They have interchangeable cases so the bolt patterns should be the same. I don't know about the input shafts though.

 

Dana 18 and Dana 20 transfer cases. Are they interchangeable?
I tried a search but got no definate answer. In other words do they have the same bolt pattern to bolt up to.... say a T18 or a T150? Are the input shafts the same? Have the oppurtunity to buy a T18 that had a Dana 18 bolted to it but I have multiople Dana 20s.....just wondering if they are interchangeable and if a 20 will just bolt right into the spot the 18 just vacated. Thanks

Are you sure its a T-18, and not a T-98?

IIRC, most 18 transfer cases have a small bore, and 20's have a large bore. I think a adapter is available for this. In the form of a ring that presses into the 20 bore.
Novak or Advance can probably help with this, more than me.
Bill

 

The Dana 18 rear output shaft is off set and is not in line with the transmission and rear differential. while the output shaft on the Dana 20 (and 300) are in the center line with the engine, transmission etc AND the real differential.

The Dana 18 transfer case has output shafts (both front and rear) that are in line and right of center.

Jeeps built with the Dana 18 have offset real axle to match the output, and Dana 20 and 300 have the rear axle with the differential in the center (more or less).

 

Later dana 18 transfer cases (I believe around 65 and later) have the same large hole, intermmediate shaft size (1 1/4") and shaft centers as later Dana 20's. In fact people have cobbled together double rear output transfer cases using a combination of Dana 18 and 20 transfer cases. The guys on the Early CJ-5 forum should have the info you want.
http://www.earlycj5.com/forums/

 

The rear differential used with a Dana 18 is offset to the right (just like the front is) and the rear differential used with the Dana 20 is centered. This is the biggest obstacle to swapping a dana 18 for a Dana 20. You will have to change the rear end too. You can swap the Dana 18 on just about all the Jeep transmissions that used the Dana 20 except for the T150. The T150 has an oil seal surface machined on the input gear and no Dana 18 input gear has that seal surface.

 

Here are a couple of photos that I took out of my photo files. One is a Dana 18 and the other is a Dana 20. Wish I has a direct over head shot, but you can still get the idea of the difference of the outputs of the two different transfer cases.

Dana 18

Dana 20

The Dana 18 also incorporated the emergency break drum at the rear output. And you can see that they are in a line which would require an offset rear axle for it to properly line up.

All that still does not address the different gears and splines, all of which could probably be made if not avalable.

Converting from a Dana 18 to a 20 (or visa-versa) is not a choice I would make.

 

The bolt pattern is the same. As 80cj mentioned, the 18 has different size input bearing holes, depending on what year it is. So, the answer is, "it depends"

 

Did some of you read the question?

The OP wants to know if a Dana 20 will bolt to the back of a transmission that currently has a Dana 18 on it. He is wanting to purchase this transmission and wants to know if the Dana 20 will work with it.

Outputs, emergency brakes, etc. don't have a thing to do with the answer to the OP's question.

80cj covered the correct answer. If the Dana 18 is the large hole version with the 1-1/4" intermediate shaft it's a direct swap.

 

I agree with above. I have a T-18 mated to a Dana 20 curently in my jeep cj7. I think this novak link may help with some of your questions. http://www.novak-adapt.com/catalog/kit_188.htm