School me on building a heavy duty Dana 18/20 hybrid

Hay folks. I am trying to find some info on building a heavy duty Dana 18 for my flatty. I am going to running about 300hp through my t/case with a V8 and 34" swampers. I am told by some that the stock 1 1/8" intermediate shaft small hole t/case will be like glass and explode by some, and others say, just rebuild it and leave it alone, as it will hold up just fine. I do know how to drive with some finesse but have the habit of feeding loud pedal on occasion. One friend says I should find a large hole 1 1/4" intermediate shaft case at least. I have a buddy that is willing to give me a Dana 20 but I want to run the offset rear for drive line angles as well as the option of the overdrive. I am thinking of using my Dana 18 and building a hybrid with the Dana 20. I just can't find any info on building one of these up. I am told the last Dana 18s use the same main case as the Dana 20. Can the parts be put together to build up a heavy duty case? Should I just do a quality rebuild on my older twin stick case and just use a good Novak kit with a quality 1 1/8" intermediate shaft? The Dana 18 I have looks to be in excellent shape right now. Can the twin stick housing and outputs be put onto the Dana 20 case? Does anybody have any thoughts on the Advanced Adapters intermediate shaft kit with tapered roller bearings? Thanks for any info.
Greg

 

I would just get a Dana 20 case with the large 1 1/4 shaft, and swap all your dana 18 stuff into it. You may want to run a girdle on it, just to give the case some extra strength. Your only other option is to go to another T-case (Dana 300), but then you are going to have to go to a centered rear end. Back in the 1970's when I used to go to the Jeep races (Hill climb, and sand drags) those guys ran some healthy motors infront of those Dana 18's and they seemed to hold up! Just my $0.05 (used to be 2 cents, then inflation hit).

 

x2

 

Greg, I built mine just as you are proposing, a 20 case with 18 guts. I did go the TerraLows route, but assembly would be the same either with those or stock gears. Perhaps it's my driving style, but I have had no problems and would guess my maximum torque is something around 450 ft lbs.

 

All right. Thanks for the info guys. Now if I wanted to build a 18/20 hybrid, could I gut my 1 1/8" shaft Dana 18 and install everything into a 20 main case? Can I use my gears, shifter housing, and outputs on the Dana 20 case? I was told the intermediate gears are the same between the 1 1/8" and 1 1/4" shafts and it is just the bearings that are different. Is this true? I would love to be able to buy some terra low gears to go with this buildup as well but they are out of my budget right now. I am also temped to just overhaul this case and leave it alone. I can just keep my eye open for a later transfer case for a spare. It just seems that Dana 18s aren't so plentiful in my area any more. They are also kind of heavy to ship across North America. I have a fairly tight budget these days so I like the simple overhaul idea the best.
Greg

 

Yes, you can use your output housings, shift rails, gears etc. in the 20 case no problem. One thing, if you are using the park brake on the transfer case make sure the 20 case has the fill plug in the side and not the back. Also make sure the case has the boss on the driver's side for the clutch pivot if you are using that style of clutch linkage. Some have the boss cast in and can be drilled and tapped, some don't have the boss, and some have the boss and are already drilled and tapped.

The 1 1/4" intermediate shaft will work fine with your intermediate gear, you just need the 1 1/4" shaft, needle bearings, and thrust washers. The id of the gear is the same.

If you wan't the strongest combination I'd recommend stepping up to the 29 tooth gear set if you can find some locally as they are a bit stronger, but if not available then the 26 tooth set will work ok.

Also remember the input hole in the 20 case is the large hole and what you have now is the small hole so this will need addressed.

The AA intermediate conversion is much quieter but requires machining of the intermediate gear which means if you do break one then you have to find and machine another gear. I like the ease of just finding another gear and putting it in. That's just me though.

The 1 1/4" intermediate shaft typically lasts longer and can be a touch quieter as the load is distributed over more surface area and more needle bearings. Less load on each individual bearing that way.

A girdle is a good thing. Tri County Gear was making them. Not sure if they still are or not.

I abuse the heck out of my Jeeps and they all have 18's except the CJ-6.

 

Just to throw this out there, you could have your 1 1/8” transfer case line bored to accept a 1 ¼” intermediate shaft.
I’m with the rest of the guys here that are saying the Dana 18 case is plenty strong. I’ve been flat mean to mine and have had very few problems. In fact it and the SM420 transmission have actually been the strongest part of my combination so far.

 

Good that was the clear answer I was looking for.

That I will have to look into as I want to run the factory park brake on the t/case.

This won't be an issue as I will be running swinging pedals along with a hydraulic pull type clutch on my SM465.

That should not be a problem as I have an Advanced Adapters sm465 to Dana 18/20 adapter kit that looks to have both rear retainers in the kit.
Thanks for the great info. There seems to be a lot of misinformation on this as I have asked around and had a lot of mixed answers. I am kind of thinking I might just grab that Dana 20 from my buddy and hoard it for now. I can do a overhaul on my small hole 1 1/8" case for now and collect the parts to build a heavy duty one later. Then maybe I can afford the terra-low gears as well. I am just not sure if I want that low of transfer case gears. At least they are 3.15:1 and not 4:1 like some other kits. To me, sometimes the low transfer case gearing is too low. It just depends what you do with the Jeep but for what I have in mind even the 2.46:1 gears should work fine. Thanks again for the info nickmil and others.
Greg

 

If you make it down for the Puyallup swap meet, I have a couple 18's in 20 cases.