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Old 03-27-2010, 09:08 PM   #1
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St-300 swap to th400

All right so just dropped a rebuilt 406 in my 66 lemans. Drove it back home the other day and bout halfway home there was alot of trans spin. therefore im concluding that the clutch has gone bad. Not too big of a deal because i was planning on putting a th400 i have from a 72 olds 98 into it. I Believe it will bolt straight on to the motor. But as far as tail shaft length goes between the two im not certain. How much longer is the th400 tail shaft then the st-300, and how much of the driveshaft will i have to cut to make it fit. also are the yokes the same or will i have to take the yoke from the olds and put it on the end of my driveshaft?
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Old 04-07-2010, 10:13 AM   #2
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Re: St-300 swap to th400


Sorry I missed this one.

Is the TH400 a "long tail"? That is, is the tail housing about 9" long? Or about 4"? The "short tail" is considered the stronger one, and what was found in all the A- and F-bodies.

Either way, it WILL "fit" with some changes. The cross member will move back about 4". The holes may need to be drilled in the '66 frame ut not a big deal. I always installed the trans to the engine, slipped the cross memebrer into the frame, bolted the mount to it, and used that as my "drill guide".

You will need a slip-yoke from the larger TH400 output shaft. Install it with about 1/2" of the "shiny" surface showing beyond the seal. With the car level and the rear "at rest" (ride height), measure from the center of the slip-yoke to the center of the "companion flange" (the rear "yoke"). That's the length you give to the driveshaft shop when they cut and balance your driveshaft.

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