The ST400 / TH400 variable Pitch Stupid Question Thread

Discussion in 'The "Juice Box"' started by Dan Hach, Oct 12, 2020.

  1. Dan Hach

    Dan Hach Well-Known Member

    So I'm rebuilding this transmission and I've never done an automatic before. So I figured rather than starting new threads every time I get stuck, I would just start this one and anyone else can dump their dumb question in here too.

    So here's today's dumb question. I'm mostly through this thing and I took the pump apart. I know the variable pitch pumps have a blow off valve for the cooler in case the lines get jammed. There's what looks like a cup pressed in there and then some kind of spring steel looking thing around the perimeter.

    My question is whether something else goes in there or if that's all of it. I'm worried that something fell out and I missed it. I can't see any reason for that steel surround to be in there.

    I've checked several other sources. I've found descriptions but no pictures. Here is a picture of the valve that I'm talking about.

    Thanks in advance.
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Dan, I took a look at my 67 Chassis Manual as well as the Ron Sessions THM400 book I have. They both mention the check valve, but the pictures and text don't mention anything other than the valve. Do you have a Chassis manual? If not, I can send you the ST400 chapter in pdf form if you e mail me at
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

  4. Dan Hach

    Dan Hach Well-Known Member

    Hi Larry, yes I have a chassis manual. It's the same as what you said. They actually show a picture of the puller used to replace the valve. I take it to be the recessed bit on the bottom. They say there's a spring. I think that is under the pressed in part. I didn't know if there was some other kind of plug or button or something that may go inside what the picture refers to as the "check valve".

    Where is the picture that you posted from? I couldn't find anything like that. At least not in the pump rebuild section of the Chassis Manual.

    So, this means that there should be nothing else in there and I didn't drop anything :).
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

  6. Jim Weise

    Jim Weise 1000+HP

    That is all that goes in there... your good..

  7. Dan Hach

    Dan Hach Well-Known Member

    Thanks guys. I have the Cliff Ruggles book, the Shop manual and the ATSG manual. I wanted to buy the Ron Sessions book, but last time I looked, I would have had to sell my first born in order to pay for it. The prices seem to vary quite a bit. The prices in that link are a little better though. I just bought my own copy of that book.

    The hard part for me has been trying even wrap my head around hw these things work. Hydraulics have always been a form of black magic to me. So when something comes up that deviates from the norm, I don't know what I'm looking at. You input is greatly appreciated.

  8. Dan Hach

    Dan Hach Well-Known Member

    Next dumb question. What's the consensus on reducing the feed hole diameter to the converter. The Cliff Ruggles book suggests doing this and gives a very definite diameter range. However, after doing some research it seems that there are many opinions on this and the real answer lies in testing to get the right pressure. Is there much risk of the torque converter ballooning on (mostly) stock 401 nailhead rebuild?
  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    I would not mess with that. I'd worry more about pushing the crank and damaging the thrust bearing.
  10. Dan Hach

    Dan Hach Well-Known Member

    It sounds like this is something you need to worry about when you get in to super high horsepower number; likely well beyond the typical nailhead. GM made millions of these things, I imagine if it was a real issue, they would have addressed it pretty early in the life of the TH400. I already assembled the pump but there was still that lingering "you're going to blow up your car" thought.
  11. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Too much pressure in the converter/cooler circuit can push the converter into the crank. It really doesn't have anything to do with HP.
    Guy Parquette and TrunkMonkey like this.
  12. Dan Hach

    Dan Hach Well-Known Member

    Maybe I misunderstood. I thought what they were saying was that the ability of a high horsepower engine to apply a high level of instant torque created higher pressures in the system. Those higher pressures ballooned the torque converter and pushed in to the crank which ruins the thrust bearing. Their solution was to restrict the orifice in to the Torque converter.
  13. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    The transmission has it's own oil pump, so I don't see what HP/TQ has to do with creating too much pressure in the system. I might be wrong. I have close to 600 ft. lbs. of torque, and a little 9.5" converter. I'm not worried.
  14. Matt69olds

    Matt69olds Well-Known Member

    If you study the flow of oil into and out of the converter, it’s pretty easy to see how converter design has a big influence on converter change pressure. Converter charge pressure comes from the pressure regulator valve, goes into the converter, and returns to the transmission thru the orifice in the input shaft and then thru various holes to lube other components.

    if the converter design limits the rate that the converter can vent fluid, naturally the charge pressure will increase. I’m guessing so many transmission builders suggest the restriction because that’s something that can be controlled on their end. The builder doesn’t have much influence over the converter, so the restriction covers their butt

    I’m willing to bet if the pump needed the restriction HydraMatic would have put one in sometime early in the 400 50 year production run.
  15. Dan Hach

    Dan Hach Well-Known Member

    That's kind of where I left it too. I think it's one of those internet panic things. Sometimes the internet is like the playground in grade school. Everybody knows a guy who had it happen, but nobody' ever met him.

    So moving on, I'm going to to air test my clutches this weekend and hopefully get my endplays set.

    One more step forward.

    BRUCE ROE Well-Known Member

    The practice is to furnace braze a converter for extreme use, to prevent ballooning
    and taking out your thrust bearing.

    The TH400 is very tough and simple, I do not need to understand every
    subtle feature, just to get it together in good operating shape. Bruce Roe
  17. Bens99gtp

    Bens99gtp Well-Known Member

    The converter feed restrictions becomes more needed when you start putting stronger pressure springs in the pump.

    Its not an internet panic thing, its not a hp thing its simple pressure.

    The main pressure in the entire system comes from the pump spring.........change this spring...or add shims to create a higher seat pressure and you change the pressures in ever system in the transmission.......all other pressures are take from this main pressure.

    The pressure rate normally is different in every gear, but highest in reverse.

    So if you put pressure spring that is higher than stk yes you in crease clamping load, yes you shift faster, but you also push harder on every working surface in the system including inside the converter........when pressurized the converter tried to push away from the into the crank......the higher the mainline pressure the more the push into the rear of the crank.

    The same thing happens to cast iron sealing rings if used inside the tranny.....the higher the pressure the more pressure there is against these......what happens when more force is used to rub two metal pieces against each other???

    The higher the pressure the harder the pump works the harder it is to spin also

    So don't go putting a 200+ psi spring in the pump without the charge restriction in place......there is normally very very little gains in going much over 160-180 psi main spring.

    Charge pressure can be measured on the outlet side running to the trans cooler........65-75 psi is a decent safe range

    Directions are out of my CK PERFORMANCE trsnsbrake kit.....but will work for all and was a quick photo to grab.

    Video shows same thing starting at 3:00 but the dude rambles a bit......TIP, USE A BRASS PLUG.....AS THIS GUY FOUND OUT THE STEEL ONES SUCK TO DRILL


    Attached Files:

  18. Dan Hach

    Dan Hach Well-Known Member

    Thanks for the thoughtful responses. That vide is the modification I was talking about. I think given the level of performance I'm going for and the level of upgraded to the trans, I will pass on this one.

    He's absolutely right about drilling the set screw. I was able to do it in my lather without too much drama. But I can see where it would have been a pain otherwise.
  19. Dan Hach

    Dan Hach Well-Known Member

    Ok, time for another dumb question. Generally when I set clearances, I go for the middle of the spec. In this case for end plays, am I better off going more towards the tight side if I can? Or should I shoot for the middle?
  20. Matt Knutson

    Matt Knutson Well-Known Member

    Shoot for the middle

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