D20 Vs D300

The next thing I want to do to the CJ7 is put a lower low range in. I have a D300 that I was going to put a 4:1 kit in, but that costs $900 plus the price of the adapter and aggravation of swapping it in. I was thinking of just putting a 3.15:1 kit in another D20 and going with that too. I know a D300 is stonger, but I think the D20 will be fine. Thoughts?

 

You can break either unit if you beat on it long enough......
Me..D20s are more plentiful than D300s
And cost less than D300s
Support for D20s is still ongoing AFAIK...D300 stuff getting harder to find.
I say D20 based on my thoughts....more bang for less buck overall.

 

Gearing ratios and Jeep use will have a huge effect on capability.
The objective is always to produce a synchronous system.
What engine, transmission and what’s your final drive ratio ?

 

360, T18 w/6.32 1st and 3.73 axles.

 

Nate here’s what I see.

360 = ample torque and HP
T18 = plenty of strength and gear spread
3.73 allows good hiway speed at proper rpm
also provides a very strong (massive) pinion.

I feel you have more than ample torque available.
Lowering gears will only serve to slow down your drive speed.
In other words lowering the speed off road simply allows the driver more time to react via steering.
This all implies there is a point diminishing return concerning drive speed which is basically quantified as crawl ratio.

75/1 crawl ratio is sufficient low for most rock crawling. But you might consider going 100/1 or slower for extreme control with steering maneuvers.
Especially if you have huge tires.

We are always aiming toward a complete and synchronous system with no weak links that are not intentional.
Because the engine and transmission are so strong and is putting out a zillion foot pounds of torque, I would lean toward the standard D300 transfer case.
Also I would stay with D300 if original to the jeep.

6.32 x 2.62 x 3.73 is at 62/1 crawl ratio.
You have an excess of torque and slow enough for any trail use but not slow enough for technical crawling.

6.32 x 4.0 x 3.73 = 94/1 crawl. With this ratio you can do it all ….

 

The kits for the D20 are hard to find. Herm the overdrive guy bought up the remaining inventory. I put one in my D20 about a year ago. My 73 is a 258, T18 6.32 1st, D20 with 3:15, 3.73 R&P with 30x9.5x15 tires. Great crawl ratio.

 

I want to run Q78 tires, which are basically 36x10.5.

Advance adapters still sells the 3.15 D20 kit. My jeep currently has a D20, so I would have to adapt the D300 to the T18.

 

Well if ya already have a D20 then it makes some sense to try and stick with it.
Yeah adapting to anything but a super rare Texas pattern D300 would create a few extra issues.
Just remember that sometimes guys have enough torque to break the D20 rear output shaft. And higher FDR will help promote that potential problem.

Let’s see 6.32 X 3.15 X 3.73 gives you a 74/1 crawl ratio. That would be a great trail speed with 36” tires but may not be ideal for technical slow crawl manuevering

 

I was texting nickmil and he said he would do a Dana 20 with the 3.15 kit.

 

I miss nickmil !!!
He's a great resource.

 

Oh no say it ain’t so.
I was wondering about Nick.
Now we lost 2 of the very best Timgr and nickmil …
We were just blessed to have those guys for so long

 

I recently finished up a 300 rebuild with parts and ( much appreciated)tech assistance from Nick. It's an awesome case with the best factory low. That said some of the factory gears are unobtanium ( notably the intermediate) and buying cores at 3-400 a piece is a losing proposition.
The JB gears look awesome but in your shoes I'd punt with the D20 ( I was already in an adapter and TH350)

 

This is a problem, output shaft is not very strong, and with 3.73's it's working hard. FWIW, we found that a salmon egg jar fit perfectly in the output hole after the shaft is gone...

 

Easy solution, carry a spare D20 output. I have about 6 of them.

 

I know it's not just lying around somewhere but it sounds like you are looking for a NV4500/Dana 300 and 4.10 or 4.27 gears.

 

Cheap improvement, put the Dana 18 gears in the 20. The Bronco gear you need to make the swap is now being reproduced.

Tim hasn't been here in a long time but I still see Nickmil every once in awhile.

 

Not a bad idea. Could do that for now until I save up for the d300 swap. Which gear is that?

 

I thought nickmil did a writeup on it but I can't find it right now. I'll see if I can find it when I get home.

 

This is what I had in my 73 with a 258 years ago on 33s. Ran around in second most of the time but had first for the technical stuff. Worked well in KY

 

I remembered that the guy I bought my scout D44s off of was selling the engine, transmission and transfer case from the scout. Transmission is a 727 auto with a TEXAS pattern D300 attached. He wants $300 for it all. Will that bolt right up to my T18?

Edit: called Novak and the short answer is no. The drive gear is the incorrect spline count for the T18.

Wonder if I my machinist buddy could re-bore and spline the automatic gear to work on the T18?