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454/th400 doubler question

R

RoTuN

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I just got a 454/th400 that has a 27 spline output with an adapter for a 203 that has a 27 spline input. I also have a 27 spline 205 that just went through a master rebuild, I know I will be changing the 205 to a 32 spline for the doubler kit.

Will the 27 spline output/input from the th400 to the 203 range box be a "weak" point in this? I'm certain it will be, and need to confirm my suspicion, but have read elsewhere that being that far up in the drivetrain it should be fine. I have never done a doubler before so this might be a stupid question?????????????

Here's the th400/203.............

IMG-20111008-00308.jpg


Heres the 205 I just finished................

IMG-20110810-00171.jpg

IMG-20110810-00170.jpg


The 205 is on a th350 I just rebuilt with shift kit, etc. but I scored the 454/th400 while searching for my D60f/14b f
 
I haven't had a problem, my 700r4 is 27 spline right there and it has worked flawlessly, I think because there isn't a lot of gear reduction right there so it doesn't break. Its definately weaker then the 32 spline setup though.
 
onetonbb74 said:
I haven't had a problem, my 700r4 is 27 spline right there and it has worked flawlessly, I think because there isn't a lot of gear reduction right there so it doesn't break. Its definately weaker then the 32 spline setup though.
Click to expand...


Good to know, I just don't want to have to do this all over again later, might as well do it now while everything is apart and easier to get too but I want to finish this thing too!
 
4X4HIGH said:
TH400's are 32 spline output not 27 spline. :deal:
Click to expand...

Yes should be if all factory, but some th375's have th400 cases and pans, w/ a 27 spline output. The 27 spline input for a np203 is a dime a dozen, so it could very well be the case here...and judging by the pic, it even looks like a spacer was installed which wouldn't be stock.
 
Yep, not factory, I do know they both are 32 spline from factory and I also counted both input/ output, they are 27 spline each and there is a spacer installed (photo). So now I need to find the short th400 output and the 32 input for the 203 or should I take the chance on the 27's that far up?
 
onetonbb74 said:
Yes should be if all factory, but some th375's have th400 cases and pans, w/ a 27 spline output. The 27 spline input for a np203 is a dime a dozen, so it could very well be the case here...and judging by the pic, it even looks like a spacer was installed which wouldn't be stock.
Click to expand...

How do I verify th400 vs. th375 without using the info on the case/ pan?
 
onetonbb74 said:
Yes should be if all factory, but some th375's have th400 cases and pans, w/ a 27 spline output. The 27 spline input for a np203 is a dime a dozen, so it could very well be the case here...and judging by the pic, it even looks like a spacer was installed which wouldn't be stock.
Click to expand...

Then it would be a TH375 NOT a TH400.

All TH375, TH400 and TH475's use the same case. The TH375 is basically a light duty TH400 with a 27 spline output, TH400's are 32 spline output and the TH475 is basically a heavy duty TH400 that uses a 32 spline output with straight cut gears.
 
depending on your motor, drive shafts and axles not sure you are going to break it so not sure I would worry about it.
 
4X4HIGH said:
Then it would be a TH375 NOT a TH400.

All TH375, TH400 and TH475's use the same case. The TH375 is basically a light duty TH400 with a 27 spline output, TH400's are 32 spline output and the TH475 is basically a heavy duty TH400 that uses a 32 spline output with straight cut gears.
Click to expand...

I was told it was a th400 from the guy I bought it from, he was told the same from whom he bought it from. Also stated it was rebuilt along with the 203 and they put a part time kit as well. Is there any chance they could have changed the output shaft to match the 203? Both shafts look new with no use as the truck was never completed with the motor/trans/ tcase swap.

Back to one of my questions......does anyone know the simplest way to tell the difference between the th400/th375 BESIDE looking at the case, pan, or tailshaft? I will be spending time researching the internals to find out but would like some info. from here as well.
 
kgblazerfive said:
depending on your motor, drive shafts and axles not sure you are going to break it so not sure I would worry about it.
Click to expand...

Thanks, I have a Gen VI 454 being torn down at the shop right now, had the peanut port heads and a TBI intake on it but will most likely be putting heads/cam/carb on the short block so I'm not sure what torque/hp I will end up with yet. Driveshafts are being donated from the '82 k3500 as well as the D60f and 14b FF with 4.10's, will be putting lockers in at some point after getting it all back together.
 
I have never heard of a TH400 having 27 splines so I am guessing its a TH375

I think you will be fine with that combo of gears and motor a TH350/ 700R4 lives with that all the time so it should not be an issue.
 
It only says 375THM on the underside of the tailshaft and on the pan, not on the case.

Im willing to bet that you have a 400 with an aftermarket output shaft installed, someone probably blew the th350 in their 1/2 ton originally and wanted to upgrade to a th400 for strength and this was their way of doing it back when It got rebuilt while still using the factory 27 spline 203 case.

The 400/203 combo is a pain to find and its only offered in 77-79, and the output for the 400/203 combo is the shortest factory 32 spline output for that trans, so the only option back then was to find the stock setup or go aftermarket output in the trans and use a spacer.
 
Thanks, you guys rock! I will research the trans I have some more, going to open it up anyway. I was thinking they changed the shaft as well but you never know? It sure seems like they did, the inside is extremely clean and the splines/shafts are still "cast" colored like when new. I'll post more as I go along.

Thanks again to all who contributed.
 
factory 375 would be long tail 27 spline.

thats a stock 27 spline 203 . and it will bolt directly to the th350 now as thay use the same shaft length from 203 and 205 on a th350.

you got a 400 with aftermarket shaft and adaptor kit.

and if me and the th350 is built good run that with the 454 just fine. this will get you 20-30 more hp to the rear wheels over a th400 and shorter driveline . and with doubler kit the rear shaft gets short as it is.

i know lots of guys with good built and cooled th350 with 454 and no problems.

just a idea for ya . :laugh:
 
I was contemplating that with the th350 already being done and painted and the hp it takes just to turn the 400. I can go ahead and run the th350 to the 203 and change out the input on the 205 with the adapter. Maybe I will do that and hang on to this 400, give me time to rebuild it anyway then I will have it on the back burner just in case.

btw..this is going in my '74 K10 with 37's and I will not be doing any rocks, just alot of pushing deep snow and trails while hunting, even when I do encounter rougher stuff it's in low and slow for me being far away from any services.

Thanks for the advise!
 
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