Drive shaft yoke spacing

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vintageman

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I have a 351C with FMX transmission, 8 inch 3.55 gears in the rear end. I pulled this transmission and driveshaft that had a 351C and 8 inch rear end as well from a 72 Mustang Coupe and it is now in in a 73 Mach 1 Fastback. I installed the drive shaft tonight and it is sitting front and rear on car dollies. I do have extended shackles on the rear (3 hole type, using the middle hole. Can't remember the amount of lift the 2nd hole gives to raising the rear end). With the driveshaft fully installed, I have about 1 1/2 inch of space that if the rear of the driveshaft were disconnected, I could probably move it all the way forward 1 1/2 inches although I did not check that. The drive shaft is engaging because I put the trans in Park and the driveshaft would only rotate a little bit, in Neutral, it would rotate. My question is: is this too much space for travel or am I good to go. I have read that you need about 3/4 of an inch for travel but I have double that. Thanks for any response and assistance. 

Tom

 

Bentworker

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Take 15 minutes to safely support the back the car in the air and remove the u-bolts on the rear axle yoke.  Slide the driveshaft forward and see how much play you actually have.  Sometimes it is not as much as you would think, as the transmission output shaft may bottom out in the slip yoke before you run out of machined surface on the slip yoke for the seal.

 

71ProjectJunk

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My understanding is that anything between 3/4" and 1 1/2" is fine.

 

vintageman

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Thanks guys for the replies, I will check again and follow back up here with the results.

Tom

 

Don C

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You also need to make sure you have enough spline contact. 3/4" should be enough play, when the rear springs are in their normal ride height. More play means less spline contact surface, which means less strength.

 

vintageman

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Thanks guys, I planned to get this checked tonight but won't be able until Friday night. More to follow. 

Tom

 

vintageman

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So, after I removed the driveshaft from the rear and pushed it all the way in then reconnected the driveshaft, I have about 1inch of play. I think I am good at the moment and hopefully will be able to drive this thing in the near future for final determination. Just in case, does anyone have a reputable shop name for lengthening drive shafts in Georgia?   Thanks.

Tom

 
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