67 Drivetrain Advice needed

[67ANNMRGT]

New to the forum...
Just acquired a 1967 Bronco, looking for some advice on drive train modifications. Won't be a daily driver, mainly a trail rig and short trips around town.

What I know right now:
67 Bronco, 302 TBI, NP435, Dana 20, 2" body, 5.5" Suspension, 37's
D44 Power Lock, 4.10 gears, 30 spline Moser axles, CTM joints, and disc brakes up front
Ford 9" Detroit Locker, 4.11 gears, 28 spline, drum brakes

Advice on gear ratio? Seems like I should be at least 4.88's with 37's?

What about an Atlas 2?

 

[Apogee]

I think most here would agree that your rig would benefit greatly from some deeper gears, at least 4.56 and more likely 4.88's or 5.13's. Your trans doesn't have an OD, however since you don't seem to want to drive it fast for long distances, you almost can't have too much gear IMO...either way the 302 won't mind doing the RPM's, but you might. Your NP435 is a glorified 3-speed with a granny gear, similar to the EB 3-speed RAT/RAN with respect to how it drives, other than shifting slower.

I'd save the ATLAS for when the rear output on the D20 lets go, since it looks like that might be your current weak link after the 28-spline rear axles.

I'm putting 4.88's in my rig with 37's, but I have a relatively torquey 351W, so that helps. I do intend to run mine on the highway for extended periods of time, so I have an NV4500/ATLAS combo going in as well. That said, I probably could/should have gone 5.13 while I was there, but before I make judgement, I'd like to drive it a few thousand miles.

 

[DirtDonk]

Hey there. Welcome to classicbroncos!
Definitely the 28 spline rear axles are a weak link. A Detroit and 28's is a bad combination usually. Especially if they're stock materials.
ARB dropped their 28spl version years ago to avoid the issue. Surprised Detroit is still selling them, but I guess there's a need still.

With axle shafts being so cheap by comparison, when you upgrade to a Detroit it would be easy to justify the extra expense. But now that you already have it, and it's 28 spline, it's a lot more work to change. Pretty much starting over in fact.
So running them and taking it easy with your right foot may let them live long and prosper.

However, if you do decide to change the gear ratios, then I would go all in and change the Detroit to at least a 31 spline unit (if not a 35 in fact) and go with new shafts as well.

Good luck. Sounds like a fun rig since unlike most, you're not trying to make a be-all and do-all rig that cruises at 80, gets 22mpg and crawls the nasty stuff with the best of them.
Being able to bias it towards off-roading only is a bonus.

Paul

 

[Pa PITT]

Welcome to the CLASSIC.

 

[ep67bro]

Yup need some lower gears for sure. And like others have said your axle shafts are a week link. Don't waste you money on an Atlas, save your pennys for new gears and axle shafts. With the NP-435 and the Dana 20 in low range you will be able to crawl good. Especially if you get the lower gears in your axles. Have fun sounds like a great rig to start with.

 

[67ANNMRGT]

Thank you for the words of wisdom. I have the small bearing rear housing, which limits my choices to a 31-spline I believe. 4.88's seem to be a good middle ground.

What are the opinions of the Grizzly Locker vs Detroit?

I could bit the bullet and upgrade the entire rear housing to the large bearing style that way I can use the tapered bearings and option up to a 35-spline. Any recommendations on a housing?

 

[sykanr0ng]

A heavy duty rear output shaft for the Dana 20 costs a lot less than an Atlas and you can keep your old one for a trail spare (then you get to be a hero to your buddy that breaks his).

 

[Stock 77]

It all depends on how hard you want to wheel your truck, but another option is to put terralow gears in the D20. That one mod will get you where you want to be and still have a good gear ratio in high range for driving on the highway. upgrading the D20 output shaft is recommended as stated above.