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What exactly is an HD Th400?

Discussion in 'The Garage' started by rjfguitar, Dec 28, 2006.

  1. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    So the Th400 in my K30 is an HD model, says the PO. I believe him because the guy is a gear head, and said he had a heck of a time finding a complete new trans for the truck when the old one blew, and thought he remembered there being something different with the gears.

    Sounds like a 475 to me that has strait cut planetaries in it, as I can also hear a slight "gear" whine when I take off. Is that what an "HD" 400 is? Let me know what you think.
     
  2. 6.2puller

    6.2puller 1/2 ton status

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    Not sure but I can tell you that the TH400 in my V3500 has straight cut planetaries. Might be an HD for all I know. I guess if their is an HD that would be the application for it. The TH400 in my old 3/4 ton sub did not have straight cut planetaries, they were helical cut.
     
  3. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    Well, that sounds right, and from what I remember from past chatter...a Th400 was called a 475 if it had strait cut planetaries. It was mainly used in RV's with 454's IIRC. I just thought it was a little od with this truck.
     
  4. 1977k5

    1977k5 3/4 ton status Vendor

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  5. JpEater

    JpEater 1/2 ton status

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    The HD Model has the bolt on dust cover with six bolts and also uses two support rods that tie in to the motor mounts. Non HD case has just a sheet metal dust cover that screws on.
     
  6. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    Ooh boo, I thought I had a 475 for a minute there....:haha:

    So it's just a heavier case, I'll go with that then.

    BUT, it still doesn't account for the strait cut gears.
     
  7. JpEater

    JpEater 1/2 ton status

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    Its not really a heavier case just a little thicker where the dust cover bolts on. The dust cover bolted on along with the two support rods is mainly where the extra strength comes from.
     
  8. 76zimmer

    76zimmer Flyin Rat Premium Member GMOTM Winner

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    thats how they relegated it being an HD?:confused:

    I had a 78 3/4t that had the cast t.c. cover on it, with the 2 rods, and got my current t400 out of a 1twrecker, and it has a sheetmetal pan, and 1 rod to the bellhousing. WTH......over:dunno:
     
  9. 79k20350

    79k20350 3/4 ton status

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    Have you pulled the tranny? No way to tell it has straight cut planetaries otherwise...no matter how good your ears are...

    No 475 were put stock into our trucks...

    Hd means 6 bolt dust cover (4wd casting)
     
  10. 76zimmer

    76zimmer Flyin Rat Premium Member GMOTM Winner

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    OK, I just don't know why they would put a HD in a 78 3/4t and a regular in a 85 1ton.
     
  11. jekbrown

    jekbrown I am CK5 Premium Member GMOTM Winner Author

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    yeppers... based on everything I have seen (including the th400s in the garage!) the only difference between the HD and non-HD th400s is the dust cover mounting flange. HD uses 6 bolts, the standard th400 uses 4 tiny screws. As noted by others, the HD used a couple of rods between the bottom of the cast aluminum dust cover and the engine mounts also.

    j
     
  12. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    I can hear what I think are strait cut gears, but I am 90% going off what the PO told me.

    It is pretty much settled, it has to be a 400 as I know this truck doesn't have a different, other than stock trans.
     
  13. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    A buddy of mine had a 78 1 ton 3+3 that had one of those HD TH400s in it stock. He blew the tranny while towing one summer, and had to get it fixed on the road. The guy who replaced the transmission put in a regular TH400, and to this day my buddy is ripped about it. I told him there is no difference less how the inspection plate bolts up, but he doesn't care, lol
     
  14. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    You ever stop to think that maybe the tranny in the 1 ton was swapped at some point in its' life. Look at all the swapping we do around here.
     
  15. u2slow

    u2slow 1/2 ton status

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    The 6bolt aluminum TC cover to designate HD is horsepoop. That's the only style TH400 case you could get in the 88+ IFS trucks - even in a 1/2-ton.

    What is more likely is the newer version is swapped into an older application... don't forget the 80-84 TH400/205 combos used a 2wd output shaft on the TH400. ;)
     
  16. roadnotca

    roadnotca 3/4 ton status

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    Over the years I've many stories of overhauls done by "shops" (engine, trans) where the vehicle owner was stuck (interstate) or didn't know they had HD/rare/HiPo components, and the repairs were completed with standard stuff; the premium parts raced on the local street or dirt the next weekend. :doah:
     
  17. 6.2puller

    6.2puller 1/2 ton status

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    I just came in from installing my TH400 after a rebuild. Was removed from my 88 V3500 3+3 long bead duallie and it was mounted with two strut rods to the dust cover and 6 bolts holding it on. Mine had straight cut planateries. The numbers on my ID tag matched the numbers for a TH400 according to my Haynes tranny manual. The TH400 rebuild kit did the trick so if it is a TH475 they must use the same internals for the most part. Mine had been rebuilt befor as it had a finish nail holding the manual shift rod. Maybe the straight planetaries fit into the TH400 from a 475 :dunno:.

    I don't see how any of this matters when the 16 roller sprague seems to always be the weak point in these as long as the strut rods are in place. I have blown two both due to the roller sprague failing and droping parts into the tranny. The cases I have never seen blow other than in non stock apps where people haven't braced them properly.
     
  18. 76zimmer

    76zimmer Flyin Rat Premium Member GMOTM Winner

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    a 400 in a 1/2t in 88?
     
  19. u2slow

    u2slow 1/2 ton status

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    Sure... OD was optional, 3spd non-OD auto is a TH400.
     

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