73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Rear Drivelines, Drive Axles and Gearing => Topic started by: Jerr-conn on February 16, 2017, 02:47:23 pm
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I thought I knew how to find rear end ratio in my 87 Chevy s.b. 2 wheel drive. It does NOT have posi.I marked driveshaft and marked tire. I started to turn shaft and about 1/2 way around the tire
Stopped turning. What happened? My speedo reads faster than I'm going. How fast I don't know.
I thought if I knew the ratio, along with my 28" tires, I could use one of the calculators to
Determine what gears I would need. Guess I need some help. Oh, it's a 350 / 700r4 if that
Makes a diff. Thanks.
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I think you have to turn the tire backwards for open diff and forwards for posi trac.
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As noted, my truck has no posi. When up in air, one wheel turns forward and one turns backward.
What do I do now?
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http://forum.73-87chevytrucks.com/smforum/index.php?topic=30546.msg256328#msg256328
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Oo oo! It's really easy. You pop the cover off the diff and count the teeth on the diff. Then drop the drive line. Remove the pinion and count the teeth on it. Then look up the ratio in a chart online.
See? Easy!
LOL
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If the axle code stamps are readable you can decode the three digit alphanumeric axle build code by using the tables in the GM parts books.
I scanned all six 1973-1991 books as searchable PDF files. You can download them from the link in my signature and type the axle code into the Search box in Adobe Reader.
You can search the online versions of my scans and more scanned GM parts books using the search function on Ian Harding's site http://gmpartswiki.com/query (http://gmpartswiki.com/query)
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Open diff procedure to determine ratio. Jack one side only.
Turn tire full 2 revolutions while counting drive shaft revolutions.
As in my case 4:10 gears, 2 revolutions of tire my shaft turns approx 4 1/8 turns.
I've done it and it works.
Like was said also, code on passenger axle tube if the gears have not been changed will give ratio.
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O k . I understand procedure now. But, why do you only have to turn a limited slip rear end
Only 1 turn to get the ratio. Is it the spider gears or something else that makes this way of
checking Different ? I always wondered about this. Explanation please. Thanks
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O K. I used the open differential procedure from BLAZER 74. It worked great and came out to
3:08, same as RPO code on glovebox door. Thanks again BLAZER74.
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In a posi you have to overcome the friction in the discs. When you jack up the rear and spin it, it is like a locked spool so you get the proper number of revolutions. In an open diff the spider gears just spin and the other tire goes backwards. You have to stop the other tire from spinning like other have said.
I have found the easiest and most reliable way is to pull the cover and rotate the ring gear until you can read the stamped ratio on the side of the ring gear!!! Plus...when was the last time you changed oil in the diff?
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In a posi you have to overcome the friction in the discs. When you jack up the rear and spin it, it is like a locked spool so you get the proper number of revolutions. In an open diff the spider gears just spin and the other tire goes backwards. You have to stop the other tire from spinning like other have said.
I have found the easiest and most reliable way is to pull the cover and rotate the ring gear until you can read the stamped ratio on the side of the ring gear!!! Plus...when was the last time you changed oil in the diff?
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I've only had the truck a couple months and it's been cold, so I only have solved a few electrical
Issues, changed cooling system hoses, replaced all engine belts. Replaced rear bumper with new
Sport type purchased from this site. I'm going to change rear end lube and grease front wheel
Bearings. I have a nice heated workshop in my pole barn, but the back is not heated, so l have
To pick decent days to work on the truck, but it's coming along fine. I also plan on lowering it also.
Maybe 2/4.