johnpas Posted October 23, 2008 Report Share Posted October 23, 2008 My friend has a 1975 cadillac coupe deville . He has a 500 ci engine with a TH400 trans . It is an older th400 with the longer tailhousing like 13 inches about. We pulled a th400 out of a 1977 sedan deville with a th400 short 4 inch tail shaft. I don't think we can change tail shaft housings from outside only . You Have to open trans and change output shaft too , right? So can we put a 9 inch longer drive shaft? Both cars had a one piece drive shaft. If we go back to salvage yard will the drive shaft work from the 77 sedan deville that had the shorter th400 4" tail? the 1975 coupe has a drive shaft 65" long . So I need a drive shaft 74" long? How much is it to lengthen a drive shaft 9 inches? Does anyone know about this stuff. We feel the trans we got out of 77 is good. Torque converter is shiny new blue paint says GMC on it too. So we think trans is new. fluid is nice and new. help please. John P. San diego. Link to comment Share on other sites More sharing options...
KHE Posted October 25, 2008 Report Share Posted October 25, 2008 The driveshaft from the '77 will most likely be too short due to the shorter wheelbase of the '77 car compared to the '75. Personally, I would rebuild the existing THM-400 and reinstall it vs. cobbling something together that may or may not work. The final drive ratios of the two transmissions are most likely different which presents another set of problems with the gearing in the rear differential. If you have the trans out, take it to a local rebuilder - I would bet they would rebuild the existing trans. for a reasonable price. Kevin '93 Fleetwood Brougham '05 Deville '04 Deville 2013 Silverado Z71 Link to comment Share on other sites More sharing options...
Cadillac Jim Posted October 25, 2008 Report Share Posted October 25, 2008 I did a quick check and a rebuild of a THM 400 to stock specs is about $850. The only thing you need to do is avoid extended warrantys and other worthless BS that drives the price up for nothing. -- Click Here for CaddyInfo page on "How To" Read Your OBD Codes-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data -- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC Yes, I was Jims_97_ETC before I changed cars. Link to comment Share on other sites More sharing options...
johnpas Posted October 27, 2008 Author Report Share Posted October 27, 2008 My friend has a 1975 cadillac coupe deville . He has a 500 ci engine with a TH400 trans . It is an older th400 with the longer tailhousing like 13 inches about. We pulled a th400 out of a 1977 sedan deville with a th400 short 4 inch tail shaft. I don't think we can change tail shaft housings from outside only . You Have to open trans and change output shaft too , right? So can we put a 9 inch longer drive shaft? Both cars had a one piece drive shaft. If we go back to salvage yard will the drive shaft work from the 77 sedan deville that had the shorter th400 4" tail? the 1975 coupe has a drive shaft 65" long . So I need a drive shaft 74" long? How much is it to lengthen a drive shaft 9 inches? Does anyone know about this stuff. We feel the trans we got out of 77 is good. Torque converter is shiny new blue paint says GMC on it too. So we think trans is new. fluid is nice and new. help please. John P. San diego. You are very right! 77 is shorter car. I went back to salvage yard with older gentleman who owns the 75 caddy. The drive shaft in 77 caddy is the same as the 75 caddy because they shortened the wheelbase. I agree he should take trans to a rebuilder . The problem is he does not have a lot of money he is older on SSI. I am a Good Samaritan helping him for free. He is an older 'spiritual Brother' from church. His drive shaft total is 65 inches. It has a double CArdan joint on trans end and a single on 3rd member end. The driveshaft itself is about 58 inches. It is 3 1/2 inches in Diameter so Max length you can have is 65 inches. 4 inch Dia. is 70 inches. If I can find a 66 to 67 inch drive shaft at salvage yard that takes a NEAPCO 2-3011 ujoint then I am good. A used shaft is 35.00 bucks . I told him my idea and he said he did not have 40.00 bucks till end of month . He has a spare brand new Battery from Kragens auto . I was thinking of buying his new battery from him to give him some money for shaft. WEll thank you guys. I am looking for a list online of Shaft lengths on different cars, but it seems that is Rare. Thank you John in San diego CA. Link to comment Share on other sites More sharing options...
