MADMAC Posted July 31, 2009 Share Posted July 31, 2009 Talking with a friend who does my tranny work, and he asks what all am I going to end up putting in it and have I considered going with the "Heavy Duty" case. One of the reasons we got to this point was because I was telling him that I plan on overbuilding (if there is such a thing) the transmission and rearend that way in the future when I'm really putting some power down I can be ready to rock; and not have to worry about finding the weak point... at least worry as much. Now I know the best way to go is with a fully built unit from a recomended shop but I know my buddy goes good work. He says the HD 400 has a thicker case, deeper pan, and 6 bolt converter. Do you guys who run highstall aftermarket converters utilize all six bolt holes the standard three? Like I said this time around I'm not really thinking I'm going to need a highstall or anything fancy because the motors just going to have a mild cam... anyways, I'm rambling. The question I have is "Is a Heavy Duty case worth looking into or just stay with what I got and avoid the extra weight of the HD case?" Thanks for any input MAC Quote Link to comment Share on other sites More sharing options...
Mike 57 Posted July 31, 2009 Share Posted July 31, 2009 Mac, Never really heard of a 'heavy duty ' case. I know that GM made a turbo 425 it is exactly like the 400 but it has straight cut planetaries that are stronger but make more whine noise. They were used in larger trucks and motor homes, they usually had an e brake hung on the back and a bolt on yoke. Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted July 31, 2009 Share Posted July 31, 2009 I've never heard of the HD case either Quote Link to comment Share on other sites More sharing options...
Shadow Gray SS Posted July 31, 2009 Share Posted July 31, 2009 I wonder if he is thinking of the 700 "K" case??? Quote Link to comment Share on other sites More sharing options...
MADMAC Posted August 2, 2009 Author Share Posted August 2, 2009 Mac, Never really heard of a 'heavy duty ' case. I know that GM made a turbo 425 it is exactly like the 400 but it has straight cut planetaries that are stronger but make more whine noise. They were used in larger trucks and motor homes, they usually had an e brake hung on the back and a bolt on yoke. Okay well I just got off the phone with my buddy and I told him you guys said he was a liar , just joking. He said he is familiar with the 425 and he also agreed that they came out of heavy trucks and motor homes. He says they are really strong but wouldn't really have a drag race application. Says there kinda hard to find too. But as far as the HD case; he tells me they come out of 4x4 vehicles only. It has HD stamped on the bell housing and some extra webbing for strength. But other wise it is identical the the regular th400. This guy builds offroad vehicles and used to do Hot Rods, so he finds all kinds of heavy duty parts to try and cut down on the trail time wrenching. (used to have a 85 MC that was a screamer) so he'll see if he can get me some pictures of HD 400's that he's built for guys in his OffRoad Club. (Midnight 4wheelers) Hopefully get some pictures soon. Quote Link to comment Share on other sites More sharing options...
Crow Posted August 5, 2009 Share Posted August 5, 2009 If its the same price, what the hell..get it, otherwise, I'm gonna say the 'HD case' is unnecessary unless you're gonna go off roading over rocks or Miatas,you want to make sure the internals are hardened where necessary for your future plans. Not a trans guy though. Quote Link to comment Share on other sites More sharing options...
MADMAC Posted August 17, 2009 Author Share Posted August 17, 2009 Okay fellas, well I was finally able to take some time and download some pics. Here is a few of an early "HD" case or better know as the th-400 "k" case. Like I said it's a little thicker, has a flywheel/starter cover thats made of the same aluminum/magnesium as the case. (Not just the regular stamped steel flywheel dust cover) As mentioned this is an early model case so it's stamped with 26 on top the later cases (I think he said 1980 and on) have the HD stamp. Otherwise it's identical to the regular 400. In some of the pics you can see the extra six bolt holes that run down the underside of the bell housing to attach the cover; on the underside of the cover you can see the two dogears. These two bolt hole locations are for two thick steel rods that attach to the cover and down the sides of the motor to the frame mounts. The rods which have plates on the end of them sandwich inbetween the framemounts and the motormounts. All they are for is to triangulate the drivetrain so you don't flex and break the bellhousing. Most likely I won't be able to utilize these bolt holes because a cars crossmember is in a different position than trucks are. Like I said maybe I'm looking into this a little too hard but I figure if anything the case is a thicker, so "in theory" it should be a little stronger. And I like how the case has the cover which encloses the flywheel, starter bendix, and torque converter. Quote Link to comment Share on other sites More sharing options...
72MC Posted August 17, 2009 Share Posted August 17, 2009 That flywheel cover looks indentical to my 4L80 tranny. - Dave Quote Link to comment Share on other sites More sharing options...
Mike 57 Posted August 17, 2009 Share Posted August 17, 2009 Thanks, Mac Quote Link to comment Share on other sites More sharing options...
monte70car Posted August 17, 2009 Share Posted August 17, 2009 Maybe I missed it but what year and wher were those trans in? I have a 400 from a 78 Surburban that was in my Monte and is going into the the Elkie and it looks like that trans. Quote Link to comment Share on other sites More sharing options...
Guest slacker Posted October 11, 2009 Share Posted October 11, 2009 Guys, I work for Frito-Lay and we've had the HD turbo 400's in our fleet for years. They came in the trucks new, behind the 6.2L, until 1992 when we went with the 4L80E. After the trucks were dogged out, We would completly rebuild the trucks and use them behind a Cummins 3.9L. I never really noticed the difference other than the output shaft was threaded so that the yoke could be bolted on. I had a 1988 Surburban 2wd that had the HD on the top of the case. This was the first time I ever noticed the HD. Which seems odd for GM to call the later cases HD when the TH-400 was only being installed in trucks in 1988.. The only thing that was different on the later year TH-400 was the bolt on flexplate cover. Which is a nice piece! Quote Link to comment Share on other sites More sharing options...
Andy's Auto Posted October 19, 2009 Share Posted October 19, 2009 As far as I know, anything with a bolt on yoke is a HD case. Both of my done Montes are sporting what should be bullet proof HD 400s built by me. The plan is to take the 71 out next Friday ans spray the snot out of it to see if it will hit low 11's with a very mild 496. Andy Quote Link to comment Share on other sites More sharing options...
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