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Old 03-29-2011, 05:12 PM
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12 bolt truck carrier id

I have a 12 bolt truck posi carrier I am trying to install in my68 Chevrolet c10. How do I identify if it is a 3 series or 4 series. I was trying to put on a set of 340 gears and not having any luck. Weird combo. 10 tooth pinion 34 tooth ring gear.....thx

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Old 03-29-2011, 06:07 PM
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What problem were you having?? If it was a 4 series carrier you wouldn't even get it in the housing with the 3.40 ring gear mounted on it, the ring gear would crash into the pinion gear because of the ring gear being 1/8" or so too close to it.
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Old 03-29-2011, 07:01 PM
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That is exactly what happend. I had bought the ring and pinion and the positrac carrier separate and used. I wasn't area at the time of the different carrier sizes. I did know about the difference between car and truck. But all good,its part of a gearhead self education. How do you properly identity between the 3 and the 4? The thickness of the flange? What size gear options are available for the 4 series?
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Old 03-29-2011, 10:59 PM
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Difference is in the distance the ring gear flange is from the center of the case. I don't know what the actual measurements are, guys that do this all the time use the casting #'s off the carrier to tell them apart, but I don't know what those are either, only ones I know are for the 8.5" corporate 10-bolt.

You might try Randy's Ring and Pinion web site(www.ringpinion.com), or the Yukon Gear and axle web site for more info.

Case breaks for gear sets are 2.76-3.42(3 series), and 3.73-up(4 series).

In the little bit of surfing I did, I couldn't find anyone with a 3.40 gear for the 12-bolt truck, only 3.42. Do they have any manufacturers identifier or logo on them??
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Old 03-30-2011, 10:40 AM
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the numbers on the ring gear are- 6270 98 10 34 gm 2 79. pinion is 339627 10 34 2 79. the casting on the carrier is 4 541 on the webbing. and there is a stamped number of a23f. I could put those 340 gears in the open carrier for now. but i dont know if they are a street gear
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Old 03-30-2011, 04:04 PM
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Numbers wise it is a factory gear set. My guess is the aftermarket just decided to do the 3.42 and not bother with the 3.40, as it is so close to the 3.42 ratio wise that it would be irrelevent to have both.

They are a street gear, you just need the right carrier.
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Old 03-30-2011, 04:27 PM
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It sounds like you have a open carrier on hand. Compare the RG Flange of your open carrier to the Posi carrier and see if they're the same.
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Old 03-30-2011, 05:29 PM
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The open carrier that I removed had 307 gears and a thinner flange. Making it a 3 series. My research shows only a 3 or 4 series was installed in the truck 12 bolts. Since I already have the 4 series posi carrier, a set of 373 gears are in order. Those 307s never really jived after I switched from a Turbo 350 to a 700r4.

Thanks for the input
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