KHE Posted October 27, 2008 Report Share Posted October 27, 2008 My friend has a 1975 cadillac coupe deville . He has a 500 ci engine with a TH400 trans . It is an older th400 with the longer tailhousing like 13 inches about. We pulled a th400 out of a 1977 sedan deville with a th400 short 4 inch tail shaft. I don't think we can change tail shaft housings from outside only . You Have to open trans and change output shaft too , right? So can we put a 9 inch longer drive shaft? Both cars had a one piece drive shaft. If we go back to salvage yard will the drive shaft work from the 77 sedan deville that had the shorter th400 4" tail? the 1975 coupe has a drive shaft 65" long . So I need a drive shaft 74" long? How much is it to lengthen a drive shaft 9 inches? Does anyone know about this stuff. We feel the trans we got out of 77 is good. Torque converter is shiny new blue paint says GMC on it too. So we think trans is new. fluid is nice and new. help please. John P. San diego. You are very right! 77 is shorter car. I went back to salvage yard with older gentleman who owns the 75 caddy. The drive shaft in 77 caddy is the same as the 75 caddy because they shortened the wheelbase. I agree he should take trans to a rebuilder . The problem is he does not have a lot of money he is older on SSI. I am a Good Samaritan helping him for free. He is an older 'spiritual Brother' from church. His drive shaft total is 65 inches. It has a double CArdan joint on trans end and a single on 3rd member end. The driveshaft itself is about 58 inches. It is 3 1/2 inches in Diameter so Max length you can have is 65 inches. 4 inch Dia. is 70 inches. If I can find a 66 to 67 inch drive shaft at salvage yard that takes a NEAPCO 2-3011 ujoint then I am good. A used shaft is 35.00 bucks . I told him my idea and he said he did not have 40.00 bucks till end of month . He has a spare brand new Battery from Kragens auto . I was thinking of buying his new battery from him to give him some money for shaft. WEll thank you guys. I am looking for a list online of Shaft lengths on different cars, but it seems that is Rare. Thank you John in San diego CA. John, Getting a driveshaft the right length is only part of the problem... The other problems are the different final drive ratios from the '77 trans compared to the original 1975 trans., speedometer mismatch, etc. The rear differential in the 1975 car so not likely to be set up to work with the 1977 trans. I realize your friend is short on money but cobbling something together will cost more money in the long run than fixing the original trans. If you have removed the trans, that saves a considerable amount of money in the rebuild process. Check with some local rebuilders and have them quote rebuilding the original trans if you bring the trans in to them. Kevin '93 Fleetwood Brougham '05 Deville '04 Deville 2013 Silverado Z71 Link to comment Share on other sites More sharing options...
johnpas Posted October 28, 2008 Author Report Share Posted October 28, 2008 My friend has a 1975 cadillac coupe deville . He has a 500 ci engine with a TH400 trans . It is an older th400 with the longer tailhousing like 13 inches about. We pulled a th400 out of a 1977 sedan deville with a th400 short 4 inch tail shaft. I don't think we can change tail shaft housings from outside only . You Have to open trans and change output shaft too , right? So can we put a 9 inch longer drive shaft? Both cars had a one piece drive shaft. If we go back to salvage yard will the drive shaft work from the 77 sedan deville that had the shorter th400 4" tail? the 1975 coupe has a drive shaft 65" long . So I need a drive shaft 74" long? How much is it to lengthen a drive shaft 9 inches? Does anyone know about this stuff. We feel the trans we got out of 77 is good. Torque converter is shiny new blue paint says GMC on it too. So we think trans is new. fluid is nice and new. help please. John P. San diego. You are very right! 77 is shorter car. I went back to salvage yard with older gentleman who owns the 75 caddy. The drive shaft in 77 caddy is the same as the 75 caddy because they shortened the wheelbase. I agree he should take trans to a rebuilder . The problem is he does not have a lot of money he is older on SSI. I am a Good Samaritan helping him for free. He is an older 'spiritual Brother' from church. His drive shaft total is 65 inches. It has a double CArdan joint on trans end and a single on 3rd member end. The driveshaft itself is about 58 inches. It is 3 1/2 inches in Diameter so Max length you can have is 65 inches. 4 inch Dia. is 70 inches. If I can find a 66 to 67 inch drive shaft at salvage yard that takes a NEAPCO 2-3011 ujoint then I am good. A used shaft is 35.00 bucks . I told him my idea and he said he did not have 40.00 bucks till end of month . He has a spare brand new Battery from Kragens auto . I was thinking of buying his new battery from him to give him some money for shaft. WEll thank you guys. I am looking for a list online of Shaft lengths on different cars, but it seems that is Rare. Thank you John in San diego CA. John, Getting a driveshaft the right length is only part of the problem... The other problems are the different final drive ratios from the '77 trans compared to the original 1975 trans., speedometer mismatch, etc. The rear differential in the 1975 car so not likely to be set up to work with the 1977 trans. I realize your friend is short on money but cobbling something together will cost more money in the long run than fixing the original trans. If you have removed the trans, that saves a considerable amount of money in the rebuild process. Check with some local rebuilders and have them quote rebuilding the original trans if you bring the trans in to them. Thank you for your input. Nice forum here. john Link to comment Share on other sites More sharing options...
clay888 Posted December 25, 2008 Report Share Posted December 25, 2008 All, Trans gears the same in '75 & '77 cars. final drive 1:1. on all. Internals of '77 could be taken out and slid into '75 case, using the planetaries and the output shaft from the '75. Internals likely the same but if different only likely to be 6 frictions in direct drum instead of 5. 5 frictions plenty good for non modified engine putting less than 600 foot lbs. Use the '75 valve body, modulator and mod valve, and the '75 govenor. Front pump seal, torque converter to pump seal and the trans extension seal a good time to do. The Ron sessions book on Turbo 400's a good buy under 25$ and almost anyone could rebuild this trans. Have done several with no special tools needed. Got Questions,? just ask! Clay888 Link to comment Share on other sites More sharing options...
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