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Hi, hoping someone can help me. I have a 71 GTO convertible which I have had for 20 years. About 15 years ago I purchased a THM 400 automatic transmission with a switch-pitch stall designed for high performance. At the time I intended to drag race the car but never did. It is just a street queen.

Anyway, this may a be a simple question for some, but not for me. The car hasn't had much use for several years so I have forgotten a lot of details about the car.

What type of transmission fluid would go in a rebuilt high performance auto transmission. The dip stick says use DEXRON 1387568, but I could swear I put O'Reilly brand Type F transmission fluid (which is not DEXRON type).

Any help appreciated as I do not know which fluid I have in the tranny and I need to add some fluid. Not sure how big a deal it would be if I accidentally had two types of fluid in there.

Thanks in advance,
Gene in Dallas
 

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I may have the answers to your questions, especially if that transmission came from Jim Burek
Hi, hoping someone can help me. I have a 71 GTO convertible which I have had for 20 years. About 15 years ago I purchased a THM 400 automatic transmission with a switch-pitch stall designed for high performance. At the time I intended to drag race the car but never did. It is just a street queen.

Anyway, this may a be a simple question for some, but not for me. The car hasn't had much use for several years so I have forgotten a lot of details about the car.

What type of transmission fluid would go in a rebuilt high performance auto transmission. The dip stick says use DEXRON 1387568, but I could swear I put O'Reilly brand Type F transmission fluid (which is not DEXRON type).

Any help appreciated as I do not know which fluid I have in the tranny and I need to add some fluid. Not sure how big a deal it would be if I accidentally had two types of fluid in there.

Thanks in advance,
Gene in Dallas
I may have an answer for you if you got that transmission from Jim Burek at PAE in El Paso or Poston performance parts when they were still open. Jim was well known for building the switch pitch variety of TH400s. He also recommended Type F for the more firm shifts. It's an old drag racer strategy which I still use. It would likely work fine with either fluid, but I would never mix them. If you ever decide to sell that switch pitch, please let me know. I'm just a bit south of you in Texas and I still love the variable stall capacity of the switch pitch.
 

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Hi Mchael, Thank you for the feedback and yes, it did come from Jim Burek and I have always been happy with it. The tranny started to trip to jump between gears and I realized the fluid was low. My only chore now is to try and determine what fluid I have in there.
Thanks,
Gene


I may have the answers to your questions, especially if that transmission came from Jim Burek

I may have an answer for you if you got that transmission from Jim Burek at PAE in El Paso or Poston performance parts when they were still open. Jim was well known for building the switch pitch variety of TH400s. He also recommended Type F for the more firm shifts. It's an old drag racer strategy which I still use. It would likely work fine with either fluid, but I would never mix them. If you ever decide to sell that switch pitch, please let me know. I'm just a bit south of you in Texas and I still love the variable stall capacity of the switch pitch.
 

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Hi Mchael, Thank you for the feedback and yes, it did come from Jim Burek and I have always been happy with it. The tranny started to trip to jump between gears and I realized the fluid was low. My only chore now is to try and determine what fluid I have in there.
Thanks,
Gene
I have not talked with Jim in many years but I spent a lot of time in his garage and shops when I was an annoying teenager. He tolerated me well. Last I heard, he still recommended Type F in all his builds, so that is probably why you recall using Type F. I also learned over the years that heat and/or low fluid levels kill automatic transmissions pretty quickly so I'd get the level up to the full mark before driving it much more. I have two of his switch pitch builds. One is in my 65 GTO and the spare, which needs a rebuild, is sitting in storage. I wish there was still support for these old inventions. It was a genius design, but GM only produced it for a few years.
 

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I am off to work now but I would like to chat with you some more about switch pitch. I was never sure I was using it correctly. Had it wired to a foot switch next to my dimmer switch.
 

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Hi, hoping someone can help me. I have a 71 GTO convertible which I have had for 20 years. About 15 years ago I purchased a THM 400 automatic transmission with a switch-pitch stall designed for high performance. At the time I intended to drag race the car but never did. It is just a street queen.

Anyway, this may a be a simple question for some, but not for me. The car hasn't had much use for several years so I have forgotten a lot of details about the car.

What type of transmission fluid would go in a rebuilt high performance auto transmission. The dip stick says use DEXRON 1387568, but I could swear I put O'Reilly brand Type F transmission fluid (which is not DEXRON type).

Any help appreciated as I do not know which fluid I have in the tranny and I need to add some fluid. Not sure how big a deal it would be if I accidentally had two types of fluid in there.

Thanks in advance,
Gene in Dallas

Howdy Gene!!

I'll second the motion for running Type-F fluid in a TH400. That's what I've got in mine now. It does firm them up a little and I like that. Type F is "grippier" than Dexron. The last time I had mine apart I put in a different converter as well, so mine is running 100% Type F at present. However, Dexron and Type F are compatible for mixing according to everything I've read. You might consider backing that Vette of yours off the lift, dropping the converter dust shield off the GTO, and turning the converter over so that you can check and see if it's one that has a drain plug since it's a 'custom' build. If not, there's a process for purging the fluid out of the converter but it takes helpers and plenty of new fluid - enough to fill the transmission, converter, trans cooler, trans cooler lines, "and then some". What you do is disconnect the return line from the cooler to the transmission and run the engine while a helper watches the fluid coming out of the return line and going into a container. At the same time, another person adds new fluid to the trans as the 'old' fluid is being pumped out -- it's a "dance" -- you have to continuously add new at the same rate that the "old" is coming out. Once you see new/fresh fluid coming out of the return, you're done. But, supposedly it's ok to mix Dexron and Type-F so you don't HAVE to do that, unless you just want to.

Ready for a rematch? :) That black '69 of mine is stronger than it was last time :)

Cheers,
Bear -aka- Rob
 

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BearGFR is probably correct about safely mixing the two fluids, but I'm old enough to have heard stories about mixtures foaming and causing trouble that I'm still cautious. For all I know, those stories could have been referring to blending synthetic and conventional motor oil, then changed over time as the story was retold to Type F and Dexron. I'm sure it's been done thousands of times, but if it were mine, I'd go all Type F as BearGFR is doing. It really does make a difference, especially if you enjoy hearing people at the track say "you chirped the tires going into third gear!"
 

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I am off to work now but I would like to chat with you some more about switch pitch. I was never sure I was using it correctly. Had it wired to a foot switch next to my dimmer switch.
I'm curious about the foot switch next to the dimmer switch. I recently changed mine from a push button on the shifter handle to a switch under the dash. The handle was ugly and I went back to a stock appearance shift knob. Plus, whenever people saw the wires running to the button on the shifter, they would assume nitrous or transmission brake. I use none of those and like it to be as stock appearing as possible. My toggle switch and the brake pedal are now my controls for the high stall setting. Is your wired into the brake light as well?
 
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