Posted by Keith on Thursday, August 26, 2004 at 11:03AM :
In Reply to: NP205/TH-400 posted by Marty on Wednesday, August 25, 2004 at 6:09PM :
Having used a TH400 in my M37 (4.89 gears), one draw back to this transmission is the not so low 2.48 first gear when using a t-case with 2 to 1 reduction (the NP200 is 1.96 to 1).
The "crawl ratio" (http://home.off-road.com/~dkf/Cal/) here would be: diff gears (4.89) x t-case (1.96) transmission first gear (2.48) = 24 - a way wimpy number as you want to be around 50 to 100. On steep technical downhill there is little to no compression braking, and with your diesel I am thinking you will be on your brakes always. A small percentage of time, on a very steep uphill, if the vehicle is at a dead stop it is hard to generate the RPMs needed to get going again.
The longest first gear I am aware of for the TH400 is still only 3.00 to 1 and costs $700+ just for the parts.
The better approach is to get the t-case low range ratio to 4 to 1, and I am not aware of anyone making NP205 gears in that ratio. Of course there is the Atlas, but that is $2,100+ in the box, and then you need an adaptor to the TH400.
Another route is to use either a doubler t-case kit from Off Road Design(http://www.offroaddesign.com/catalog/doubler.htm :) which uses the strong side of a NP 203 without the chain, or a Terra Flex conversion kit for an NP231 or, better yet the Dana 300 as it also can be fitted with a disc e-brake on the rear out put shaft. See: www.teraflx.com/index2.htm
See also:
http://www.novak-adapt.com/knowledge/th400.htm
http://www.offroaddesign.com/catalog/doubler.htm :
"GM TH400 Automatic Transmission:
If you have a TH400, there are three options for mating the 203 to your trans.
1: Use a factory configuration. The TH400 was available with the NP203 in heavier duty trucks (3/4 and 1-ton generally) in approximately '78-'79. The factory configuration uses a 1 3/8" stickout 32 spline shaft in the TH400, a 32 spline input gear in the NP203 and a 1.1" adapter plate between the two, somewhat similar to the TH350 adapter plate. This is the shortest way to mate the TH400 and NP203.
2: Use a TH400 that originally had an NP208 mated to it. This TH400 uses a 32 spline shaft that sticks out of the case about 2.5" and has an adapter housing that's about 2.5" thick. You'll use a TH400 version 32 spline input gear in the NP203 and re-drill and tap the face of the NP203 with the round bolt pattern to match up to the NP208 adapter housing. Benefits of this kit are the parts are quite common since the NP208 was very widely used with TH400 trannies. The drawback is the adapter is about 1.5" longer than the shorter factory configuration.
3: Use a TH400 that originally had a NP205 behind it. This TH400 uses a 32 spline shaft that sticks out about 4.5". You'll need to use a factory TH400 to round pattern NP205 adapter housing that is 4" thick and once again, re-drill the front face of the NP203 to match the round pattern. The input gear will be the TH400 version NP203 32 spline input. Drawbacks to this system are the overall length with the 4" housing between the two and the expense of buying the factory housing. They're not common as used parts and typically must be purchased new. ORD can supply this adapter."