[runningman]

After several years of procrastination I'm finally doing the auto to 4 speed muncie swap on my 67 GTO. I've seen the driveshaft scenario covered several times here with the answer usually being the TH400 driveshaft is simply "too short" to use. So doing some simple math the TH400 u-joint center-to-center dimension is 59.34" long. The factory assembly manual states the M20 driveshaft is 60.00" with a 4.95" yoke so a total length of 64.95". Using a Neapco N3R-3-6081X yoke that's 5.50" long makes the TH400 driveshaft 64.84" - less than an 1/8" shorter than the "60 inch" M20 driveshaft.

So question is does this slightly shorter 1/8" length matter? Does it matter the u-joint sits back roughly 5/8" more from the tranny?

The fact I couldn't find this outlined on the forum anywhere makes me think I might be missing something.

thanks in advance for the comments....

[Goatracer1]

I did something similar 14 years ago when I removed my T400 and installed a 200-4R. Haven't had any problem.

[connman]

The longer the yoke is from the end of the trans will cause a lot of stress on the rear bearing and tail shaft. By the time you buy the yoke and worry about what is going to happen when you drop the hammer on the 4 speed it will be worth the price to have a new drive shaft made. The factory could have done that and NOT made 2 different drive shafts so consider that. I had a new drive shaft made for my TKO600 that with yoke ends and 1350 Ford ujoints only cost me just less than $300. I had it made at a local big truck drive shaft shop. The yoke its self cost $100. You will reuse your ends so hat should save you about half what I paid. Dennys and another advertised drive shaft shop wanted almost $600 and up to 5 weeks delivery time.

[importsmasher]

I changed my 73 Ventura from a TH400 to a 4 speed 20 years ago and did not change the drive shaft (I had a shop shorten a random drive shaft I had laying around when I put the car together, as the Ventura didn't come with a TH400, and my car didn't come with a drive shaft anyway). I remember being surprised that it fit when I swapped in the 4 speed, as I had heard that it would need a longer shaft. I have beaten the snot out of it for many years, running upper 12s at the track. In fact, the entire drive train from the Ventura (engine, trans, driveshaft, and rear axle) is now in my 74 GTO, and still taking the abuse.

That being said, it is probably best to get the correct length; I am just a cheapskate....

Importsmasher

[Goatracer1]

I suppose I should tell you that my car runs high 10's and never spit out the driveshaft.