12 bolt carrier question

[rtm]

I wish to change my VERY steep 4:11 gearset, to a more practical 3:42.
I am running a turbo 350 trans. with 26" dia. tyres.
First gear is useless, I am immediately shifting to second gear.
I'm looking for a more streetable setup.
My problem lies with my G.M. 12 bolt (from a late 60's early 70's impala, now narrowed)
All the gearsets I find @ JEGS in my desired ratio (3:42) call for a series 3 carrier.
Since my 12 bolt came stock with a 4:11 ratio, I have to assume it is a series 4 carrier?
4:11 is stamped into the third member housing, near the cover, (by G.M.?)
I can go numerically higher, (lower gearset) lots of options available.
I want to go numerically lower (higher gearset) .
How do I install a 3 series gearset into a 4 series carrier?
Hope this makes sense to someone here .
thanx in advance,
HAPPY HOLIDAYZ,
RICK THE MASON

[Kenova]

I think you are pretty well stuck with changing your carrier.
Have you considered an overdrive transmission?

Ken

[chevyart]

rick isnt there any way to get a taller tire under there?.i know 4.11 is a little much gear, but it makes that baby sing. first gear is always a waste and you have to get out of it right away, but it gets you going real good. you should check with the major manufacturers and see if they make a gear that is thinner(on mating surface to carrier) that will work with the 4 series carrier, but i wouldnt go all the way down to 342. it would take too much away from that nice setup of yours and maybe even require you to change the converter(?). a 373 gear is the absolute best gear to use to get the best of both worlds for a cruising setup and a hi-po setup . i have a NASCAR type ratio of 3.80 for my ford rear that is going in the vega. that was my limit for my street vega(with the big engine). keep us posted art

[72 SS PNL]

4.11 is aftermarket ratio , 4.10 is a factory ratio

factory would only be on a 4 series carrier

aftermarket can be on either a 3 or 4 series , you would need to pull the cover and check the gears and their numbers and contact the company that made them , usually a 4.11 would be a thick gear to be on the 3 series , or a thin gear with a spacer

the spacer can also be on a factory gear with the other carrier

you can only put a higher numerically ratio on the lower carrier with the spacer or thicker gear , you can not go the other way

[monzaz]

Double that.

NO WAY 3.73 is the only gear you can fit on the 4 series carrier. YOU have to have a 3 series carrier or a 2 series carrier with a spacer.
IF you can find a 3 series case you can take all the guts from your 4 series carrier and install all the parts into the 3 or 2 series.

That would be your cheapest way to go. Look on Ebay for the 2 or 3 series case and try that. Jim

[rtm]

thanx 4 pics jim.
here is an old pic of my diff.

[monzaz]

[quote="72 SS PNL"]4.11 is aftermarket ratio , 4.10 is a factory ratio

This info is incorrect- 9 -37 and 10 -41 were BOTH factory ratios. 9.37 were early gears they they switched to 10 - 41 which gave more tooth contact as there are more teeth for almost the same ratio. 1 more tooth on the pinion and 4 more on the ring. Helped bleed heat off and give more strength to the ratio.

in the early Chevy 55-64 10 bolt 8.2 third member drop outs there was ONLY 4.11 (9 - 37) 3.70 (10 - 37 ) Now we use 3.73 11 - 41 more teeth again.

Jim
JD

[72 SS PNL]

but those were different rear ends than the 12bolt , and they only listed 4.10 as the OE ratio

just like a 10 bolt used 3.36 , and 12bolt was 3.31 , though both had 3.55's , and the 8.5 10bolt had 3.23 and 3.42

[MonzaRacer]

As for the myth you need 4.11s to make a good launch, thats BS. My 77 Monza had 3.42 s10 rearend and I used 15x8.5 wheels and Hoosier radial GT 275/60/15 that came in around 27-27.25 tall and it cruised fine with my th350 and was a bullet off the line.
If you go with more torque and less rough cam you will get so much better launch.
And not blow off the tires so easy. Also if boost of any kind is added then you will need even less gearing as turbo cars 500-1000 lb heavier are using 3.00-3.23 with wild abandon,,,I my self am still looking for a 3.08 8.5 s10 4x4 or zr2 rear end. I want newer offset wheels. The minor differences in ratios had to do with gear failures, tooth count/engagement/manufacturer changes only. Heck if you need new gears look for a 2 series carrier and use a spacer or spacer gears. I have built many such 2 series LSD centers with either spacer gears or spacers and no issues, heck got a 12 bolt running 14/32s and a 2 series LSD and a spacer and he runs 1.02 60fts.