12-bolt Rearend. 3 series or 4 series??

[Bazooka]

How can I tell if my 12-bolt rearend is a 3 or 4 series? My rear is in bad shape and needs a complete rebuild including ring and pinion.. It has 3:73 gears right now..

Thanks for the help

[Alan's Classic]

I think a 3:73 and up (numerically) is a 4 series, but I'm not a specialist.

[Bazooka]

Originally Posted by Alan's Classic

I think a 3:73 and up (numerically) is a 4 series, but I'm not a specialist.

Thanks Alan.

This seems to be true from what info I have been able to find online..

[198plus]

If it's a 373 gear it would be a 3-Series.......3.07 thru 3.90

[Bazooka]

Originally Posted by 198plus

If it's a 373 gear it would be a 3-Series.......3.07 thru 3.90

Just curious where you heard this info? Pretty sure this isn't correct. All info I can find online says 3:73 is a 4 series but there is aftermarket 3:73 gears that will fit into a 3 series case.

I hope this isnt true I already ordered my gears

[198plus]

A 373 12 bolt......came factory with a 3 series carrier.

12 bolt ED32088 ..............2 series......... 2.73:1 & down
12 bolt 30140PM1.............3 series........3.08:1 to 3.73:1
12 bolt EDB30174............. 4 series....... 3.90:1 & up

Or by measurement:

Originally Posted by Bazooka

Just curious where you heard this info? Pretty sure this isn't correct. All info I can find online says 3:73 is a 4 series but there is aftermarket 3:73 gears that will fit into a 3 series case.

I hope this isnt true I already ordered my gears

[Bazooka]

Randys Ring and Pinion sight says 3:42 and down (numerically) carriers are the thick gear sets which Im assuming is 3 series. That would make 3:73 and higher (numerically) 4 series.

[Bazooka]

I think what your saying is correct for passenger cars. The passenger car and truck rears are different in that respect right?

Car 3 series 3.90 and down

Truck 3 series 3.42 and down

Truck 4 series 3.73/4.10/4.56... 3.90s were not offered

I just looked at several parts suppliers websites and 3.90 is only offered for passenger cars..

Somebody else want to clear this up??

[198plus]

I wouldn't think there would be a difference in the car or truck 12 bolts.....I'll try to find out

Originally Posted by Bazooka

I think what your saying is correct for passenger cars. The passenger car and truck rears are different in that respect right?

Car 3 series 3.90 and down

Truck 3 series 3.42 and down

Truck 4 series 3.73/4.10/4.56... 3.90s were not offered

I just looked at several parts suppliers websites and 3.90 is only offered for passenger cars..

Somebody else want to clear this up??

[wild one]

The truck twelve bolt IS different than a passenger car twelve bolt - every aspect from the diff cover to drums is different!

The factory truck carriers were offered in either a three or four series with the factory ratios of 3.73 and up are ALL 4 series. The trucks that are 3.42 and lower are 3 series.

The aftermarket DOES offer a 3.73 gear that will work on a 3 series carrier - the ring gear is thicker to account for the extra offset of the carrier.

DO NOT use shims to put a 4 series ring on a 3 series carrier - if any of the bolts happen to back out (due to excessive deflection of the ring gear since it is not securely bolted to the carrier) the results WILL be catastrophic!

[Tomsriv]

Not to hijack the thread, but what different ratios were available on 1/2 and 3/4 ton trucks? I thought most were 3:73.

[wild one]

Originally Posted by Tomsriv

Not to hijack the thread, but what different ratios were available on 1/2 and 3/4 ton trucks? I thought most were 3:73.

I can only quote the 1/2 ton twelve bolts, ratio's as follows:

3.07, 3.40 or 3.42 (depends on what year 12 bolt you source them from) 3.73, 4.10 or 4.11. those are the only ratio's that I am familar with.

The two most common for 1964 - 1972 are the 3.07 and 3.73's.

You can find the 3.40-3.42's in the mid 70's trucks, mine came in a donor 1975 GMC that I bought for $150.00 and DROVE home!