Transmission swap.

john87442

Apprentice
Supporting Member
Mar 9, 2021
62
18
I was searching the forum about this and thought to ask a few questions and start a separate conversation.
I’m installing a built 403 in my 87 442,I believe about 475 to 500 hp. (Don’t have dyno results yet).
My old 200r4 trans will be replaced with a th400. I know it will handle the power but what else have I not considered? Drive shaft length? No overdrive (I do drive freeway sometimes) different cable length? Speedo issue? Always appreciate the knowledge here. Thanks in advance!
 

69hurstolds

Geezer
Supporting Member
Jan 2, 2006
6,838
113
Using a tire diameter of 26.02" (Eagle GT stock diameter) there is NO driven gear for a TH400 that will net you where you want to go without a gear ratio adapter. If you use different tires, ensure you know the exact diameter or rolling circumfrence of the tire, usually in inches. You need to convert to revolutions per mile is the idea.

Diameter would use 20,168 and divide that by the diameter to get the revs per mile. 20,168/26.02 = 775.09 revs/mile .If using rolling circumfrence, you'll need to convert to feet per one revolution, then divide that into a mile in feet (5,280). For the Eagle GT 215/65-15, it's 6.812 feet rolling circumference. Thus, 5280/6.812 = 775.14 revs per mile.

Revs per mile x final drive ratio x drive teeth and divide that by 1001 = the number of driven teeth needed. TH400 uses 34-45 teeth. (NOTE: 18 teeth is the typical factory TH400 drive gear, but others are available. Changing it will require some slight tail end invasive work to do on a short shaft TH400.)

Anything you get outside the 34-45 tooth driven gear range would need changing the drive gear or getting an adapter.

This guy here, whoever he is, :LOL: thinks he knows what he's doing by installing his drive gear. Actually, oldsofb does an excellent how-to video on how to do this, as all his vids are excellent. Ironically it's going in an 87 442. He installs a 15 tooth drive gear, and if you used that same 15 tooth drive gear with 3.73 and stock Eagle GTs, you'd need a 43 tooth (p/n 1362196) without an adapter. He's using different tires, and all that, so his setup is likely different than what you're seeking perhaps, so just use the procedure within regardless of which gear set you use.


Now, assuming you have an 18 tooth drive gear and don't want to change it out, you will get 37.5457 gear needed by using a recommended 0.7222 adapter. If you use a 0.7333 adapter, you get 38.1223. Keep in mind, the further you are from a whole number, the more the speedo error may occur.

Red 37 tooth or Blue 38 tooth should get you started in this case. I'd probably go with the 38, but that's up to you. Blue 38 is p/n 1359272. Red 37 is p/n 1359271.

There are two speedo gear housings that go into the side of the trans tail for the driven gear. 34-39 tooth version and a 40-45 tooth version. You would use the 34-39 tooth version with the adapter mentioned. GM p/n 397807. The 40-45 tooth version is GM p/n 397806.

Good luck.
 
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pagrunt

Geezer
Sep 14, 2014
8,046
113
Elderton, Pa
Using a tire diameter of 26.02" (Eagle GT stock diameter) there is NO driven gear for a TH400 that will net you where you want to go without a gear ratio adapter. If you use different tires, ensure you know the exact diameter or rolling circumfrence of the tire, usually in inches. You need to convert to revolutions per mile is the idea.
Not quite at 26.02", but '70-'72 Z28's & '71-'72 Chevelle SS's used F60-15 tires sitting at 25.90". They may be a good base to see about getting a driven gear to work.
 
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stew86MCSS396

Apprentice
Aug 1, 2022
81
18
(808)
I've done this sort of swap back in the day of slow 56k dial-up internet in a '81 Regal. Donor trans came from a '72 Buick Skylark which reportedly had some 3.90ish rear gears. I paid no attention to drive and driven gears so needless to say, speedo was off by a lot as I was still using the original 2.41 peg leg, but thank you^^^for posting. I used the dual hump crossmember out of the '72 and drilled holes to where it all lined up. In your case, most internet searches say they are the same or +1/16" as the TH-2004r. I seem to recall (it's all a blur) reusing the original driveshaft with the correct 32 splined yoke, however internet searches do say that the 4" tail housing TH-400s are 3/4" longer than the TH-2004r. Also I have a friend who claimed that he reused the original DS w TH-400 yoke. In the end, best to measure and shorten as needed.

Fast forward today, I may do the same to my '85 Caballero. I treat Cabby like a truck so hardly any daily driving so I don't mind the no OD. The thought was to rebuild a TH-2004r to replace the TH-250c but if I'm doing the work...it may be an eternity before that happens. :sleep:
 

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