Dana 18 gears in a dana 20 case

Hey guys, I wanted to get firsthand opinions on swapping d-18 gears into a d-20 case. first of all, is it really worth it? Or is it one of those hypothetical situations where the dana 20 case MAY be stronger? I have a D-20 case that was mated to the t-18 I'm building. I also have a recently rebuilt d-18 transfer case that is currently mounted to my 134/T-90/d-18 drivetrain.

For the drivetrain that I'm currently swapping to (225/granny t-18/ offset transfer case) should I simply swap the rebuilt D-18 over to the new drivetrain, or is it worth the effort to pull all the guts from one and swap it into the other? Are there any hangups with the two different cases? D-18 is out of a 1957 CJ-5, D-20 case is out of a late 70's J20 pickup.

 

I thought I remembered somewhere that the case from a '57 was a "small hole" case, and I wasn't sure if it would directly bolt up to the back of my Jeep T-18. Would that be reason enough to swap the guts over?

 

All 20's are large hole and 1 1/4" intermediate shaft. If stock your '57 transfer case would be a small hole 1 1/8" intermediate shaft.
If the T-18 had a 20 on the back you will need a large hole transfer case housing. All 20 cases I've seen had a stronger casting. Some just a little heavier, some a lot.
Watch for fill plug location. Many 20 cases had the full plug on the back and would be covered by the stock park brake. Some were on the side just like the 18. Some mild grinding on the pan rail of the 20 case may be necessary to install the intermediate gear upon final assembly. This is easy. Use the gasket as a template. The spot where it rounds outward (inside of gasket) right by the mounting ear to mount the transfer case to the transmission is what may need ground. Some do, some don't.
The switch to the 1 1/4" intermediate shaft is an upgrade. Better load distribution on the bearings.


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do you have an offset rear diff or centered ? wouldn't that have more to do with transfer case choice than anything?

 

ohh I get it

 

I'm 99.9% positive that when I rebuilt my transfer case last year the intermediate shaft measured 1 1/4". I'm pretty sure it was a small hole though, because I remember having to take the gear off to pull the trans and transfer cases apart. I took a look at my d-20 case and the fill plug is on the rear, so I won't be using it. However, I have both a '67 and '69 dana 18 cases, which should be a large hole and larger intermediate shaft, correct? Is the tranny/t-case mounting pattern the same between the d-20 and d-18 so I can use whichever I choose?

 

The 1957 D18 should have the 1.25" shaft as that switch was midway through the 1955 production. However, if it is already mounted to a T18, my guess is that it would be a large index case and if that is what it is, you may or may not gain some strength from a 20 case. In my experience, there is not too much difference-if any- between the large index cases for 20's or 18's. All are significantly more robust than the small index 18's and if a small index is what you have, yes, do the switch. I don't recall having to grind the pan rails when I built my D18 using a 20 case.

 

My d-18 is currently mounted to my f-head/t-90 combo. I have two other drivetrains from the late 60s. The Dana 20 came from a 70's j20 pickup, but has the fill hole in the rear so I wont be using it. I'm going to take the fresh gears from my hurricane drivetrain and toss them in one of the big hole D-18s. Thanks guys!