sm420, 350, and 2.73 axle gears comments, suggestions

[Cheepjeep85]

like the title says i need some advice on this combination. i'm dropping my 350 off at the machine shop next week and getting ready to do the conversion, i'm trying to decide on a tranny that will work well with my Dana 300 , i know the costs of the adapters and such, but i'm going back and forth on keeping my T-176 and rebuilding it, or adapting a sm420 to my Transfer Case . my main concern is how the gearing of the sm420 is going to match the high crappy gearing in the axles until i can afford to regear (which could be a long time down the road) this is my daily driver so i need something that works good. im thinking of the sm420 for offroad use of course (granny gear) but it has to be usable on the street. id like to hear everyone's suggestions, comments, experiences, or concerns.

 

[BusaDave9]

The SM420 is an excellent Transmission . It has the lowest gearing of any tranny that can be installed in a CJ. You can keep your tall axle gearing and just start out in first. The only downside is your drivetrain from the tranny to the differential will experience higher torque than if you re-geared the diffs. But that's not a big issue. I did the same thing with a CJ7 I used to have. In that case I put in a Ford T-18 and kept the tall axle gears. If this is your daily driver you could just keep it like that.
Installing the SM420 into your CJ isn't hard. Check out this link: Adapting the SM420 to the Jeep Dana 300 Transfer Case You have 2 kits that could be used to make it work. You can get a TCase adapter shaft or a new output shaft for the SM420.

Do you already have the SM420? If not another option is you could get a Ford T-18 to swap in. The advantage of the T-18 is your drive shafts would not have to be modified for length.
The disadvantage to the T-18 is it doesn't have quite as low of a first (6.32 instead of 7).

 

[Cheepjeep85]

I can get an sm420 from a friend for $30 but the Transfer Case adapter is more expensive which is fine, but the cost of buying a T-18 and an adapter for my Dana 300 would offset what id spend on drive shafts. Plus I'm doing the sbc swap at the same time. I guess if I could find an np205 at the same time that might be ideal. My main concern is the gearing, I guess. I know what motor I'm going to use I just need figure out the rest of the drivetrain, that'll match my axle gears. But you say the gearing works pretty well that's makes me feel a little better.
Droooiiddd 2

 

[Old Dog]

should work with the granny 1st. as long as you don't mind spending the money on the adapter.
I had 2:73's with my 350 before I regeared, it still had good highway power and it got good mpg. I say go for it.

 

[BusaDave9]

What size tires do you have?

 

[BusaDave9]

First gear would be twice as low as your T-176 (7 instead of 3.57) When you start out in first with your SM420 it would be like having 5.46 gears in your axles. Tire size wont matter. Unless you have 60 inch tractor tires this will be much lower. But top gear would still be the same 1 to 1. If you have oversized tires it would be like having an Overdrive when you are on the highway.
This SM420 will be much better for off roading. Second gear in the SM420 is the same as first gear in the T-176 (3.5).

 

[Cheepjeep85]

I have 33x12.50 right now, with the possibilty of 35s way down the road. I think I'm gonna go with the 420, I have to drive on the intestate everyday, and with the tall gears it does good with the AMC 258 i6 / 4.2l but its way to high geared for any technical stuff offroad. So I'm thinkin with the added power of the sbc, with this tranny ill be pretty happy with it on and off road. Ill probably wind up selling my AMC 258 i6 / 4.2l complete for the adapter, its only got about 25k miles on it, ill sell it with everything including the carb, intake, exhaust, all accesories and ignition for $500 so that should get my adapter. Thanks for the advice fellas.

Droooiiddd 2

 

[1986cj7]

I have 33x12.50 right now, with the possibilty of 35s way down the road. I think I'm gonna go with the 420, I have to drive on the intestate everyday, and with the tall gears it does good with the AMC 258 i6 / 4.2l but its way to high geared for any technical stuff offroad. So I'm thinkin with the added power of the sbc, with this tranny ill be pretty happy with it on and off road. Ill probably wind up selling my AMC 258 i6 / 4.2l complete for the adapter, its only got about 25k miles on it, ill sell it with everything including the carb, intake, exhaust, all accesories and ignition for $500 so that should get my adapter. Thanks for the advice fellas.

Droooiiddd 2

Have you sold your AMC 258 i6 / 4.2l yet if not let me take a look at it with a few pic and I might take it off your hands!!!!!

 

[Cheepjeep85]

Its gonna be a little while on it, I still haven't got my sbc built yet, I hope to have it done and running in my jeep by early - mid dec. I have to get everything ready and do the swap in a weekend (atleast that's the plan) cause its my dd.

Droooiiddd 2