turns plus not quite a 1/4 turn would be 2.23-1, 3 full turns plus not quite 3/4 turn would be 3.73-1 etc.
If you have help, that would be quickest and easiest.
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Wow never thought of that haha I'll probay do that it sounds pretty easy. When I do it I'll post what I get so you guys can help me figure out the exact ratio
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Instead of rolling the truck, save yourself some energy, and jack the wheel off the ground.
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Taking the diff cover off and seeing what is written on the ring gear would surely be the easiest option?!? It takes half an hour max! All you need is one socket and an oil tray.
Once you've taken the diff cover off rotate the crown wheel until you see 8 numbers stamped into the edge, in groups of 2. I think mine said summin like:
12 41 11 87
This means 12 teeth on the pinion, 41 on the ring gear, 11 - month of manufacture, 87 - year of manufacture.
41 divided by 12 equals a 3:42 for me.
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If you jack it up and try to turn the wheels on an open diff. you end up spinning the spider gears in the differental and it does not give you usable info. Whichever wheel you spin the other turns the other direction and the driveshaft does not turn. It would work on a posi or locked locker.
As far as which is easiest I guess it's just a matter of preference. Pulling the cover would be 100% accurate. Doing it this way just saves jacking it up, the mess, the time to clean gasket surfaces and resealing and refilling.
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When was the last time the gear oil was changed in the diff? Pull the cover, inspect the oil for metal shavings, count the teeth or read the stamped ratio, new gasket and install cover, fill and now you know! :D
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Sorry I haven't been on in a few days but i'm currently taking finals for school i restart work on the truck monday, thats the first on my list and pretty sure the oil was never changed.
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If you jack it up you only jack one wheel. Think thats why he wrote " jack the wheel " singular.
Best way is to pull the cover. Buy a few quarts of 80 / 90 gear oil, several cans of brake clean, a high quality silicone to make the new gasket. and spend 30 minutes under there.